hubeny

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Author name code: hubeny
ADS astronomy entries on 2022-09-14
author:"Hubeny, Ivan"

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Title: Properties of Primitive Galaxies
Authors: Heap, Sara R.; Hubeny, I.; Bouret, J. -C.; Lanz, T.;
Brinchmann, J.
2022arXiv220704553H Altcode:
We report on a study of 9 nearby star-forming, very low-metallicity
galaxies observed by Hubble's COS far-UV spectrograph that can serve
as templates of high-z galaxies to be observed by JWST. We find
that the nebular spectra of these primitive galaxies show evidence
of irradiation by X-ray emitters. Following Thuan et al. (2004),
we identify the sources of X-ray emission as massive X-ray binaries
containing a massive accreting stellar black hole. We further find
that the lower the metallicity, the higher the probability of strong
X-irradiation. Following Heger et al. (2003), we suggest that these
accreting black holes are produced by direct collapse of stars having
initial masses greater than $\sim50\, M_\odot$. Our models of young
star clusters with an embedded stellar black hole produce effects on
the surrounding gaseous medium that are consistent with the observed
spectra. We conclude that primitive galaxies are qualitatively different
from more metal-rich galaxies in showing evidence of hard radiation
that can best be explained by the presence of one or more embedded
stellar black holes.

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Title: How Do Spitzer IRAC Fluxes Compare to HST CALSPEC?
Authors: Bohlin, Ralph C.; Krick, Jessica E.; Gordon, Karl D.;
Hubeny, Ivan
2022AJ....164...10B Altcode: 2022arXiv220508322B
An accurate tabulation of stellar brightness in physical units is
essential for a multitude of scientific endeavors. The HST/CALSPEC
database of flux standards contains many stars with spectral coverage
in the 0.115-1 μm range, with some extensions to longer wavelengths of
1.7 or 2.5 μm. Modeled flux distributions to 32 μm for calibration of
JWST complement the shorter-wavelength HST measurements. Understanding
the differences between IRAC observations and CALSPEC models is
important for science that uses IR fluxes from multiple instruments,
including JWST. The absolute flux of Spitzer IRAC photometry at 3.6-8
μm agrees with CALSPEC synthetic photometry to 1% for the three prime
HST standards: G191B2B, GD153, and GD71. For a set of 17-22 A-star
standards, the average IRAC difference rises from agreement at 3.6 μm
to 3.4% ± 0.1% brighter than CALSPEC at 8 μm. For a smaller set of
G-type stars, the average of the IRAC photometry falls below CALSPEC
by as much as 3.7% ± 0.3% for IRAC1, while one G-type star, P330E,
is consistent with the A-star ensemble of IRAC/CALSPEC ratios.

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Title: WD 1145+017: Alternative models of the atmosphere, dust clouds,
and gas rings
Authors: Budaj, J.; Maliuk, A.; Hubeny, I.
2022A&A...660A..72B Altcode: 2022arXiv220307520B
Context. WD 1145+017 (WD1145) is the first white dwarf known to be
orbited by disintegrating exoasteroids. It is a DBZ-type white dwarf
with strongly variable broad circumstellar lines and variable shallow
ultraviolet (UV) transits. Various models of the dust clouds and gaseous
rings have been proposed as an explanation for this behavior. 
Aims: We aim to revisit these observations and propose alternative
or modified models of the atmosphere of this white dwarf, its dust
clouds, and gas rings. Methods: The simple radiative transfer
code Shellspec was modified for this purpose and used for testing
the new dust cloud and gas disk models. We used modified TLUSTY
and SYNSPEC codes to calculate atmosphere models assuming the local
thermodynamical equilibrium (LTE) or nonLTE (NLTE), and to calculate
the intrinsic spectrum of the star. We then used these atmosphere
models to estimate the mass of the radiative and convective zones and
NLTE spectrum synthesis to estimate their chemical composition. 
Results: We offer an alternative explanation of some (not all) shallow
UV transits. These may be naturally caused by the optical properties of
the dust grains: opacities and mainly phase functions as a result of the
forward scattering. The latter is much stronger in UV compared to the
optical region, leaving more UV photons in the original direction during
the transit. We also developed an alternative model of the gaseous disk,
consisting of an inner, hotter, and almost circular disk and an outer,
cooler, and eccentric disk. The structure precesses with a period of
3.83 ± 0.12 yr. We demonstrate that it fits the observed circumstellar
lines reasonably well. These alternative models solve a few drawbacks
that might be associated with the previous models, but they also have
their own disadvantages. We confirm that the chemical composition of
the atmosphere is similar to that of CI chondrites but carbon, nitrogen,
and sulfur are significantly underabundant and much closer to the bulk
Earth composition. This is a strong argument that the star has recently
encountered and accreted material from a body of Earth-like composition.

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Title: High Resolution Spectroscopy of the Unusual Spectral Feature
in the Central Compact Object PSR J0821-4300
Authors: Gotthelf, Eric; Bogdanov, Slavko; Paerels, Frits; Groger,
John; Helfand, David; Halpern, Jules; Hubeny, Ivan; Lanz, Thierry
2022HEAD...1911006G Altcode:
We present Chandra LETG high-resolution X-ray spectroscopy of the
unusual line feature seen in the blackbody emission from the central
compact object (CCO) in supernova remnant (SNR) Puppis~A. This feature
is likely associated with one of the antipodal hot-spots on the NS
surface and may provide the first detection of atomic photospheric
absorption lines, allowing a model independent, direct measurement of
the mass-radius relation, a strong constrain on the equation of state
of nuclear matter. If generated by cyclotron resonance scattering or
possibly from accretion of supernova debris, the line emission may
provide a localized measurement of the magnetic field, important for
reconciling the observed bright surface hot-spots unprecedented for
a neutron star with such a weak (2.8E-10 G) dipole magnetic field.

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Title: Accurate Metallicities for Very Metal-poor Stars from the Ca
II Infrared Triplet
Authors: Osorio, Yeisson; Aguado, David S.; Prieto, Carlos Allende;
Hubeny, Ivan; González Hernández, Jonay I.
2022ApJ...928..173O Altcode: 2022arXiv220301257O
The Ca II H and K lines are among the few features available to infer
the metallicity of extremely metal-poor stars from medium-resolution
spectroscopy. Unfortunately, these lines can overlap with absorption
produced in the intervening interstellar medium, introducing systematic
errors in the derived metallicities. The strength of the Ca II infrared
triplet lines can also be measured at extremely low metallicities,
and it is not affected by interstellar absorption, but it suffers
significant departures from local thermodynamic equilibrium (LTE). We
investigate the feasibility of adopting the Ca II infrared triplet as a
metallicity indicator in extremely metal-poor stars using state-of-the
art non-LTE models including the most recent atomic data. We find
that the triplet lines exhibit non-LTE abundance corrections that
can exceed 0.5 dex. When interstellar absorption affecting the Ca II
resonance lines is accounted for using high-resolution observations,
the agreement between non-LTE abundances for the triplet and those
for the resonance lines, with only minor departures from LTE, is
excellent. Non-LTE effects strengthen the Ca II IR triplet lines,
facilitating measurements at very low metallicities, compared with LTE
estimates, down to [Fe/H] = -6.0. This result has important implications
for the discovery of primitive stars in our Galaxy and others, since
instruments are most sensitive at red/near-infrared wavelengths, and
tens of millions of spectra covering the Ca II IR triplet will soon
become available from the Gaia, DESI, WEAVE, and PFS missions.

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Title: The Photospheric X-ray Spectrum of the Neutron Star in Puppis A
Authors: Groger, John; Paerels, Frits; Gotthelf, Eric; Halpern, Jules;
Helfand, David; Bogdanov, Slavko; Hubeny, Ivan; Lanz, Thierry
2022HEAD...1910107G Altcode:
We have collected 352.6 ksec (as of September 7, 2021) of a 400 ksec
Chandra observation of the central compact object PSR J0821-4300
in the supernova remnant Puppis A using the Low Energy Transmission
Grating Spectrometer. We have clearly detected photospheric radiation
from the neutron star with a temperature of approximately 2 million
degrees. We anticipate that we will have the sensitivity to detect
absorption features from highly-ionized metals that may be present at
the surface of the star. In this poster, we discuss our preliminary
analysis of the spectral data.

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Title: The Neutron Star in Puppis A: Atomic Photospheric Spectroscopy
at Last?
Authors: Paerels, Frits; Groger, John John; Helfand, David; Bogdanov,
Slavko; Gotthelf, Eric; Halpern, Jules; Hubeny, Ivan; Lanz, Thierry
2022HEAD...1910106P Altcode:
We have accumulated 400 ksec of exposure time with the Chandra Low
Energy Transmission Grating Spectrometer on the neutron star in the
supernova remnant Puppis A. We clearly detect photospheric radiation
from a ~ 2 million degree star. We anticipate that the spectrum will
have sufficient sensitivity for us to detect atomic absorption features
from highly ionized mid-Z atoms, which may be present at the surface
of the star. We discuss the data and our preliminary spectroscopic
analysis.

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Title: Simulation of Stark-broadened Hydrogen Balmer-line Shapes
for DA White Dwarf Synthetic Spectra
Authors: Cho, P. B.; Gomez, T. A.; Montgomery, M. H.; Dunlap, B. H.;
Fitz Axen, M.; Hobbs, B.; Hubeny, I.; Winget, D. E.
2022ApJ...927...70C Altcode:
White dwarfs (WDs) are useful across a wide range of astrophysical
contexts. The appropriate interpretation of their spectra relies on the
accuracy of WD atmosphere models. One essential ingredient of atmosphere
models is the theory used for the broadening of spectral lines. To date,
the models have relied on Vidal et al., known as the unified theory
of line broadening (VCS). There have since been advancements in the
theory; however, the calculations used in model atmosphere codes have
only received minor updates. Meanwhile, advances in instrumentation
and data have uncovered indications of inaccuracies: spectroscopic
temperatures are roughly 10% higher and spectroscopic masses are roughly
0.1 M ⊙ higher than their photometric counterparts. The
evidence suggests that VCS-based treatments of line profiles may be at
least partly responsible. Gomez et al. developed a simulation-based
line-profile code Xenomorph using an improved theoretical treatment
that can be used to inform questions around the discrepancy. However,
the code required revisions to sufficiently decrease noise for use in
model spectra and to make it computationally tractable and physically
realistic. In particular, we investigate three additional physical
effects that are not captured in the VCS calculations: ion dynamics,
higher-order multipole expansion, and an expanded basis set. We also
implement a simulation-based approach to occupation probability. The
present study limits the scope to the first three hydrogen Balmer
transitions (Hα, Hβ, and Hγ). We find that screening effects and
occupation probability have the largest effects on the line shapes and
will likely have important consequences in stellar synthetic spectra.

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Title: All-Order Full-Coulomb Quantum Spectral Line-Shape Calculations
Authors: Gomez, T. A.; Nagayama, T.; Cho, P. B.; Zammit, M. C.;
Fontes, C. J.; Kilcrease, D. P.; Bray, I.; Hubeny, I.; Dunlap, B. H.;
Montgomery, M. H.; Winget, D. E.
2021PhRvL.127w5001G Altcode:
Understanding how atoms interact with hot dense matter is essential for
astrophysical and laboratory plasmas. Interactions in high-density
plasmas broaden spectral lines, providing a rare window into
interactions that govern, for example, radiation transport in
stars. However, up to now, spectral line-shape theories employed
at least one of three common approximations: second-order Taylor
treatment of broadening operator, dipole-only interactions between
atom and plasma, and classical treatment of perturbing electrons. In
this Letter, we remove all three approximations simultaneously for
the first time and test the importance for two applications: neutral
hydrogen and highly ionized magnesium and oxygen. We found 15%-50%
change in the spectral line widths, which are sufficient to impact
applications including white-dwarf mass determination, stellar-opacity
research, and laboratory plasma diagnostics.

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Title: The Stars Like Dust: Inferring a Probabilistic Extinction
Law from STIS UV Spectroscopy of the Cosmic Flux Standards
Authors: Narayan, Gautham; Calamida, Annalisa; Axelrod, Tim; Bohlin,
Ralph C.; Deustua, Susana E.; Holberg, Jay B.; Hubeny, Ivan; MacKenty,
John W.; Matheson, Thomas; Olszewski, Edward W.; Points, Sean D.;
Rauch, Thomas; Rest, Armin; Sabbi, Elena; Saha, Abhijit; Stubbs,
Christopher W.
2021hst..prop16764N Altcode:
Our understanding of dust, encoded in our knowledge of the extinction
law, is one of the leading sources of systematic bias impacting
cosmology today. We propose to measure the extinction law, and its
variation, directly through STIS FUV and NUV spectroscopy of 21 faint
white dwarf stars, the Cosmic Flux Standards. These standards have
SEDs that are exceptionally well-constrained by high S/N legacy HST
panchromatic imaging, detailed ground-based optical spectroscopy, and
temporal monitoring to assess variability. These Cosmic Flux Standards
extend the CALSPEC system down to V~19.5 mag, and has been demonstrated
that measurements of these standards will allow 1% absolute photometric
calibration and 0.5% relative calibration, suitable for major upcoming
projects such as the Vera Rubin Observatory and Nancy Grace Roman
Space Telescope. These observations will allow a direct measurement
of the extinction law by comparing the STIS spectrophotometry to the
unreddened SEDs of the Cosmic Flux Standards, allowing a population
inference of the extinction law, as well as line-of-sight estimates
to each object. This probabilistic model of the extinction law will
directly address dust as a source of systematic error, and the STIS
FUV and NUV observations will also extend the validity of the standards
to 1,150 Angstrom in preparation for future UV space-missions.

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Title: An improved model for the spectra of discs of nova-like
variables
Authors: Hubeny, Ivan; Long, Knox S.
2021MNRAS.503.5534H Altcode: 2021arXiv210309341H; 2021MNRAS.tmp..821H
The spectra arising from the discs of nova-like variables show many of
the features seen in stellar atmospheres. They are typically modelled
either from an appropriated weighted set of stellar atmospheres or a
disc atmosphere with energy is dissipated near the disc plane, with
the effective temperature distribution expected from a steady-state
accretion disc. However, these models generally overpredict the depth of
the Balmer jump and the slope of the spectrum in the ultraviolet. The
problem is likely due to energy dissipation in the disc atmosphere,
which produces a flatter vertical temperature profile than is observed
in stars. Here, we provide validation for this hypothesis in the
form of spectra generated using the stellar atmosphere code TLUSTY
using a parametric prescription for energy dissipation as a function
of depth and closely match the spectrum of the nova-like IX Vel over
the wavelength range 1150-6000 Å.

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Title: TLUSTY and SYNSPEC Users's Guide IV: Upgraded Versions 208
and 54
Authors: Hubeny, Ivan; Allende Prieto, Carlos; Osorio, Yeisson;
Lanz, Thierry
2021arXiv210402829H Altcode:
We present a brief description of the newly upgraded versions of the
programs TLUSTY, version 208, and SYNSPEC, version 54. TLUSTY is used
to generate model stellar atmospheres or accretion disks, and SYNSPEC
produces detailed synthetic spectra and/or opacity tables. This paper
complements published guides that provide a detailed description
of earlier versions of the codes, TLUSTY205, and SYNSPEC51. The
main upgrades include the flexible construction of opacity tables
in SYNSPEC, and their use in producing hybrid models with TLUSTY},
with important species treated in NLTE, while the bulk of opacity of
atomic and molecular lines and continua are considered in LTE using
a pre-calculated opacity table. There is also a number of additional
changes and upgrades that increase the versatility and flexibility of
these codes.

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Title: Massive stars in the Small Magellanic Cloud. Evolution,
rotation, and surface abundances
Authors: Bouret, J. -C.; Martins, F.; Hillier, D. J.; Marcolino,
W. L. F.; Rocha-Pinto, H. J.; Georgy, C.; Lanz, T.; Hubeny, I.
2021A&A...647A.134B Altcode: 2021arXiv210109269B
Context. The evolution of massive stars depends on several physical
processes and parameters. Metallicity and rotation are among the most
important, but their quantitative effects are not well understood. Aims: To complement our earlier study on main-sequence stars,
we study the evolutionary and physical properties of evolved O stars
in the Small Magellanic Cloud (SMC). We focus in particular on their
surface abundances to further investigate the efficiency of rotational
mixing as a function of age, rotation, and global metallicity. 
Methods: We analysed the UV and optical spectra of 13 SMC O-type giants
and supergiants using the stellar atmosphere code CMFGEN to derive
photospheric and wind properties. We compared the inferred properties to
theoretical predictions from evolution models. For a more comprehensive
analysis, we interpret the results together with those we previously
obtained for O-type dwarfs. Results: Most dwarfs of our sample
lie in the early phases of the main sequence. For a given initial mass,
giants are farther along the evolutionary tracks, which confirms that
they are indeed more evolved than dwarfs. Supergiants have higher
initial masses and are located past the terminal-age main-sequence in
each diagram. We find no clear trend of a mass discrepancy, regardless
of the diagram that was used to estimate the evolutionary mass. Surface
CNO abundances are consistent with nucleosynthesis from the CNO
cycle. Comparisons to theoretical predictions reveal that the initial
mixture is important when the observed trends in the N/C versus N/O
diagram are to be reproduced. A trend for stronger chemical evolution
for more evolved objects is observed. Above about 30 M⊙,
more massive stars are on average more chemically enriched at a given
evolutionary phase. Below 30 M⊙, the trend vanishes. This
is qualitatively consistent with evolutionary models. A principal
component analysis of the abundance ratios for the whole (dwarfs and
evolved stars) sample supports the theoretical prediction that massive
stars at low metallicity are more chemically processed than their
Galactic counterparts. Finally, models including rotation generally
reproduce the surface abundances and rotation rates when different
initial rotational velocities are considered. Nevertheless, for some
objects, a stronger braking and/or more efficient mixing is required. This research is based on observations made with the NASA/ESA Hubble
Space Telescope obtained from the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS 5-26555. These observations
are associated with programmes GO 7437, GO 9434, and GO 11625. Based on data products from observations made with ESO Telescopes
at the La Silla Paranal Observatory under programmes ID 67.D-0238,
70.D-0164, 074.D-0109, 079.D-0073, and 079.D-0562.

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Title: All-Order Full-Coulomb Quantum Spectral Line Shape Calculations
Authors: Gomez, Thomas; Nagayama, Taisuke; Cho, Patricia; Zammit,
Mark; Fontes, Chris; Kilcrease, David; Bray, Igor; Hubeny, Ivan;
Dunlap, Bart; Montgomery, Michael; Winget, Don
2021APS..DPPPO3009G Altcode:
Understanding how atoms interact with hot dense matter (HDM) is
essential for astrophysical and laboratory plasmas modeling and
analysis. In high density plasmas, spectral lines are significantly
pressure broadened. Line shape calculations and measurements thus
provide a rare window that lets us examine how atoms interact in dense
plasmas, for example inside stars. Up to now, spectral line-shape
theories employed at least one of the following approximations:
dipole, 2nd-order perturbation, and semi-classical. Here,
we remove all three approximations simultaneously for the first time
and test the importance for two applications: neutral hydrogen and
highly-ionized magnesium. We found 15-50% change in the spectral line
widths. We will discuss potential impact on white-dwarf spectroscopy,
and plasma diagnostics. SNL is managed and operated by NTESS
under DOE NNSA contract DE-NA0003525.

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Title: Mass-Metallicity Trends in Transiting Exoplanets from
Atmospheric Abundances of H2O, Na, and K
Authors: Welbanks, Luis; Madhusudhan, Nikku; Allard, Nicole F.;
Hubeny, Ivan; Spiegelman, Fernand; Leininger, Thierry
2020EPSC...14..168W Altcode:
Atmospheric compositions can provide powerful diagnostics of formation
and migration histories of planetary systems. In this talk, I will
present the results of our latest survey of atmospheric compositions
focused on atmospheric abundances of H2O, Na, and K. We employ a sample
of 19 exoplanets spanning from cool mini-Neptunes to hot Jupiters,
with equilibrium temperatures between ~300 and 2700 K. We employ the
latest transmission spectra, new H2 broadened opacities of Na and K,
and homogeneous Bayesian retrievals. We confirm detections of H2O in 14
planets and detections of Na and K in 6 planets each. Among our sample,
we find a mass-metallicity trend of increasing H2O abundances with
decreasing mass, spanning generally substellar values for gas giants
and stellar/superstellar for Neptunes and mini-Neptunes. However,
the overall trend in H2O abundances, is significantly lower than
the mass-metallicity relation for carbon in the solar system giant
planets and similar predictions for exoplanets. On the other hand, the
Na and K abundances for the gas giants are stellar or superstellar,
consistent with each other, and generally consistent with the solar
system metallicity trend. The H2O abundances in hot gas giants are
likely due to low oxygen abundances relative to other elements rather
than low overall metallicities, and provide new constraints on their
formation mechanisms. Our results show that the differing trends in
the abundances of species argue against the use of chemical equilibrium
models with metallicity as one free parameter in atmospheric retrievals,
as different elements can be differently enhanced.

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Title: VizieR Online Data Catalog: Candidate spectrophotometric
standard DA WDs (Calamida+, 2019)
Authors: Calamida, A.; Matheson, T.; Saha, A.; Olszewski, E.; Narayan,
G.; Claver, J.; Shanahan, C.; Holberg, J.; Axelrod, T.; Bohlin, R.;
Stubbs, C. W.; Deustua, S.; Hubeny, I.; MacKenty, J.; Points, S.;
Rest, A.; Sabbi, E.
2020yCat..18720199C Altcode:
Photometric data discussed in this investigation were collected with
the WFC3 UVIS and IR cameras on board the HST during Cycle 20 and 22
(proposals GO-12967 and GO-13711, PI: Saha). Observations were taken in
five filters in Cycle 20, namely F336W, F475W, F625W, F775W, F160W. In
Cycle 22 the near-UV filter F275W was added. Nine of the candidate
DA white dwarfs (DAWDs) are distributed along the celestial equator
and were observed in both Cycle 20 and Cycle 22, while the other 14
DAWDs and the three HST primary CALSPEC standards were observed only
in Cycle 22. Table 4 lists the log of the observations for Cycle 20
and 22. Observations span a time interval of about 1yr for Cycle 20
(2012 November until 2013 September) and about 1.3yr for Cycle 22
(2014 September until 2016 January). The final magnitudes for the 23
candidate spectrophotometric standard DAWDs and the HST primary CALSPEC
WDs are listed in Table 9. We used two different facilities to
obtain spectra of our standard star candidates. As part of the HST
photometry proposal, we were awarded Gemini/GMOS time. This amounted
to 43hr from Cycle 20 (split over Gemini semesters 2013A and 2013B) and
28.1hr from Cycle 22 (split over Gemini semesters 2015A and 2015B). The
final spectra are continuous from 3500 to 6360Å. Alternate spectra were
obtained in 2015 at the Multiple Mirror Telescope (MMT) with the Blue
Channel spectrograph (wavelength coverage from 3400 to 8400Å). Details
of the observations with both facilities are presented in Table 5. (6 data files).

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Title: New Grids of Pure-hydrogen White Dwarf NLTE Model Atmospheres
and the HST/STIS Flux Calibration
Authors: Bohlin, Ralph C.; Hubeny, Ivan; Rauch, Thomas
2020AJ....160...21B Altcode: 2020arXiv200510945B
Nonlocal thermodynamic equilibrium (NLTE) calculations of hot white
dwarf (WD) model atmospheres are the cornerstone of modern flux
calibrations for the Hubble Space Telescope and for the CALSPEC
database. These theoretical spectral energy distributions (SEDs)
provide the relative flux versus wavelength, and only the absolute
flux level remains to be set by reconciling the measured absolute flux
of Vega in the visible with the Midcourse Space Experiment values for
Sirius in the mid-IR. The most recent SEDs calculated by the TLUSTY
and TMAP NLTE model atmosphere codes for the primary WDs G191-B2B,
GD 153, and GD 71 show improved agreement to 1% from 1500 Å to 30
μm, in comparison to the previous 1% consistency only from 2000 Å
to 5 μm. These new NLTE models of hot WDs now provide consistent flux
standards from the far UV to the mid-IR.

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Title: H-He collision-induced satellite in the Lyman α profile of
DBA white dwarf stars
Authors: Allard, Nicole F.; Kielkopf, John F.; Xu, Siyi; Guillon,
Grégoire; Mehnen, Bilel; Linguerri, Roberto; Al Mogren, Muneerah
Mogren; Hochlaf, Majdi; Hubeny, Ivan
2020MNRAS.494..868A Altcode: 2020arXiv200305110A; 2020MNRAS.tmp..656A
The spectra of helium-dominated white dwarf stars with hydrogen in
their atmosphere present a distinctive broad feature centred around 1160
Å in the blue wing of the Lyman α line. It is extremely apparent in
WD 1425+540 recently observed with Hubble Space Telescope(HST) Cosmic
Origins Spectrograph (COS). With new theoretical line profiles based on
ab initio atomic interaction potentials we show that this feature is
a signature of a collision-induced satellite due to an asymptotically
forbidden transition. This quasi-molecular spectral satellite is
crucial to understanding the asymmetrical shape of Lyman α seen
in this and other white dwarf spectra. Our previous work predicting
this absorption feature was limited by molecular potentials that were
not adequate to follow the atomic interactions with spectroscopic
precision to the asymptotic limit of large separation. A new set of
potential energy curves and electronic dipole transition moments for
the lowest electronic states of the H-He system were developed to
account accurately for the behaviour of the atomic interactions at
all distances, from the chemical regime within 1 Å out to where
the radiating H atoms are not significantly perturbed by their
neighbours. We use a general unified theory of collision-broadened
atomic spectral lines to describe a rigorous treatment of hydrogen
Lyman α with these potentials and present a new study of its broadening
by radiative collisions of hydrogen and neutral helium. These results
enable ab initio modelling of radiative transport in DBA white dwarf
atmospheres.

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Title: NLTE for APOGEE: simultaneous multi-element NLTE radiative
transfer
Authors: Osorio, Y.; Allende Prieto, C.; Hubeny, I.; Mészáros, Sz.;
Shetrone, M.
2020A&A...637A..80O Altcode: 2020arXiv200313353O
Context. Relaxing the assumption of local thermodynamic equilibrium
(LTE) in modelling stellar spectra is a necessary step to determine
chemical abundances to better than about 10% in late-type stars. 
Aims: We describe our multi-element (Na, Mg, K, and Ca) non-LTE (NLTE)
calculations, which can be applied to the APOGEE survey. Methods:
The new version of TLUSTY allows for the calculation of restricted
NLTE in cool stars using pre-calculated opacity tables. We demonstrate
that TLUSTY gives consistent results with MULTI, a well-tested code
for NLTE in cool stars. We used TLUSTY to perform LTE and a series
of NLTE calculations that simultaneously used all combinations of
one, two, three and four of the elements in NLTE. Results: We
take into account that departures from LTE in one element can affect
others through changes in the opacities of Na, Mg, K, and Ca. We
find that atomic Mg, which provides strong UV opacity and exhibits
significant departures from LTE in the low-energy states, can affect
the NLTE populations of Ca, leading to abundance corrections as large
as 0.07 dex. The differences in the derived abundances between the
single-element and the multi-element cases can exceed those between the
single-element NLTE determinations and an LTE analysis. We therefore
caution that this is not always a second-order effect. Based on
detailed tests for three stars with reliable atmospheric parameters
(Arcturus, Procyon, and the Sun), we conclude that our NLTE calculations
provide abundance corrections that can in the optical amount to 0.1,
0.2, and 0.7 dex for Ca, Na and K, but LTE is a good approximation
for Mg. In the H-band, NLTE corrections are much smaller and always
lower than 0.1 dex. The derived NLTE abundances in the optical and
in the IR are consistent. In all three stars, NLTE line profiles
fit the observations better than the LTE counterparts for all four
elements. Conclusions: The atomic elements in ionisation stages
where over-ionisation is an important NLTE mechanism are likely affected
by departures from LTE in Mg. Particular care must be taken with the
collisions that are adopted for high-lying levels when NLTE profiles
of lines in the H-band are calculated. The derived NLTE corrections in
the optical and in the H-band differ, but the derived NLTE abundances
are consistent between the two spectral regions.

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Title: Supernovae Chemical Yields in Magellanic Cloud Environments
Authors: Peters, G. J.; Lanz, T.; Bouret, J.; Proffitt, C. R.; Adelman,
S. J.; Hubeny, I.
2020AAS...23511025P Altcode:
Recently there has been interest in the abundance of Mn and other
Fe group elements as diagnostics for determining the progenitors of
SNe Ia and their role in the chemical evolution of a galaxy. We have
combined recent spectroscopic observations from the Cosmic Origins
Spectrograph (COS) on the Hubble Space Telescope with archival data from
the Far Ultraviolet Spectroscopic Explorer (FUSE) and ESO's VLT/UVES to
determine the abundances of the Fe group elements (Ti, V, Cr, Mn, Fe,
Co, &amp; Ni) in main sequence B stars in the Magellanic Clouds. Here we
report results for NGC 1818-D1 (LMC) and AV 304 (SMC). The analysis was
carried through using the Hubeny/Lanz NLTE programs TLUSTY/SYNSPEC. The
COS observations were secured with the G130M, G160M, G185M, and G225M
gratings. Combined with the FUSE data, we have achieved spectral
coverage in the UV from 950 to 2400 A. Measurable lines from the Fe
group, except for a very few multiplets of Fe II, III are not found
in optical spectra. The following stellar parameters were adopted:
Teff = 24700 K, log g = 4.0 cm/s2, Vturb =
0 km/s, and v sin i = 30 km/s for NGC 1818-D1, and Teff =
27500 K, log g = 3.7 cm/s2, Vturb = 1 km/s, and v sin i =
8 km/s for AV 304. Both stars show Fe group abundances about 0.2-0.3
dex smaller than their lighter elements when compared to solar values
and the LMC/SMC averages. However Ti, an alpha-process element, appears
to have an abundance more in line with LMC/SMC values. [Mn/Fe] ranges
from 0.2 dex (NGC 1818-D1) to 0.1 dex (AV 304) with an uncertainty of
0.2 dex, and implies that the progenitors that produced their Fe group
material probably had Chandrasekhar masses (cf. Seitenzahl et al. 2013,
A&amp;A, 559, L5). Support from STScI grants HST-GO-14081.002 and
HST-GO-13346.022, and USC's Women in Science and Engineering (WiSE)
program are greatly appreciated.

---------------------------------------------------------
Title: Mass-Metallicity Trends in Transiting Exoplanets from
Atmospheric Abundances of H2O, Na, and K
Authors: Welbanks, Luis; Madhusudhan, Nikku; Allard, Nicole F.;
Hubeny, Ivan; Spiegelman, Fernand; Leininger, Thierry
2019ApJ...887L..20W Altcode: 2019arXiv191204904W
Atmospheric compositions can provide powerful diagnostics of formation
and migration histories of planetary systems. We investigate constraints
on atmospheric abundances of H2O, Na, and K, in a sample
of transiting exoplanets using the latest transmission spectra and
new H2 broadened opacities of Na and K. Our sample of
19 exoplanets spans from cool mini-Neptunes to hot Jupiters, with
equilibrium temperatures between ∼300 and 2700 K. Using homogeneous
Bayesian retrievals we report atmospheric abundances of Na, K, and
H2O, and their detection significances, confirming 6 planets
with strong Na detections, 6 with K, and 14 with H2O. We find
a mass-metallicity trend of increasing H2O abundances with
decreasing mass, spanning generally substellar values for gas giants
and stellar/superstellar for Neptunes and mini-Neptunes. However,
the overall trend in H2O abundances, from mini-Neptunes
to hot Jupiters, is significantly lower than the mass-metallicity
relation for carbon in the solar system giant planets and similar
predictions for exoplanets. On the other hand, the Na and K abundances
for the gas giants are stellar or superstellar, consistent with each
other, and generally consistent with the solar system metallicity
trend. The H2O abundances in hot gas giants are likely due
to low oxygen abundances relative to other elements rather than low
overall metallicities, and provide new constraints on their formation
mechanisms. The differing trends in the abundances of species argue
against the use of chemical equilibrium models with metallicity as
one free parameter in atmospheric retrievals, as different elements
can be differently enhanced.

---------------------------------------------------------
Title: New insight on accretion shocks onto young stellar
objects. Chromospheric feedback and radiation transfer
Authors: de Sá, L.; Chièze, J. -P.; Stehlé, C.; Hubeny, I.; Lanz,
T.; Cayatte, V.
2019A&A...630A..84D Altcode:
Context. Material accreted onto classical T Tauri stars is expected
to form a hot quasi-periodic plasma structure that radiates
in X-rays. Simulations of this phenomenon only partly match
observations. They all rely on a static model for the chromosphere
and on the assumption that radiation and matter are decoupled. 
Aims: We explore the effects of a shock-heated chromosphere and of
the coupling between radiation and hydrodynamics on the structure
and dynamics of the accretion flow. Methods: We simulated
accretion columns that fall onto a stellar chromosphere using the
1D ALE code AstroLabE. This code solves the hydrodynamics equations
along with the first two moment equations for radiation transfer,
with the help of a dedicated opacity table for the coupling between
matter and radiation. We derive the total electron and ion densities
from collisional-radiative model. Results: The chromospheric
acoustic heating affects the duration of the cycle and the structure
of the heated slab. In addition, the coupling between radiation
and hydrodynamics leads to a heating of the accretion flow and of
the chromosphere: the whole column is pushed up by the inflating
chromosphere over several times the steady chromosphere thickness. These
last two conclusions are in agreement with the computed monochromatic
intensity. Acoustic heating and radiation coupling affect the amplitude
and temporal variations of the net X-ray luminosity, which varies
between 30 and 94% of the incoming mechanical energy flux, depending
on which model is considered.

---------------------------------------------------------
Title: Stars and Stellar Black Holes in the Low-metallicity Galaxy
I Zw 18
Authors: Heap, S. R.; Hubeny, I.; Lanz, T. M.
2019ASPC..519..267H Altcode:
I Zw 18 is a star-forming dwarf galaxy having a very low metal content,
O/H ∼ 1/50 solar (Skillman &amp; Kennicutt 1993). While galaxies
with such low metallicity are rare in the low-redshift universe,
they are likely to be common in galaxies at cosmic dawn. Thus,
I Zw 18 is a “living” template for z &gt; 6 galaxies. We have
obtained HST/COS far-UV spectra of the northwest star cluster in I
Zw 18 and have compared them to SYNSPEC model spectra by Lanz &amp;
Hubeny (2003, 2007) in order to determine the properties of the stellar
population. We have also compared the observed spectra of I Zw 18-NW to
the CLOUDSPEC models (Hubeny et al. 2000) of the stellar cluster with
an embedded ultra-luminous X-ray source (ULX). This comparison reveals
feedback of the stellar black hole in the form of photoionization and
heating. Such models can be used as starting points to explore the
physical conditions in which stars and black holes form and evolve in
an extremely low-metallicity environment at high redshift.

---------------------------------------------------------
Title: The Radial Oxygen Abundance Gradient from OB Stars in the
Outer Galactic Disk
Authors: Daflon, Simone; Bragança, Gustavo A.; Lanz, Thierry; Cunha,
Katia; Hubeny, Ivan; Bensby, Thomas; McMillan, Paul J.; Garmany,
Kate; Glaspey, John W.; Borges Fernandes, Marcelo; Oey, Sally
2019ASPC..519..213D Altcode:
Metallicity gradients are important constraints for models of chemical
evolution of the Galaxy. We present the current radial gradient of
oxygen abundances using a sample of main-sequence OB stars located
in the outer Galactic Disk, considering non-NLTE abundances, and
distances based on GAIA DR2 results. We compare the obtained gradient
with results from other young objects of the Galactic Disk.

---------------------------------------------------------
Title: 3D Spectral Radiative Transfer and Perspectives for
Spectroscopic Diagnostics
Authors: Ibgui, Laurent; Hubeny, Ivan; Lanz, Thierry; González,
Matthias; Stehlé, Chantal; Orlando, Salvatore; Colombo, Salvatore
2019ASPC..519...21I Altcode:
We present the features of the three-dimensional (3D) radiative
transfer code IRIS, which synthesizes spectra emitted from hydrodynamics
structures. We discuss our first application of IRIS to the analysis
of a laboratory radiative shock. We demonstrate, with the help
of the radiation hydrodynamics (RHD) code HERACLES, the existence
of a radiative precursor. We validate the gray approximated model
M1 used by HERACLES. We show a couple of synthesized X-UV
spectra of such a shock. We finally open up our discussion to the future
possibilities of spectroscopic diagnostics of accreting streams onto
the surface of Classical T Tauri stars (CTTSs).

---------------------------------------------------------
Title: Radiation Transport in Astronomical Objects: Yesterday, Today,
and Tomorrow
Authors: Hubeny, Ivan
2019ASPC..519...75H Altcode:
First, I present a briefly description of some highlights of research
that my collaborators and I achieved in the field of astrophysical
radiative transfer and its application to modeling stellar and
planetary atmospheres and accretion disks. On the application side,
an emphasis is given to the computer programs TLUSTY for computing
model stellar atmospheres and disks, COOLTLUSTY, its variant for
modeling atmospheres of giant planets and brown dwarfs, and SYNSPEC,
a general spectrum synthesis program. Then, I briefly summarize the
present state and current upgrades of these codes. Finally, I outline
several general topics that I think will be pursued in the near future,
such as 3-D radiation hydrodynamic description of atmospheres and disks,
and complex non-equilibrium models of exoplanetary atmospheres.

---------------------------------------------------------
Title: Radiation Feedback in Accretion Shocks on Young Stars
Authors: de Sá, Lionel; Stehlé, Chantal; Chièze, Jean-Pierre;
Hubeny, Ivan; Lanz, Thierry; Colombo, Salvatore; Cayatte, Véronique;
Ibgui, Laurent; Orlando, Salvatore
2019ASPC..519..281D Altcode:
The first stages of pre-main sequence stars evolution are governed
by exchanges of mass and momentum between the proto-star and its
accretion disk. These quantities remain uncertain due to numerous
unanswered questions concerning the topology of the accretion flow,
its temperature, and its dynamics. In this work, we first present 1D
simulations of accretion columns falling onto a a stellar chromosphere;
the goal is to inspect the feedback of the dynamics of the chromosphere
on the accretion flow. Then, we analyze the effect of radiation
absorption and emission on the flow. The simulations take benefit of
a newly generated base of opacities computed by SYNSPEC code for the
conditions of the present astrophysical process. We finally present
our perspectives on this topic.

---------------------------------------------------------
Title: Radial abundance gradients in the outer Galactic disk as
traced by main-sequence OB stars
Authors: Bragança, G. A.; Daflon, S.; Lanz, T.; Cunha, K.; Bensby,
T.; McMillan, P. J.; Garmany, C. D.; Glaspey, J. W.; Borges Fernandes,
M.; Oey, M. S.; Hubeny, I.
2019A&A...625A.120B Altcode: 2019arXiv190404340B
Context. Elemental abundance gradients in galactic disks are important
constraints for models of how spiral galaxies form and evolve. However,
the abundance structure of the outer disk region of the Milky Way
is poorly known, which hampers our understanding of the spiral
galaxy that is closest to us and that can be studied in greatest
detail. Young OB stars are good tracers of the present-day chemical
abundance distribution of a stellar population and because of their
high luminosities they can easily be observed at large distances,
making them suitable to explore and map the abundance structure and
gradients in the outer regions of the Galactic disk. Aims: Using
a sample of 31 main-sequence OB stars located between galactocentric
distances 8.4-15.6 kpc, we aim to probe the present-day radial abundance
gradients of the Galactic disk. Methods: The analysis is
based on high-resolution spectra obtained with the MIKE spectrograph
on the Magellan Clay 6.5-m telescope on Las Campanas. We used a
non-NLTE analysis in a self-consistent semi-automatic routine based
on TLUSTY and SYNSPEC to determine atmospheric parameters and chemical
abundances. Results: Stellar parameters (effective temperature,
surface gravity, projected rotational velocity, microturbulence, and
macroturbulence) and silicon and oxygen abundances are presented for 28
stars located beyond 9 kpc from the Galactic center plus three stars
in the solar neighborhood. The stars of our sample are mostly on the
main-sequence, with effective temperatures between 20 800-31 300 K,
and surface gravities between 3.23-4.45 dex. The radial oxygen and
silicon abundance gradients are negative and have slopes of -0.07
dex kpc-1 and -0.09 dex kpc-1, respectively,
in the region 8.4 ≤ RG ≤ 15.6 kpc. Conclusions:
The obtained gradients are compatible with the present-day oxygen and
silicon abundances measured in the solar neighborhood and are consistent
with radial metallicity gradients predicted by chemodynamical models
of Galaxy Evolution for a subsample of young stars located close to
the Galactic plane. Based on data obtained with the Magellan Clay
telescope at the Las Campanas observatory and the ESO/MPI telescope
at La Silla under the ESO-ON agreement.

---------------------------------------------------------
Title: Subpercent Photometry: Faint DA White Dwarf Spectrophotometric
Standards for Astrophysical Observatories
Authors: Narayan, Gautham; Matheson, Thomas; Saha, Abhijit; Axelrod,
Tim; Calamida, Annalisa; Olszewski, Edward; Claver, Jenna; Mandel,
Kaisey S.; Bohlin, Ralph C.; Holberg, Jay B.; Deustua, Susana; Rest,
Armin; Stubbs, Christopher W.; Shanahan, Clare E.; Vaz, Amali L.;
Zenteno, Alfredo; Strampelli, Giovanni; Hubeny, Ivan; Points, Sean;
Sabbi, Elena; Mackenty, John
2019ApJS..241...20N Altcode: 2018arXiv181112534N
We have established a network of 19 faint (16.5 mag &lt; V &lt; 19 mag)
northern and equatorial DA white dwarfs (WDs) as spectrophotometric
standards for present and future wide-field observatories. Our analysis
infers spectral energy distribution (SED) models for the stars that
are tied to the three CALSPEC primary standards. Our SED models are
consistent with panchromatic Hubble Space Telescope photometry to
better than 1%. The excellent agreement between observations and
models validates the use of non-LTE DA WD atmospheres extinguished
by interstellar dust as accurate spectrophotometric references. Our
standards are accessible from both hemispheres and suitable for
ground- and space-based observatories covering the ultraviolet to the
near-infrared. The high precision of these faint sources makes our
network of standards ideally suited for any experiment that has very
stringent requirements on flux calibration, such as studies of dark
energy using the Large Synoptic Survey Telescope and the Wide-field
Infrared Survey Telescope.

---------------------------------------------------------
Title: New insight on Young Stellar Objects accretion shocks --
a claim for NLTE opacities
Authors: de Sá, Lionel; Chièze, Jean-Pierre; Stehlé, Chantal;
Hubeny, Ivan; Lanz, Thierry; Cayatte, Véronique
2019arXiv190409156D Altcode:
Context. Accreted material onto CTTSs is expected to form a hot
quasi-periodic plasma structure that radiates in X-rays. Simulations
of this phenomenon only partly match with observations. They all rely
on a static model for the chromosphere model and on the assumption
that radiation and matter are decoupled. Aims. We explore the effects
on the structure and on the dynamics of the accretion flow of both a
shock-heated chromosphere and of the coupling between radiation and
hydrodynamics. Methods. We simulate accretion columns falling onto
a stellar chromosphere using the 1D ALE code AstroLabE. This code
solves the hydrodynamics equations along with the two first momenta
equations for radiation transfer, with the help of a dedicated opacity
table for the coupling between matter and radiation. We derive the
total electron and ions densities from collisional-radiative NLTE
ionisation equilibrium. Results. The chromospheric acoustic heating
has an impact on the duration of the cycle and on the structure
of the heated slab. In addition, the coupling between radiation
and hydrodynamics leads to a heating of the accretion flow and the
chromosphere, inducing a possible unburial of the whole column. These
two last conclusions are in agreement with the computed monochromatic
intensity. Both effects (acoustic heating and radiation coupling)
have an influence on the amplitude and temporal variations of the
net X-ray luminosity, which varies between 30 and 94% of the incoming
mechanical energy flux, depending on the model considered.

---------------------------------------------------------
Title: Photometry and Spectroscopy of Faint Candidate
Spectrophotometric Standard DA White Dwarfs
Authors: Calamida, Annalisa; Matheson, Thomas; Saha, Abhijit;
Olszewski, Edward; Narayan, Gautham; Claver, Jenna; Shanahan, Clare;
Holberg, Jay; Axelrod, Tim; Bohlin, Ralph; Stubbs, Christopher W.;
Deustua, Susana; Hubeny, Ivan; Mackenty, John; Points, Sean; Rest,
Armin; Sabbi, Elena
2019ApJ...872..199C Altcode: 2018arXiv181200034C
We present precise photometry and spectroscopy for 23 candidate
spectrophotometric standard white dwarfs. The selected stars are
distributed in the Northern hemisphere and around the celestial equator,
and are all fainter than r ∼ 16.5 mag. This network of stars,
when established as standards and together with the three Hubble
Space Telescope primary CALSPEC white dwarfs, will provide a set of
spectrophotometric standards to directly calibrate data products to
better than 1%. In future deep photometric surveys and facilities,
these new faint standard white dwarfs will have enough signal-to-noise
ratio to be measured accurately while still avoiding saturation. They
will also fall within the dynamic range of large telescopes and
their instruments for the foreseeable future. This paper discusses
the provenance of the observational data for our candidate standard
stars. A comparison with models, reconciliation with reddening, and the
consequent derivation of the full spectral energy density distributions
for each of them is reserved for a subsequent paper.

---------------------------------------------------------
Title: Modelling the Accretion on Young Stars, Recent Results and
Perspectives
Authors: de Sá, L.; Stehlé, C.; Chièze, J. P.; Hubeny, I.; Lanz,
T.; Colombo, S.; Ibgui, L.; Orlando, S.
2019ASSP...55...29D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A collection of model stellar spectra for spectral types B
to early-M
Authors: Allende Prieto, C.; Koesterke, L.; Hubeny, I.; Bautista,
M. A.; Barklem, P. S.; Nahar, S. N.
2018A&A...618A..25A Altcode: 2018arXiv180706049A
Context. Models of stellar spectra are necessary for interpreting light
from individual stars, planets, integrated stellar populations, nebulae,
and the interstellar medium. Aims: We provide a comprehensive
and homogeneous collection of synthetic spectra for a wide range of
atmospheric parameters and chemical compositions. Methods:
We compile atomic and molecular data from the literature. We adopt
the largest and most recent set of ATLAS9 model atmospheres, and use
the radiative code ASSɛT. Results: The resulting collection
of spectra is made publicly available at medium and high-resolution
(R ≡ λ/δλ = 10 000, 100 000 and 300 000) spectral grids,
which include variations in effective temperature between 3500 K
and 30 000 K, surface gravity (0 ≤ log g ≤ 5), and metallicity
(-5 ≤ [Fe/H] ≤ +0:5), spanning the wavelength interval 120-6500
nm. A second set of denser grids with additional dimensions, [α/Fe]
and micro-turbulence, are also provided (covering 200-2500 nm). We
compare models with observations for a few representative cases. Data files are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/618/A25">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/618/A25</A>

---------------------------------------------------------
Title: VizieR Online Data Catalog: Model stellar spectra for B to
early-M (Allende Prieto+, 2018)
Authors: Allende Prieto, C.; Koesterke, L.; Hubeny, I.; Bautista,
M.; Barklem, P. S.; Nahar, S. N.
2018yCat..36180025A Altcode:
The following text describes the data files available from
ftp://carlos:allende@ftp.ll.iac.es/collection These are in ASCII
(but bzip2 compressed), and ready to be used with the FERRE code
(github.com/callendeprieto/ferre). The format includes a header
and as many rows as model spectra. See the FERRE manual for more
details (github.com/callendeprieto/ferre/docs/ferre.pdf). There
are two main families of files. The 'coarse' (nsc) grids consider
three atmospheric parameters ([Fe/H], Teff and logg) and typically
include hundreds of models on a very coarse network, while the
'large' (ns) grids consider five (the previous three plus [alpha/Fe]
and microturbulence), and include tens to hundreds of thousands of
models. The files are numbered 1-5 depending on the Teff range
they span. There are files (ns and nsc) smoothed with a Gaussian
kernal to R=10,000, while nsc files are provided for R=100,000
and 200,000 as well. The spectral range is 0.12-6.6um for the nsc
files and 0.2-2.5um for the ns files. for the nsc files and
0.2-2.5um for the ns files. nsc ('coarse' grids): [Fe/H], Teff,
logg size filename Teff range R 158004765 f_nsc1.dat.bz2
3500&lt;= Teff&lt;=6000 10000 128868874 f_nsc2.dat.bz2 5750&lt;=
Teff&lt;=8000 102539424 f_nsc3.dat.bz2 7000&lt;= Teff&lt;=12000 77786534
f_nsc4.dat.bz2 10000&lt;=Teff&lt;=20000 26089034 f_nsc5.dat.bz2
20000&lt;=Teff&lt;=30000 1430284037 f_hnsc1.dat.bz2 3500&lt;=
Teff&lt;=6000 100000 1159906320 f_hnsc2.dat.bz2 5750&lt;= Teff&lt;=8000
913976329 f_hnsc3.dat.bz2 7000&lt;= Teff&lt;=12000 695827559
f_hnsc4.dat.bz2 10000&lt;=Teff&lt;=20000 234598400 f_hnsc5.dat.bz2
20000&lt;=Teff&lt;=30000 2596690648 f_uhnsc1.dat.bz2 3500&lt;=
Teff&lt;=6000 300000 2067630039 f_uhnsc2.dat.bz2 5750&lt;= Teff&lt;=8000
1605227763 f_uhnsc3.dat.bz2 7000&lt;= Teff&lt;=12000 1221970143
f_uhnsc4.dat.bz2 10000&lt;=Teff&lt;=20000 411096450 f_uhnsc5.dat.bz2
20000&lt;=Teff&lt;=30000 ns ('large' grids): [Fe/H], [alpha/Fe],
log(micro), Teff, logg 30936708717 f_ns1.dat.bz2 3500&lt;=
Teff&lt;=6000 10000 22957349531 f_ns2.dat.bz2 5750&lt;= Teff&lt;=8000
19640724566 f_ns3.dat.bz2 7000&lt;= Teff&lt;=12000 14267448478
f_ns4.dat.bz2 10000&lt;=Teff&lt;=20000 8622793847 f_ns5.dat.bz2
20000&lt;=Teff&lt;=30000 (2 data files).

---------------------------------------------------------
Title: The Abundances of the Fe Group Elements in AV 304, an Abundance
Standard in the Small Magellanic Cloud
Authors: Peters, Geraldine J.; Lanz, Thierry; Bouret, Jean-Claude;
Proffitt, Charles R.; Adelman, Saul J.; Hubeny, Ivan
2018AAS...23231704P Altcode:
AV 304 is a B0.5 IV field star in the Small Magellanic Cloud with
ultra-sharp spectral lines that has emerged as an abundance standard. We
have combined recent spectroscopic observations from the Cosmic Origins
Spectrograph (COS) on the Hubble Space Telescope with archival data from
the Far Ultraviolet Spectroscopic Explorer (FUSE) and ESO’s VLT/UVES
to determine the abundances of the Fe group elements (Ti, V, Cr, Mn, Fe,
Co, &amp; Ni). The analysis was carried through using the Hubeny/Lanz
NLTE programs TLUSTY/SYNSPEC. The COS observations were secured with the
G130M, G160M, G185M, and G225M gratings. Combined with the FUSE data, we
have achieved spectral coverage in the UV from 950 to 2400 A. Measurable
lines from the Fe group, except for a very few multiplets of Fe II, III
are not observed in optical spectra. The following stellar parameters
were found: Teff = 27500±500 K, log g = 3.7±0.1 cm/s2,
Vturb= 1±1 km/s, and v sin i = 8 ±2 km/s. The Fe abundance appears
to be only slightly lower than the mean depletion in the SMC, but the
other Fe group elements are underabundant by 0.3 dex or more. This study
confirmed the low abundance of nitrogen (-1.25 dex relative to the solar
value) that was reported by Peters &amp; Adelman (ASP Conf. Series, 348,
p. 136, 2006). Whereas the light elements are delivered to the ISM by
core-collapse supernovae (CCSNe), the Fe group elements are believed
to come mostly from low/intermediate mass binaries containing white
dwarfs that undergo SNe Ia explosions. A single SNe Ia can deliver
0.5 solar masses of pure Fe (and maybe Mn) to the ISM compared with
about 0.07 solar masses from a CCSNe. It appears that there is very
little processed material from its interior in the atmosphere of AV
304 and that the star did not form from an interstellar cloud that
was enriched by material from earlier supernova activity. Support from
STScI grants HST-GO-14081.002 and HST-GO-13346.022, and USC’s Women
in Science and Engineering (WiSE) program is greatly appreciated.

---------------------------------------------------------
Title: Sub-percent Photometry: Faint DA White Dwarf Spectrophotometric
Standards for Astrophysical Observatories
Authors: Narayan, Gautham; Axelrod, Tim; Calamida, Annalisa; Saha,
Abhijit; Matheson, Thomas; Olszewski, Edward; Holberg, Jay; Holberg,
Jay; Bohlin, Ralph; Stubbs, Christopher W.; Rest, Armin; Deustua,
Susana; Sabbi, Elena; MacKenty, John W.; Points, Sean D.; Hubeny, Ivan
2018AAS...23135417N Altcode:
We have established a network of faint (16.5 &lt; V &lt; 19) hot DA
white dwarfs as spectrophotometric standards for present and future
wide-field observatories. Our standards are accessible from both
hemispheres and suitable for ground and space-based covering the
UV to the near IR. The network is tied directly to the most precise
astrophysical reference presently available - the CALSPEC standards -
through a multi-cycle program imaging using the Wide-Field Camera 3
(WFC3) on the Hubble Space Telescope (HST). We have developed two
independent analyses to forward model all the observed photometry and
ground-based spectroscopy and infer a spectral energy distribution
for each source using a non-local-thermodynamic-equilibrium (NLTE)
DA white dwarf atmosphere extincted by interstellar dust. The
models are in excellent agreement with each other, and agree with the
observations to better than 0.01 mag in all passbands, and better than
0.005 mag in the optical. The high-precision of these faint sources,
tied directly to the most accurate flux standards presently available,
make our network of standards ideally suited for any experiments that
have very stringent requirements on absolute flux calibration, such
as studies of dark energy using the Large Synoptic Survey Telescope
(LSST) and the Wide-Field Infrared Survey Telescope (WFIRST).

---------------------------------------------------------
Title: Modelling ultraviolet-line diagnostics of stars, the ionized
and the neutral interstellar medium in star-forming galaxies
Authors: Vidal-García, A.; Charlot, S.; Bruzual, G.; Hubeny, I.
2017MNRAS.470.3532V Altcode: 2017arXiv170510320V
We combine state-of-the-art models for the production of stellar
radiation and its transfer through the interstellar medium (ISM) to
investigate ultraviolet-line diagnostics of stars, the ionized and
the neutral ISM in star-forming galaxies. We start by assessing the
reliability of our stellar population synthesis modelling by fitting
absorption-line indices in the ISM-free ultraviolet spectra of 10
Large Magellanic Cloud clusters. In doing so, we find that neglecting
stochastic sampling of the stellar initial mass function in these young
(∼10-100 Myr), low-mass clusters affects negligibly ultraviolet-based
age and metallicity estimates but can lead to significant overestimates
of stellar mass. Then, we proceed and develop a simple approach,
based on an idealized description of the main features of the ISM,
to compute in a physically consistent way the combined influence of
nebular emission and interstellar absorption on ultraviolet spectra
of star-forming galaxies. Our model accounts for the transfer of
radiation through the ionized interiors and outer neutral envelopes of
short-lived stellar birth clouds, as well as for radiative transfer
through a diffuse intercloud medium. We use this approach to explore
the entangled signatures of stars, the ionized and the neutral ISM in
ultraviolet spectra of star-forming galaxies. We find that, aside from
a few notable exceptions, most standard ultraviolet indices defined in
the spectra of ISM-free stellar populations are prone to significant
contamination by the ISM, which increases with metallicity. We also
identify several nebular-emission and interstellar-absorption features,
which stand out as particularly clean tracers of the different phases
of the ISM.

---------------------------------------------------------
Title: Model atmospheres of sub-stellar mass objects
Authors: Hubeny, Ivan
2017MNRAS.469..841H Altcode: 2017arXiv170309283H
We present an outline of basic assumptions and governing structural
equations describing atmospheres of sub-stellar mass objects, in
particular the extrasolar giant planets and brown dwarfs. Although most
of the presentation of the physical and numerical background is generic,
details of the implementation pertain mostly to the code cooltlusty. We
also present a review of numerical approaches and computer codes devised
to solve the structural equations, and make a critical evaluation of
their efficiency and accuracy.

---------------------------------------------------------
Title: The Validity of 21 cm Spin Temperature as a Kinetic Temperature
Indicator in Atomic and Molecular Gas
Authors: Shaw, Gargi; Ferland, G. J.; Hubeny, I.
2017ApJ...843..149S Altcode: 2017arXiv170601425S
The gas kinetic temperature (T K ) of various interstellar
environments is often inferred from observations that can deduce
level populations of atoms, ions, or molecules using spectral
line observations; H I 21 cm is perhaps the most widely used,
and has a long history. Usually the H I 21 cm line is assumed
to be in thermal equilibrium and the populations are given by the
Boltzmann distribution. A variety of processes, many involving Lyα,
can affect the 21 cm line. Here we show how this is treated in the
spectral simulation code Cloudy, and present numerical simulations of
environments where this temperature indicator is used, with a detailed
treatment of the physical processes that determine level populations
within H0. We discuss situations where this temperature
indicator traces T K , cases where it fails, as well as
the effects of Lyα pumping on the 21 cm spin temperature. We also
show that the Lyα excitation temperature rarely traces the gas
kinetic temperature.

---------------------------------------------------------
Title: A Grid of Synthetic Spectra for Hot DA White Dwarfs and Its
Application in Stellar Population Synthesis
Authors: Levenhagen, Ronaldo S.; Diaz, Marcos P.; Coelho, Paula R. T.;
Hubeny, Ivan
2017ApJS..231....1L Altcode: 2017arXiv170704299L
In this work we present a grid of LTE and non-LTE synthetic spectra
of hot DA white dwarfs (WDs). In addition to its usefulness for the
determination of fundamental stellar parameters of isolated WDs and
in binaries, this grid will be of interest for the construction of
theoretical libraries for stellar studies from integrated light. The
spectral grid covers both a wide temperature and gravity range, with
17,000 K ≤ T eff ≤ 100,000 K and 7.0 ≤ {log}g ≤
9.5. The stellar models are built for pure hydrogen and the spectra
cover a wavelength range from 900 Å to 2.5 μm. Additionally, we derive
synthetic HST/ACS, HST/WFC3, Bessel UBVRI, and SDSS magnitudes. The
grid was also used to model integrated spectral energy distributions
of simple stellar populations and our modeling suggests that DAs might
be detectable in ultraviolet bands for populations older than ∼8 Gyr.

---------------------------------------------------------
Title: VizieR Online Data Catalog: Hot DA white dwarfs grid of
synthetic spectra (Levenhagen+, 2017)
Authors: Levenhagen, R. S.; Diaz, M. P.; Coelho, P. R. T.; Hubeny, I.
2017yCat..22310001L Altcode:
Files tTTTgGGA.dat concern non-LTE DA White Dwarf synthetic spectra. TTT
corresponds to the effective temperature, GG corresponds to log(g)
and nl (for NLTE) flag. Files bessel.dat, hstacs.dat, hstwfc3.dat,
sdss.dat are respectively synthetic magnitude tables in the Bessel,
HST/ACS, HST/WFC3 and SLOAN/SDSS systems. (8 data files).

---------------------------------------------------------
Title: TLUSTY User's Guide II: Reference Manual
Authors: Hubeny, Ivan; Lanz, Thierry
2017arXiv170601935H Altcode:
This is the second part of a three-volume guide to TLUSTY and
SYNSPEC. It presents a detailed reference manual for TLUSTY, which
contains a detailed description of basic physical assumptions and
equations used to model an atmosphere, together with an overview of
the numerical methods to solve these equations.

---------------------------------------------------------
Title: A brief introductory guide to TLUSTY and SYNSPEC
Authors: Hubeny, Ivan; Lanz, Thierry
2017arXiv170601859H Altcode:
This is the first of three papers that present a detailed guide for
working with the codes {\sc tlusty} and {\sc synspec} to generate
model stellar atmospheres or accretion disks, and to produce detailed
synthetic spectra. In this paper, we present a very brief manual
intended for casual users who intend to use these codes for simple,
well defined tasks. This paper does not present any background theory,
or a description of the adopted numerical approaches, but instead uses
simple examples to explain how to employ these codes. In particular,
it shows how to produce a simple model atmosphere from the scratch,
or how to improve an existing model by considering more extended model
atoms. This paper also presents a brief guide to the spectrum synthesis
program {\sc synspec}.

---------------------------------------------------------
Title: TLUSTY User's Guide III: Operational Manual
Authors: Hubeny, Ivan; Lanz, Thierry
2017arXiv170601937H Altcode:
This paper presents a detailed operational manual for TLUSTY. It
provides a guide for understanding the essential features and the basic
modes of operation of the program. To help the user, it is divided
into two parts. The first part describes the most important input
parameters and available numerical options. The second part covers
additional details and a comprehensive description of all physical
and numerical options, and a description of all input parameters,
many of which needed only in special cases.

---------------------------------------------------------
Title: Effective temperatures of cataclysmic-variable white dwarfs
as a probe of their evolution
Authors: Pala, A. F.; Gänsicke, B. T.; Townsley, D.; Boyd, D.;
Cook, M. J.; De Martino, D.; Godon, P.; Haislip, J. B.; Henden,
A. A.; Hubeny, I.; Ivarsen, K. M.; Kafka, S.; Knigge, C.; LaCluyze,
A. P.; Long, K. S.; Marsh, T. R.; Monard, B.; Moore, J. P.; Myers,
G.; Nelson, P.; Nogami, D.; Oksanen, A.; Pickard, R.; Poyner, G.;
Reichart, D. E.; Rodriguez Perez, D.; Schreiber, M. R.; Shears, J.;
Sion, E. M.; Stubbings, R.; Szkody, P.; Zorotovic, M.
2017MNRAS.466.2855P Altcode: 2017arXiv170102738P
We present HST spectroscopy for 45 cataclysmic variables (CVs), observed
with HST/COS and HST/STIS. For 36 CVs, the white dwarf is recognisable
through its broad Ly α absorption profile and we measure the white
dwarf effective temperatures (Teff) by fitting the HST data
assuming log g = 8.35, which corresponds to the average mass for CV
white dwarfs (≃0.8 M⊙). Our results nearly double the
number of CV white dwarfs with an accurate temperature measurement. We
find that CVs above the period gap have, on average, higher temperatures
(&lt;Teff&gt; ≃ 23 000 K) and exhibit much more scatter
compared to those below the gap (&lt;Teff&gt; ≃ 15 000
K). While this behaviour broadly agrees with theoretical predictions,
some discrepancies are present: (I) all our new measurements above the
gap are characterized by lower temperatures (Teff ≃ 16
000-26 000 K) than predicted by the present-day CV population models
(Teff ≃ 38 000-43 000 K); (II) our results below the
gap are not clustered in the predicted narrow track and exhibit in
particular a relatively large spread near the period minimum, which
may point to some shortcomings in the CV evolutionary models. Finally,
in the standard model of CV evolution, reaching the minimum period,
CVs are expected to evolve back towards longer periods with mean
accretion rates dot{M}≲ 2 × 10^{-11} M_{⊙} yr^{-1}, corresponding
to Teff ≲ 11 500 K. We do not unambiguously identify any
such system in our survey, suggesting that this major component of
the predicted CV population still remains elusive to observations.

---------------------------------------------------------
Title: Hot DA white dwarf model atmosphere calculations: including
improved Ni PI cross-sections
Authors: Preval, S. P.; Barstow, M. A.; Badnell, N. R.; Hubeny, I.;
Holberg, J. B.
2017MNRAS.465..269P Altcode: 2016arXiv161009662P
To calculate realistic models of objects with Ni in their atmospheres,
accurate atomic data for the relevant ionization stages need to
be included in model atmosphere calculations. In the context of
white dwarf stars, we investigate the effect of changing the Ni
IV-VI bound-bound and bound-free atomic data on model atmosphere
calculations. Models including photoionization cross-section (PICS)
calculated with AUTOSTRUCTURE show significant flux attenuation of up
to ∼80 per cent shortward of 180 Å in the extreme ultraviolet (EUV)
region compared to a model using hydrogenic PICS. Comparatively, models
including a larger set of Ni transitions left the EUV, UV, and optical
continua unaffected. We use models calculated with permutations of these
atomic data to test for potential changes to measured metal abundances
of the hot DA white dwarf G191-B2B. Models including AUTOSTRUCTURE
PICS were found to change the abundances of N and O by as much as
∼22 per cent compared to models using hydrogenic PICS, but heavier
species were relatively unaffected. Models including AUTOSTRUCTURE PICS
caused the abundances of N/O IV and V to diverge. This is because the
increased opacity in the AUTOSTRUCTURE PICS model causes these charge
states to form higher in the atmosphere, more so for N/O V. Models
using an extended line list caused significant changes to the Ni IV-V
abundances. While both PICS and an extended line list cause changes
in both synthetic spectra and measured abundances, the biggest changes
are caused by using AUTOSTRUCTURE PICS for Ni.

---------------------------------------------------------
Title: Probing Supernovae Chemical Yields in Low Metallicity
Environments with UV Spectroscopy of Magellanic Cloud B-type Stars
Authors: Lanz, Thierry; Adelman, Saul J.; Bouret, Jean-Claude;
Gulliver, Austin F.; Hill, Graham; Hubeny, Ivan; Peters, Geraldine J.;
Pickering, Juliet C.; Proffitt, Charles R.
2016hst..prop14081L Altcode:
Spectrum synthesis studies of the UV spectra of sharp-lined
main sequence B stars provide astronomers with some of the best
determinations of the abundances of the light, Fe group, and neutron
capture elements. B stars are therefore best-suited to study the
chemical evolution of the Magellanic Clouds. But the HST archive
is virtually devoid of high resolution spectra of such objects. We
propose FUV and NUV observations with the COS G130M, G160M, G185M,
and G225M gratings. The four program stars have been observed with
the FUSE spacecraft, hence this project will produce continuous
high-resolution spectral coverage from 950 to 2400 A and provide a
permanent archive of fundamental spectra from which ground-breaking
studies of the Magellanic Clouds can be performed in the decades
to come. This limited program aims at producing an extragalactic,
low-metallicity counterpart to the bright star library of early B
stars that is currently being obtained as part of the HST Cycle 21
Treasury program "Advanced Spectral Library II: Hot Stars" (GO 13346,
PI T. Ayres). Spectral lines from most Fe group and s-process
elements are found only in the UV region in B stars and information
on their abundances is important for studying the chemical evolution
of a galaxy, computing opacities for stellar evolution calculations,
and assessing the validity of theoretical calculations of explosive
nucleosynthesis. Comparing the derived abundances of iron-peak and
heavier elements in galactic and Magellanic Cloud B main sequence
stars will provide an empirical probe of chemical yields ejected by
evolved stars and supernovae in different environments.

---------------------------------------------------------
Title: Spectroscopy from the Hubble Space Telescope Cosmic Origins
Spectrograph of the Southern Nova-like BB Doradus in an Intermediate
State
Authors: Godon, Patrick; Sion, Edward M.; Gänsicke, Boris T.; Hubeny,
Ivan; de Martino, Domitilla; Pala, Anna F.; Rodríguez-Gil, Pablo;
Szkody, Paula; Toloza, Odette
2016ApJ...833..146G Altcode: 2017arXiv170105222G
We present a spectral analysis of the spectrum from the Hubble Space
Telescope Cosmic Origins Spectrograph (HST/COS) of the southern
VY Scl nova-like variable BB Doradus, obtained as part of a Cycle
20 HST/COS survey of accreting white dwarfs (WDs) in cataclysmic
variables. BB Dor was observed with COS during an intermediate state
with a low mass accretion rate, thereby allowing an estimate of the
WD temperature. The results of our spectral analysis show that the
WD is a significant far-ultraviolet (FUV) component of the spectrum
with a temperature of about 35,000-50,000 K, assuming a WD mass of
0.80 {M}⊙ ({log}(g)=8.4). The disk, with a mass accretion
rate of ≈ {10}-10 {M}⊙ {{yr}}-1,
contributes about 1/5 to 1/2 of the FUV flux.

---------------------------------------------------------
Title: GW Librae: a unique laboratory for pulsations in an accreting
white dwarf
Authors: Toloza, O.; Gänsicke, B. T.; Hermes, J. J.; Townsley, D. M.;
Schreiber, M. R.; Szkody, P.; Pala, A.; Beuermann, K.; Bildsten, L.;
Breedt, E.; Cook, M.; Godon, P.; Henden, A. A.; Hubeny, I.; Knigge,
C.; Long, K. S.; Marsh, T. R.; de Martino, D.; Mukadam, A. S.; Myers,
G.; Nelson, P.; Oksanen, A.; Patterson, J.; Sion, E. M.; Zorotovic, M.
2016MNRAS.459.3929T Altcode: 2016arXiv160402162T; 2016MNRAS.tmp..639T; 2016MNRAS.tmp..623T
Non-radial pulsations have been identified in a number of accreting
white dwarfs in cataclysmic variables. These stars offer insight
into the excitation of pulsation modes in atmospheres with mixed
compositions of hydrogen, helium, and metals, and the response of
these modes to changes in the white dwarf temperature. Among all
pulsating cataclysmic variable white dwarfs, GW Librae stands out by
having a well-established observational record of three independent
pulsation modes that disappeared when the white dwarf temperature rose
dramatically following its 2007 accretion outburst. Our analysis of
Hubble Space Telescope (HST) ultraviolet spectroscopy taken in 2002,
2010, and 2011, showed that pulsations produce variations in the white
dwarf effective temperature as predicted by theory. Additionally in
2013 May, we obtained new HST/Cosmic Origin Spectrograph ultraviolet
observations that displayed unexpected behaviour: besides showing
variability at ≃275 s, which is close to the post-outburst
pulsations detected with HST in 2010 and 2011, the white dwarf exhibits
high-amplitude variability on an ≃4.4 h time-scale. We demonstrate
that this variability is produced by an increase of the temperature of
a region on white dwarf covering up to ≃30 per cent of the visible
white dwarf surface. We argue against a short-lived accretion episode
as the explanation of such heating, and discuss this event in the
context of non-radial pulsations on a rapidly rotating star.

---------------------------------------------------------
Title: Wide band spectra of nova-like variables: A confrontation of
observations with theory
Authors: Long, Knox S.; Higginbottom, Nick; Hubeny, Ivan; Knigge,
Christian; Matthews, James; Sim, Stuart A.
2016hst..prop14637L Altcode:
Nova-like variables are the best and nearest examples of steady-state
disk accretion onto a compact object. Emission in these system arises
from a large number of components, including the WD and secondary
star, the accretion disk through which mass is accreted, and a wind
through which mass is lost. However, the only wide band ”spectra”
of these variable objects have been constructed by stitching together
spectra obtained at different times, and as a result it is difficult to
conclude exactly what is causing the departures from the predictions of
steady-state accretion disk models that are observed, or to establish
the geometry of the wind in these systems. This is a project to
use HST/STIS to obtain single-epoch, FUV-near IR spectra of five
bright nova-like variables, with varying inclinations, to remedy this
problem. We will compare this benchmark set of spectra to new models of
the disks and winds for this group of cataclysmic variables in order to
measure the temperature profiles of the disks, to determine mass loss
rates, collimation, and other physical characteristics of the winds,
and to assess the overall effect of the dense regions of the wind on
the spectral energy distribution.

---------------------------------------------------------
Title: Division G Commission 36: Theory of Stellar Atmospheres
Authors: Puls, Joachim; Hubeny, Ivan; Asplund, Martin; Allard, France;
Allende Prieto, Carlos; Ayres, Thomas R.; Carlsson, Mats; Gustafsson,
Bengt; Kudritzki, Rolf-Peter; Ryabchikova, Tatiana A.
2016IAUTA..29..453P Altcode:
Different from previous triennial reports, this report covers the
activities of IAU Commission 36 `Theory of Stellar Atmospheres'
over the past six years†, and will be the last report from the
`old' Commission 36. After the General Assembly in Honolulu (August
2015), a new Commission `Stellar and Planetary Atmospheres' (C.G5,
under Division G, `Stars and Stellar Physics') has come into life,
and will continue our work devoted to the outer envelopes of stars,
as well as extend it to the atmospheres of planets (see Sect. 4).

---------------------------------------------------------
Title: Does star formation proceed differently in metal-poor galaxies?
Authors: Lebouteiller, vianney; Aloisi, Alessandra; Barlow, Michael J.;
Cormier, Diane; Galliano, Frederic; Glover, Simon; Heap, Sara Ridgway;
Hubeny, Ivan; James, Bethan Lesley; Karczewski, Oskar; Kunth, Daniel;
Le Bourlot, Jacques; Le Petit, Franck; Lehner, Nicolas; Madden,
Suzanne; Remy, Aurelie
2015hst..prop13697L Altcode:
The importance of molecular gas in the star-formation process has been
questioned by recent theoretical studies. When metals are scarce,
star formation could proceed before the molecular fraction becomes
significant, making cold atomic gas the key pre-requisite for star
formation. The best case studies are blue compact dwarf galaxies (BCDs),
with their prominent star-formation episode and yet with little or no
evidence of molecular gas. Current observations do not provide strong
constraints on the presence of dense atomic gas in BCDs nor on the
fraction of molecular gas. We propose to examine the HI region
of 9 nearby BCDs selected from the Herschel Dwarf Galaxy Survey. Our
program relies on the synergy of Hubble and Herschel, by calculating
the gas cooling rate from the fine-structure level of ionized carbon,
a parameter that can be determined both in the FUV with COS (probing
the diffuse gas through the 1335.7A CII* absorption) and in the FIR with
Herschel (probing the denser gas through the [CII] 157um emission). This
comparison allows us to constrain the volume filling factor of dense
vs. diffuse gas. The program we propose will allow us to examine how
this fraction varies with metallicity, star-formation rate, and total
gas mass. We will also be able to quantify the mass of molecular gas and
evaluate its actual importance for star formation. Finally, a secondary
objective is to characterize the main gas heating mechanisms in the
HI region of BCDs and in particular the validity of the photoelectric
effect paradigm in sources with a low dust-to-gas ratio, with potential
implications for high-redshift galaxies.

---------------------------------------------------------
Title: Tables of phase functions, opacities, albedos, equilibrium
temperatures, and radiative accelerations of dust grains in exoplanets
Authors: Budaj, J.; Kocifaj, M.; Salmeron, R.; Hubeny, I.
2015MNRAS.454....2B Altcode: 2015arXiv150508013B
There has been growing observational evidence for the presence of
condensates in the atmospheres and/or comet-like tails of extrasolar
planets. As a result, systematic and homogeneous tables of dust
properties are useful in order to facilitate further observational
and theoretical studies. In this paper we present calculations and
analysis of non-isotropic phase functions, asymmetry parameter (mean
cosine of the scattering angle), absorption and scattering opacities,
single scattering albedos, equilibrium temperatures, and radiative
accelerations of dust grains relevant for extrasolar planets. Our
assumptions include spherical grain shape, Deirmendjian particle
size distribution, and Mie theory. We consider several species:
corundum/alumina, perovskite, olivines with 0 and 50 per cent iron
content, pyroxenes with 0, 20, and 60 per cent iron content, pure iron,
carbon at two different temperatures, water ice, liquid water, and
ammonia. The presented tables cover the wavelength range of 0.2-500 μm
and modal particle radii from 0.01 to 100 μm. Equilibrium temperatures
and radiative accelerations assume irradiation by a non-blackbody source
of light with temperatures from 7000 to 700 K seen at solid angles from
2π to 10-6 sr. The tables are provided to the community
together with a simple code which allows for an optional, finite,
angular dimension of the source of light (star) in the phase function.

---------------------------------------------------------
Title: The Wind and Photosphere of the Unique DO White Dwarf RE
J0503-289
Authors: Barstow, Martin; Holberg, Jay B.; Hubeny, Ivan; Lanz, Thierry;
Sion, Edward M.
2015hst..prop.6628B Altcode:
We propose to obtain high S/N GHRS spectra of the key He-richwhite dwarf
RE J0503-289. These observations have twoprincipal aims. The first
is to investigate the phenomenon ofmass loss in this star. Excluding
planetary nebulae nuclei, REJ0503-289 is the only white dwarf in which
evidence of ongoingmass loss has been observed with IUE. However, the
reporteddetection is near the limit of the sensitivity of IUEpreventing
study of the wind during its less intense periods.Mass loss may well be
the missing ingredient in thetheoretical description of heavy element
abundances in whitedwarfs. RE J0503-289 is the only known white dwarf
where thiseffect can be studied as an on going phenomenon. The secondaim
is to search for Fe and Ni in the photosphere of thisstar. The presence
of these elements is required to explainthe large flux deficit observed
in the EUV. Detailed models,including those elements so far detected (C,
N, O and Si),substantially over predict EUV flux levels. RE J0503-289
isalso the only DO white dwarf for which a joint analysis of EUVand UV
data will be possible and for which we can conduct adefinitive study
of the composition and structure of thephotosphere.

---------------------------------------------------------
Title: GW Lib: a Unique Laboratory for White Dwarf Pulsations
Authors: Toloza, Odette; Gänsicke, Boris T.; Hermes, JJ; Townsley,
Dean M.; Szkody, Paula; Beuermann, K.; Bildsten, Lars; de Martino,
D.; Godon, Patrick; Henden, Arne A.; Hubeny, Ivan; Knigge, Christian;
Long, Knox S.; Marsh, T. R.; Patterson, Joseph; Schreiber, M. R.;
Sion, Edward M.; Zorotovic, Monica
2015ASPC..493..253T Altcode:
Non-radial pulsations have been identified in a number of accreting
white dwarfs in cataclysmic variables. These stars offer insight into
the excitation of pulsation modes in mixed H/He/Z atmospheres, and the
response of these modes to changes in the white dwarf temperature. Among
all pulsating cataclysmic variable white dwarfs, GW Lib stands out by
having a well-established observational record of three independent
pulsation modes that were wiped out during its 2007 outburst. We
have obtained new HST ultraviolet observations in May 2013 that show
an unexpected behaviour: besides some activity near the ∼280 s
period that has been observed in the past, the white dwarf underwent
a large-amplitude brightening. We demonstrate that the brightening is
related to an increase of the photospheric temperature, argue against
an accretion episode as explanation, and discuss this event in the
context of non-radial pulsations on a rapidly rotating star.

---------------------------------------------------------
Title: A Possible Solution to the Lyman/Balmer Line Problem in Hot
DA White Dwarfs
Authors: Preval, Simon P.; Barstow, Martin A.; Badnell, Nigel R.;
Holberg, Jay B.; Hubeny, Ivan
2015ASPC..493...15P Altcode: 2014arXiv1410.0811P
Arguably, the best method for determining the effective temperature
(Teff) and surface gravity (log g) of a DA white dwarf is
by fitting the Hydrogen Lyman and Balmer absorption features. However,
as has been shown for white dwarfs with Teff&gt;50,000K,
the calculated value from the Lyman and Balmer lines are discrepant,
which worsens with increasing temperature. Many different solutions
have been suggested, ranging from the input physics used to calculate
the models, to interstellar reddening. We will focus on the former,
and consider three variables. The first is the atomic data used, namely
the number of transitions included in line blanketing treatments and
the photoionization cross sections. The second is the stark broadening
treatment used to synthesise the Lyman and Balmer line profiles, namely
the calculations performed by Lemke (1997) and Tremblay &amp; Bergeron
(2009). Finally, the third is the atmospheric content. The model
grids are calculated with a pure H composition, and a metal polluted
composition using the abundances of Preval et al. (2013). We present
the preliminary results of our analysis, whereby we have determined the
Teff for a small selection of white dwarfs. We plan to extend
our analysis by allowing metallicity to vary in future model grids.

---------------------------------------------------------
Title: An Intensive HST/COS Study of 31 CV WDs
Authors: Pala, A. F.; Gänsicke, B. T.; Beuermann, K.; Bildsten, L.;
De Martino, D.; Godon, P.; Henden, A. A.; Hubeny, I.; Knigge, C.;
Long, K. S.; Marsh, T. R.; Patterson, J.; Schreiber, M. R.; Sion,
E. M.; Szkody, P.; Townsley, D.; Zorotovic, M.
2015ASPC..493..521P Altcode:
Accretion is the key ingredient in the evolution of white dwarfs
(WDs) in cataclysmic variables (CVs), with important implications for
their potential as progenitors for Type Ia Supernovae. The effective
temperature (Teff) gives a direct measure of the average
accretion rate and we report here the Teff measurement
for 31 CV WDs, observed as a part of a 122 orbit HST program. The HST
spectra alone cannot resolve the degeneracy between Teff
and the surface gravity log g, and we will use phase-resolved X-Shooter
observations to (i) measure the mass ratio of the CVs from the reflex
motion of both the WD and the donor star (ii) to establish the spectral
energy distribution (SED), which constrains both Teff
and log g. Combining the X-Shooter and HST data, we can measure the
WD masses to a few percent, and will be able to answer the question
whether accreting CV WDs grow in mass.

---------------------------------------------------------
Title: Refining our Knowledge of the White Dwarf Mass-Radius Relation
with HST Observations of Sirius-type Binaries
Authors: Barstow, M. A.; Bond, H. E.; Burleigh, M. R.; Casewell,
S. L.; Farihi, J.; Holberg, J. B.; Hubeny, I.
2015ASPC..493..307B Altcode:
The presence of a white dwarf in a resolved binary system, such as
Sirius, provides an opportunity to combine dynamical information about
the masses, from astrometry and spectroscopy, with a gravitational
red-shift measurement and spectrophotometry of the white dwarf
atmosphere to provide a test of theoretical mass-radius relations of
unprecedented accuracy. We demonstrated this with the first Balmer
line spectrum of Sirius B to be obtained free of contamination from
the primary, with STIS on HST. However, we also found an unexplained
discrepancy between the spectroscopic and gravitational red-shift
mass determinations. With the recovery of STIS, we have been able to
revisit our observations of Sirius B with an improved observation
strategy designed to reduce systematic errors on the gravitational
red-shift measurement. We provide a preliminary report on the refined
precision of the Sirius B mass-radius measurements and the extension
of this technique to a larger sample of white dwarfs in resolved
binaries. Together these data can provide accurate mass and radius
determinations capable of testing the theoretical mass-radius relation
and distinguishing between possible structural models.

---------------------------------------------------------
Title: Refining our knowledge of the white dwarf mass-radius relation
Authors: Barstow, M. A.; Bond, H. E.; Burleigh, M. R.; Casewell,
S. L.; Farihi, J.; Holberg, J. B.; Hubeny, I.
2015arXiv150503427B Altcode:
The presence of a white dwarf in a resolved binary system, such as
Sirius, provides an opportunity to combine dynamical information about
the masses, from astrometry and spectroscopy, with a gravitational
red-shift measurement and spectrophotometry of the white dwarf
atmosphere to provide a test of theoretical mass-radius relations of
unprecedented accuracy. We demonstrated this with the first Balmer
line spectrum of Sirius B to be obtained free of contamination from
the primary, with STIS on HST. However, we also found an unexplained
discrepancy between the spectroscopic and gravitational red-shift
mass determinations. With the recovery of STIS, we have been able to
revisit our observations of Sirius B with an improved observation
strategy designed to reduce systematic errors on the gravitational
red-shift measurement. We provide a preliminary report on the refined
precision of the Sirius B mass-radius measurements and the extension
of this technique to a larger sample of white dwarfs in resolved
binaries. Together these data can provide accurate mass and radius
determinations capable of testing the theoretical mass-radius relation
and distinguishing between possible structural models.

---------------------------------------------------------
Title: Population III Stars in I Zw 18
Authors: Heap, Sally; Bouret, Jean-Claude; Hubeny, Ivan
2015arXiv150402742H Altcode:
Ultraviolet and 21-cm observations suggest that the extremely
low-metallicity galaxy, I Zw 18, is a stream-fed galaxy containing
a "pocket" of pristine stars responsible for producing nebular He
II recombination emission observed in I Zw18-NW. Far-UV spectra by
Hubble/COS and the Far Ultraviolet Spectroscopic Explorer (FUSE)
make this suggestion conclusive by demonstrating that the spectrum
of I Zw 18-NW shows no metal lines like O VI 1032, 1038 of comparable
ionization as the He II recombination emission.

---------------------------------------------------------
Title: Non-LTE Abundances in OB stars: Preliminary Results for 5
Stars in the Outer Galactic Disk
Authors: Bragançca, G. A.; Lanz, T.; Daflon, S.; Cunha, K.; Garmany,
C. D.; Glaspey, J. W.; Borges Fernandes, M.; Oey, M. S.; Bensby, T.;
Hubeny, I.
2015IAUS..307...90B Altcode: 2014arXiv1408.5807B
The aim of this study is to analyse and determine elemental abundances
for a large sample of distant B stars in the outer Galactic disk in
order to constrain the chemical distribution of the Galactic disk and
models of chemical evolution of the Galaxy. Here, we present preliminary
results on a few stars along with the adopted methodology based on
securing simultaneous O and Si ionization equilibria with consistent
NLTE model atmospheres.

---------------------------------------------------------
Title: A large HST program: effective temperatures of cataclysmic
variable white dwarfs
Authors: Pala, A. F.; Gaensicke, B.; Beuermann, K.; Bildsten, L.;
de Martino, D.; Godon, P.; Henden, A.; Hubeny, I.; Knigge, C.; Long,
K.; Marsh, T.; Patterson, J.; Schreiber, M. R.; Sion, E. M.; Szkody,
P.; Townsley, D.; Zorotovic, M.
2015gacv.workE..32P Altcode: 2015PoS...255E..32P
No abstract at ADS

---------------------------------------------------------
Title: 3D Gray Radiative Properties of a Radiation Hydrodynamic
Model of a YSO Accretion Shock
Authors: Ibgui, L.; de Sá, L.; Stehlé, C.; Chièze, J. -P.; Orlando,
S.; Hubeny, I.; Lanz, T.; Matsakos, T.; González, M.; Bonito, R.
2014ASPC..488...83I Altcode:
We present preliminary results of radiative properties of a 1D gray
radiation hydrodynamic (RHD) model of an accretion shock on a young
stellar object (YSO). This model takes into account the transition
between the collisional equilibrium regime (local thermodynamic
equilibrium, LTE), and the coronal equilibrium regime. Based on the
1D planar structure, we built a 3D cylindrical one. Most notably,
the post-shock region obtained in our case is far less extended (by
a factor of 10 000) than the typical one obtained with models that
assume gray optically thin radiative losses. Moreover, we find that
the column is optically thin in its longitudinal dimension, and in the
transverse dimension, except over an extremely narrow region (≲ 700
m). Consequently, still under the gray assumption, the photons emitted
by the hot slab can propagate through the column and escape freely in
all directions, including towards the chromosphere. The radiation flux
has therefore components that are perpendicular to the accretion column,
which demonstrates that a multidimensional (2D or 3D) radiative model
is necessary for such a cylindrical structure. This study needs to
be taken forward and expanded, by improving the radiative treatment
of the RHD model, through relaxation of both the gray and the LTE
approximations for the calculation of opacities, in order to clarify
the structure of the post-shock region, which is a major source of
emission probed by observations.

---------------------------------------------------------
Title: Evidence for an external origin of heavy elements in hot DA
white dwarfs
Authors: Barstow, M. A.; Barstow, J. K.; Casewell, S. L.; Holberg,
J. B.; Hubeny, I.
2014MNRAS.440.1607B Altcode: 2014MNRAS.tmp..544B; 2014arXiv1402.2164B
We present a series of systematic abundance measurements for 89 hydrogen
atmosphere (DA-type) white dwarfs with temperatures spanning 16 000-77
000 K drawn from the FUSE spectral archive. This is the largest study to
date of white dwarfs where radiative forces are significant, exceeding
our earlier work, based mainly on International Ultraviolet Explorer
and Hubble Space Telescope data, by a factor 3. Using heavy element
blanketed non-local thermodynamic equilibrium stellar atmosphere
calculations, we have addressed the heavy element abundance patterns
making completely objective measurements of abundance values and
their error ranges using a χ2 fitting technique. We
are able to establish the broad range of abundances seen in a given
temperature range and establish the incidence of stars which appear,
in the optical, to be atmospherically devoid of any material other
than H. We compare the observed abundances to predictions of radiative
levitation calculations, revealing little agreement. We propose that
the supply of heavy elements is accreted from external sources rather
than being intrinsic to the star. These elements are then retained
in the white dwarf atmospheres by radiative levitation, a model that
can explain both the diversity of measured abundances for stars of
similar temperature and gravity, including cases with apparently pure
H envelopes, and the presence of photospheric metals at temperatures
where radiative levitation is no longer effective.

---------------------------------------------------------
Title: The Discovery of Pulsating Hot Subdwarfs in NGC 2808
Authors: Brown, T.; Landsman, W.; Randall, S.; Sweigart, A.; Lanz,
T.; Hubeny, I.
2014ASPC..481...29B Altcode:
We present preliminary results of a Hubble Space Telescope program
to search for pulsating hot subdwarfs in the core of NGC 2808. These
observations, obtained in March of 2013, were motivated by the recent
discovery of such stars in the outskirts of ω Cen. Both ω Cen and
NGC 2808 are massive globular clusters exhibiting complex stellar
populations and large numbers of extreme horizontal branch stars. Our
far-UV photometric monitoring of over 100 UV-bright stars has revealed
at least six pulsating subdwarfs with periods ranging from 100 to
150 seconds. In the UV color-magnitude diagram of NGC 2808, all six
of these stars lie immediately below the canonical horizontal branch,
a region populated by the subluminous “blue hook” stars. Three of
these six pulsators also have low-resolution far-UV spectroscopy that
is sufficient to broadly constrain their atmospheric abundances and
effective temperatures. Curiously, the spectroscopic and photometric
data do not exhibit the uniformity one might expect from a well-defined
instability strip.

---------------------------------------------------------
Title: Synthetic Spectra for O and B Type Subdwarf Stars
Authors: Nemeth, P.; Östensen, R.; Tremblay, P.; Hubeny, I.
2014ASPC..481...95N Altcode: 2013arXiv1308.0252N
We present a grid of optical (3200-7200 Å) synthetic spectra calculated
with TLUSTY/SYNSPEC. The new NLTE model atmospheres include the most
recent hydrogen Stark broadening profiles; were calculated in opacity
sampling and limited to pure H/He composition. The grid covers the
observed parameter space of (He-)sdB and (He-)sdO stars, therefore it
is suitable for the homogeneous spectral analyses of such evolved stars.

---------------------------------------------------------
Title: 3D Gray Radiative Properties of Accretion Shocks in Young
Stellar Objects
Authors: Ibgui, L.; Orlando, S.; Stehlé, C.; Chièze, J. -P.; Hubeny,
I.; Lanz, T.; de Sá, L.; Matsakos, T.; González, M.; Bonito, R.
2014EPJWC..6404005I Altcode:
We address the problem of the contribution of radiation to the structure
and dynamics of accretion shocks on Young Stellar Objects. Solving the
3D RTE (radiative transfer equation) under our "gray LTE approach",
i.e., using appropriate mean opacities computed in local thermodynamic
equilibrium, we post-process the 3D MHD (magnetohydrodynamic) structure
of an accretion stream impacting the stellar chromosphere. We find a
radiation flux of ten orders of magnitude larger than the accreting
energy rate, which is due to a large overestimation of the radiative
cooling. A gray LTE radiative transfer approximation is therefore
not consistent with the given MHD structure of the shock. Further
investigations are required to clarify the role of radiation, by
relaxing both the gray and LTE approximations in RHD (radiation
hydrodynamics) simulations. Post-processing the obtained structures
through the resolution of the non-LTE monochromatic RTE will provide
reference radiation quantities against which RHD approximate solutions
will be compared.

---------------------------------------------------------
Title: Theory of Stellar Atmospheres
Authors: Hubeny, Ivan; Mihalas, Dimitri
2014tsa..book.....H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Accretion shock stability on a dynamically heated YSO
atmosphere with radiative transfer
Authors: de Sá, Lionel; Chièze, Jean-Pierre; Stehlé, Chantal;
Matsakos, Titos; Ibgui, Laurent; Lanz, Thierry; Hubeny, Ivan
2014EPJWC..6404002D Altcode:
Theory and simulations predict Quasi-Periodic Oscillations of
shocks which develop in magnetically driven accretion funnels
connecting the stellar disc to the photosphere of Young Stellar
Objects (YSO). X-ray observations however do not show evidence of
the expected periodicity. We examine here, in a first attempt, the
influence of radiative transfer on the evolution of material impinging
on a dynamically heated stellar atmosphere, using the 1D ALE-RHD code
ASTROLABE. The mechanical shock heating mechanism of the chromosphere
only slightly perturbs the flow. We also show that, since the impacting
flow, and especially the part which penetrates into the chromosphere,
is not treated as a purely radiating transparent medium, a sufficiently
efficient coupling between gas and radiation may affect or even suppress
the oscillations of the shocked column. This study shows the importance
of the description of the radiation effects in the hydrodynamics and
of the accuracy of the opacities for an adequate modeling.

---------------------------------------------------------
Title: DISKSPEC: A Tool for Analyzing Observed Spectra of Accretion
Disk Systems
Authors: Hubeny, Ivan
2013hst..prop13251H Altcode:
Accretion disks are ubiquitous in astronomy, associated with AGN,
stellar mass black holes, X-ray binaries, progenitors of Ia supernovae,
X-ray binaries, cataclysmic variables {CVs} {ordinary, recurrent, and
dwarf novae, novalikes}, pre-main sequence stars. An understanding of
the nature of, and physical processes in, accretion disks is one of
the central themes in astrophysics. A large number of past, present,
and future HST observational programs were/are/will be devoted to
astronomical systems that harbor an accretion disk. We propose to
develop a package DISKSPEC, which enables the user to generate a
synthetic spectum of an accretion disk for essentially any combination
of input parameters. The strategy is to use a sufficiently dense and
extended grid of models for genericdisk rings, and to develop a code
that computes a spectrum of the whole disk by interpolating the generic
grid spectra to the actual spectra emergent from the individual radial
rings of the disk, and integrates over the disk taking into account
Doppler shifts due to disk rotation. We believe that the package will
serve many researchers using new or archival HST data to be able to
perform a spectroscopic analysis of objects that contain an accretion
disk in a fast and flexible way, and thus contribute significantly to
a better use of past, present, and future HST observations.

---------------------------------------------------------
Title: Binary Star Synthetic Photometry and Distance Determination
Using BINSYN
Authors: Linnell, Albert P.; DeStefano, Paul; Hubeny, Ivan
2013AJ....146...68L Altcode:
This paper extends synthetic photometry to components of binary star
systems. The paper demonstrates accurate recovery of single star
photometric properties for four photometric standards, Vega, Sirius,
GD153, and HD209458, ranging over the HR diagram, when their model
synthetic spectra are placed in fictitious binary systems and subjected
to synthetic photometry processing. Techniques for photometric distance
determination have been validated for all four photometric standards.

---------------------------------------------------------
Title: Massive stars at low metallicity. Evolution and surface
abundances of O dwarfs in the SMC
Authors: Bouret, J. -C.; Lanz, T.; Martins, F.; Marcolino, W. L. F.;
Hillier, D. J.; Depagne, E.; Hubeny, I.
2013A&A...555A...1B Altcode: 2013arXiv1304.6923B
 Aims: We aim to study the properties of massive stars at
low metallicity, with an emphasis on their evolution, rotation, and
surface abundances. We focus on O-type dwarfs in the Small Magellanic
Cloud. These stars are expected to have weak winds that do not remove
significant amounts of their initial angular momentum. Methods:
We analyzed the UV and optical spectra of twenty-three objects using
the NLTE stellar atmosphere code cmfgen and derived photospheric and
wind properties. Results: The observed binary fraction of the
sample is ≈26%, which is consistent with more systematic studies if
one considers that the actual binary fraction is potentially larger
owing to low-luminosity companions and that the sample was biased
because it excluded obvious spectroscopic binaries. The location of
the fastest rotators in the Hertzsprung-Russell (H-R) diagram built
with fast-rotating evolutionary models and isochrones indicates that
these could be several Myr old. The offset in the position of these
fast rotators compared with the other stars confirms the predictions
of evolutionary models that fast-rotating stars tend to evolve more
vertically in the H-R diagram. Only one star of luminosity class
Vz, expected to best characterize extreme youth, is located on the
zero-age main sequence, the other two stars are more evolved. We
found that the distribution of O and B stars in the ɛ(N) - vsin i
diagram is the same, which suggests that the mechanisms responsible
for the chemical enrichment of slowly rotating massive stars depend
only weakly on the star's mass. We furthermore confirm that the group
of slowly rotating N-rich stars is not reproduced by the evolutionary
tracks. Even for more massive stars and faster rotators, our results
call for stronger mixing in the models to explain the range of
observed N abundances. All stars have an N/C ratio as a function of
stellar luminosity that match the predictions of the stellar evolution
models well. More massive stars have a higher N/C ratio than the less
massive stars. Faster rotators show on average a higher N/C ratio
than slower rotators, again consistent with the expected trend of
stronger mixing as rotation increases. When comparing the N/O versus
N/C ratios with those of stellar evolution models, the same global
qualitative agreement is reached. The only discrepant behavior is
observed for the youngest two stars of the sample, which both show
very strong signs of mixing, which is unexpected for their evolutionary
status. Based on observations made with the NASA-ESA Hubble Space
Telescope (program GO 11625), obtained at STScI, which is operated
by AURA, Inc., under NASA contract NAS 5-26555. Based on observations
collected at the European Southern Observatory Very Large Telescope,
program 079.D-0073.Appendix B is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: Chemical enrichment and physical conditions in I Zw 18
Authors: Lebouteiller, V.; Heap, S.; Hubeny, I.; Kunth, D.
2013A&A...553A..16L Altcode: 2013arXiv1302.4746L
Context. Low-metallicity star-forming dwarf galaxies are prime targets
to understand the chemical enrichment of the interstellar medium. The
H i region contains the bulk of the mass in blue compact dwarfs, and
it provides important constraints on the dispersal and mixing of heavy
elements released by successive star-formation episodes. The metallicity
of the H i region is also a critical parameter to investigate the future
star-formation history, as metals provide most of the gas cooling that
will facilitate and sustain star formation. Aims: Our primary
objective is to study the enrichment of the H i region and the interplay
between star-formation history and metallicity evolution. Our secondary
objective is to constrain the spatial- and time-scales over which the H
i and H ii regions are enriched, and the mass range of stars responsible
for the heavy element production. Finally, we aim to examine the gas
heating and cooling mechanisms in the H i region. Methods: We
observed the most metal-poor star-forming galaxy in the Local Universe,
I Zw 18, with the Cosmic Origin Spectrograph onboard Hubble. The
abundances in the neutral gas are derived from far-ultraviolet
absorption-lines (H i, C ii, C ii*, N i, O i, ...) and are compared to
the abundances in the H ii region. Models are constructed to calculate
the ionization structure and the thermal processes. We investigate the
gas cooling in the H i region through physical diagnostics drawn from
the fine-structure level of C+. Results: We find that
H i region abundances are lower by a factor of ~2 as compared to the H
ii region. There is no differential depletion on dust between the H i
and H ii region. Using sulfur as a metallicity tracer, we calculate a
metallicity of 1/46 Z⊙ (vs. 1/31 Z⊙ in the
H ii region). From the study of the C/O, [O/Fe], and N/O abundance
ratios, we propose that C, N, O, and Fe are mainly produced in
massive stars. We argue that the H i envelope may contain pockets of
pristine gas with a metallicity essentially null. Finally, we derive
the physical conditions in the H i region by investigating the C ii*
absorption line. The cooling rate derived from C ii* is consistent
with collisions with H0 atoms in the diffuse neutral
gas. We calculate the star-formation rate from the C ii* cooling rate
assuming that photoelectric effect on dust is the dominant gas heating
mechanism. Our determination is in good agreement with the values in
the literature if we assume a low dust-to-gas ratio (~2000 times lower
than the Milky Way value). Appendix A is available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>

---------------------------------------------------------
Title: 3D Spectral Radiative Transfer with IRIS: Application to
the Simulation of Laboratory Models of Accretion Shocks in Young
Stellar Objects
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.; González,
M.; Chièze, J. P.
2013ASPC..474...66I Altcode:
IRIS is a new three-dimensional radiative transfer code that provides
spectroscopic diagnostics of hydrodynamics structures. We present its
major numerical features and capabilities. We investigate the dynamical,
radiative and spectral properties of laboratory generated radiative
shocks, with the aim of improving our understanding of the mechanisms of
accretion in young stellar objects. We employed IRIS to generate X-UV
theoretical spectra, while three-dimensional non-stationary radiation
hydrodynamics simulations were performed with the code HERACLES.

---------------------------------------------------------
Title: Stellar Atmospheres
Authors: Hubeny, Ivan
2013pss4.book...51H Altcode:
Basic concepts of the stellar atmospheres theory are briefly
outlined. After discussing essential assumptions, approximations, and
basic structural equations describing a stellar atmospheres, emphasis
is given to describing efficient numerical methods developed to deal
with the stellar atmosphere problem, namely the method of complete
linearization and its recent variants, and the whole class of methods
known by name Accelerated Lambda Iteration. The existing computer codes,
and some of the most useful grids of model atmospheres that are publicly
available, are briefly summarized. Some interesting properties of newly
computed NLTE models atmospheres and their comparison to LTE models
are shown. Finally, it is briefly shown how the model atmospheres are
used to determine basic stellar parameters.

---------------------------------------------------------
Title: IRIS: a generic three-dimensional radiative transfer code
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.
2013A&A...549A.126I Altcode: 2012arXiv1211.4870I
Context. For most astronomical objects, radiation is the only probe of
their physical properties. Therefore, it is important to have the most
elaborate theoretical tool to interpret observed spectra or images,
thus providing invaluable information to build theoretical models of
the physical nature, the structure, and the evolution of the studied
objects. Aims: We present IRIS, a new generic three-dimensional
(3D) spectral radiative transfer code that generates synthetic spectra,
or images. It can be used as a diagnostic tool for comparison with
astrophysical observations or laboratory astrophysics experiments. Methods: We have developed a 3D short-characteristic solver that
works with a 3D nonuniform Cartesian grid. We have implemented a
piecewise cubic, locally monotonic, interpolation technique that
dramatically reduces the numerical diffusion effect. The code takes
into account the velocity gradient effect resulting in gradual Doppler
shifts of photon frequencies and subsequent alterations of spectral
line profiles. It can also handle periodic boundary conditions. This
first version of the code assumes local thermodynamic equilibrium
(LTE) and no scattering. The opacities and source functions are
specified by the user. In the near future, the capabilities of IRIS
will be extended to allow for non-LTE and scattering modeling. 
Results: IRIS has been validated through a number of tests. We provide
the results for the most relevant ones, in particular a searchlight
beam test, a comparison with a 1D plane-parallel model, and a test of
the velocity gradient effect. Conclusions: IRIS is a generic
code to address a wide variety of astrophysical issues applied to
different objects or structures, such as accretion shocks, jets in
young stellar objects, stellar atmospheres, exoplanet atmospheres,
accretion disks, rotating stellar winds, cosmological structures. It
can also be applied to model laboratory astrophysics experiments,
such as radiative shocks produced with high power lasers.

---------------------------------------------------------
Title: Hydrodynamic modeling of accretion shocks on a star with
radiative transport and a chromospheric model
Authors: de Sá, L.; Chièze, J. -P.; Stehlé, C.; Hubeny, I.;
Delahaye, F.; Lanz, T.
2012sf2a.conf..309D Altcode:
The aim of the project (ANR STARSHOCK) is to understand the dynamics
and the radiative properties of accretion columns, linking the
circumstellar disk to the surface photosphere of Young Stellar
Objects. The hydrodynamics is computed first, using a high resolution
hydrodynamic 1D ALE code (ASTROLABE) coupled to radiative transfer
and line cooling, along with a model for the acoustic heating of
the chromospheric plasma. Spectra are then post-processed with a 1D
radiative transfer code (SYNSPEC), using DFE solver and an extended
atomic database covering a wavelength range from X rays to visible.

---------------------------------------------------------
Title: 3D numerical simulations of laboratory models of accretion
shocks in young stellar objects
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.; González,
M.; Chièze, J. -P.
2012sf2a.conf..347I Altcode:
We show preliminary results of our numerical simulations of laboratory
experiments of radiative shocks. Such experiments aim at understanding
accretion shocks in young stellar objects. Three-dimensional
non-stationary radiation hydrodynamics calculations were performed
with the code HERACLES. X-UV spectra were then generated with the new
three-dimensional radiative transfer code IRIS.

---------------------------------------------------------
Title: An Online Catalog of Cataclysmic Variable Spectra from the
Far-Ultraviolet Spectroscopic Explorer
Authors: Godon, Patrick; Sion, Edward M.; Levay, Karen; Linnell, Albert
P.; Szkody, Paula; Barrett, Paul E.; Hubeny, Ivan; Blair, William P.
2012ApJS..203...29G Altcode: 2012arXiv1210.1118G
We present an online catalog containing spectra and supporting
information for cataclysmic variables that have been observed with
the Far-Ultraviolet Spectroscopic Explorer (FUSE). For each object
in the catalog we list some of the basic system parameters such as
(R.A., decl.), period, inclination, and white dwarf mass, as well as
information on the available FUSE spectra: data ID, observation date
and time, and exposure time. In addition, we provide parameters needed
for the analysis of the FUSE spectra such as the reddening E(B - V),
distance, and state (high, low, intermediate) of the system at the time
it was observed. For some of these spectra we have carried out model
fits to the continuum with synthetic stellar and/or disk spectra using
the codes TLUSTY and SYNSPEC. We provide the parameters obtained from
these model fits; this includes the white dwarf temperature, gravity,
projected rotational velocity, and elemental abundances of C, Si,
S, and N, together with the disk mass accretion rate, the resulting
inclination, and model-derived distance (when unknown). For each object
one or more figures are provided (as gif files) with line identification
and model fit(s) when available. The FUSE spectra and the synthetic
spectra are directly available for download as ASCII tables. References
are provided for each object, as well as for the model fits. In this
article we present 36 objects, and additional ones will be added to
the online catalog in the future. In addition to cataclysmic variables,
we also include a few related objects, such as a wind-accreting white
dwarf, a pre-cataclysmic variable, and some symbiotics.

---------------------------------------------------------
Title: Modeling periodic media with the three-dimensional radiative
transfer code IRIS
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.
2012sf2a.conf..343I Altcode:
We describe the implementation in our generic three-dimensional
radiative transfer code, IRIS, of an algorithm that allows the
modeling of periodic infinite media. We show how this algorithm has
been validated by comparison with well-established 1D plane-parallel
models. A particularly interesting astrophysical application will be
the calculation of synthetic spectra of the fully three-dimensional
solar atmosphere.

---------------------------------------------------------
Title: BINSYN: Simulating Spectra and Light Curves of Binary Systems
with or without Accretion Disks
Authors: Linnell, Albert P.; DeStefano, Paul; Hubeny, Ivan
2012ascl.soft08002L Altcode:
The BINSYN program suite is a collection of programs for analysis
of binary star systems with or without an optically thick accretion
disk. BINSYN produces synthetic spectra of individual binary star
components plus a synthetic spectrum of the system. If the system
includes an accretion disk, BINSYN also produces a separate synthetic
spectrum of the disk face and rim. A system routine convolves the
synthetic spectra with filter profiles of several photometric standards
to produce absolute synthetic photometry output. The package generates
synthetic light curves and determines an optimized solution for system
parameters.

---------------------------------------------------------
Title: BINSYN: A Publicly Available Program for Simulating Spectra
and Light Curves of Binary Systems with or without Accretion Disks
Authors: Linnell, Albert P.; DeStefano, Paul; Hubeny, Ivan
2012PASP..124..885L Altcode:
The BINSYN program suite, a collection of programs for analysis of
binary star systems with or without an optically thick accretion disk,
is available for download from a wiki. This article describes the
package, including download instructions. BINSYN produces synthetic
spectra of individual binary star components plus a synthetic spectrum
of the system. If the system includes an accretion disk, BINSYN also
produces a separate synthetic spectrum of the disk face and rim. A
system routine convolves the synthetic spectra with filter profiles of
several photometric standards to produce absolute synthetic photometry
output. The package generates synthetic light curves and determines
an optimized solution for system parameters. This article includes
illustrative literature references that have used the suite, including
mass transfer rates in several cataclysmic binary systems.

---------------------------------------------------------
Title: Chemical abundances of hot post-AGB stars
Authors: Mello, D. R. C.; Daflon, S.; Pereira, C. B.; Hubeny, I.
2012A&A...543A..11M Altcode:
Context. Hot post-AGB stars are luminous objects of low- and
intermediate mass (0.8-8 M⊙) in the final stage
of evolution, moving between the asymptotic giant branch (AGB)
stars and planetary nebulae. The absorption lines observed in
their spectra are typical of early-type stars and their abundance
pattern may be associated with the occurence of one or more mixing
processes during the previous AGB phase. Aims: To better
constrain their observed chemical pattern and evolutionary status,
we determined chemical abundances for a sample of hot post-AGB
stars selected according to spectroscopic criteria. Methods:
The observational data are high-resolution spectra obtained with the
FEROS spectrograph. The stellar parameters and chemical composition
were obtained from fully consistent non-LTE synthesis. Results:
The general abundance pattern reveals relevant nitrogen enrichment,
slight depletion in carbon and sulfur and mild excess in helium
for most of the objects. One notable exception is LSE 148, with Z =
0.001, which is likely to be a metal-poor object at high galactic
latitude. The atmospheric parameters and chemical abundances obtained
are discussed in the context of evolutionary models. Mixing processes
like the second/third dredged-up and "hot bottom-burning" are invoked to
explain the obtained results. Based on observations made with the
2.2 m telescope at the European Southern Observatory (La Silla, Chile).

---------------------------------------------------------
Title: From Interacting Binaries to Exoplanets: Essential Modeling
Tools
Authors: Richards, Mercedes T.; Hubeny, Ivan
2012IAUS..282.....R Altcode:
Preface; 1. Multiwavelength photometry and spectroscopy of interacting
binaries; 2. Observations and analysis of exoplanets and brown dwarfs;
3. Imaging techniques; 4. Model atmospheres of stars, interacting
binaries, disks, exoplanets, and brown dwarfs; 5. Synthetic light
curves and velocity curves, synthetic spectra of binary stars and their
accretion; 6. Analysis of spectra and light curves; 7. Formation and
evolution of binary stars, brown dwarfs, and planets; 8. Hydrodynamic
simulations of exoplanets and mass transfer in interacting binaries;
Summary; Index.

---------------------------------------------------------
Title: From Interacting Binaries to Exoplanets (IAU S282)
Authors: Richards, Mercedes T.; Hubeny, Ivan
2012fibe.book.....R Altcode:
Preface; 1. Multiwavelength photometry and spectroscopy of interacting
binaries; 2. Observations and analysis of exoplanets and brown dwarfs;
3. Imaging techniques; 4. Model atmospheres of stars, interacting
binaries, disks, exoplanets, and brown dwarfs; 5. Synthetic light
curves and velocity curves, synthetic spectra of binary stars and their
accretion; 6. Analysis of spectra and light curves; 7. Formation and
evolution of binary stars, brown dwarfs, and planets; 8. Hydrodynamic
simulations of exoplanets and mass transfer in interacting binaries;
Summary; Index.

---------------------------------------------------------
Title: Modeling Atmospheres of Brown Dwarfs and Extrasolar Giant
Planets
Authors: Hubeny, Ivan
2012AAS...22041402H Altcode:
A brief review of spectroscopic studies of subsolar mass objects, namely
brown dwarfs and extrasolar giant planets, is given, with emphasis on
physics and chemistry of their atmospheres, their predicted spectra,
and a comparison of theory with observations. Besides mentioning some
recent highlights, main emphasis is given to discussing the role of
uncertainties in the molecular opacity data, cloud formation physics,
and departures from chemical equilibrium, on model atmospheres and
theoretical spectra, and consequently on deducing basic physical
properties of these objects.

---------------------------------------------------------
Title: The distribution of metals in hot DA white dwarfs
Authors: Dickinson, N. J.; Barstow, M. A.; Hubeny, I.
2012MNRAS.421.3222D Altcode: 2012MNRAS.tmp.2468D; 2012arXiv1201.2940D
The importance to stellar evolution of understanding the metal
abundances in hot white dwarfs is well known. Previous work has
found the hot DA white dwarfs REJ 1032+532, REJ 1614-085 and GD
659 to have highly abundant, stratified photospheric nitrogen,
due to the narrow absorption-line profiles of the far-ultraviolet
(FUV) N V doublet and the lack of extreme-ultraviolet (EUV) continuum
absorption. A preliminary analysis of the extremely narrow, deep line
profiles of the photospheric metal absorption features of PG 0948+534
suggested a similar photospheric metal configuration. However, other
studies have found that REJ 1032+532, REJ 1614-085 and GD 659 can
be well described by homogeneous models, with nitrogen abundances
more in keeping with those of white dwarfs with higher effective
temperatures. Here, a re-analysis of the nitrogen absorption features
seen in REJ 1032+532, REJ 1614-085 and GD 659 is presented, with the
aim of better understanding the structure of these stars, to test
which models better represent the observed data and apply the results
to the line profiles seen in PG 0948+534. A degeneracy is seen in the
modelling of the nitrogen absorption-line profiles of REJ 1032+532,
REJ 1614-085 and GD 659, with low-abundance, homogeneously distributed
nitrogen models most likely being a better representation of the
observed data. In PG 0948+534, no such degeneracy is seen, and the
enigmatically deep line profiles could not be modelled satisfactorily.

---------------------------------------------------------
Title: Panel Discussion III
Authors: Allard, F.; Batten, A.; Budding, E.; Devinney, E.; Eggleton,
P.; Hatzes, A.; Hubeny, I.; Kley, W.; Lammer, H.; Linnell, A.; Trimble,
V.; Wilson, R. E.
2012IAUS..282..501A Altcode:
I. Hubeny Does anyone from the panel have a theme question to start with
today? V. Trimble It's another one-liner: From an active galaxy
meeting many years ago when people talked about spiral structure. I
was reminded by Dr. Rucinski's talk of Lodewijk Woltjer's remark:
“The larger our ignorance, the stronger the magnetic field.”

---------------------------------------------------------
Title: Flash Mixing on the White Dwarf Cooling Curve: Spectroscopic
Confirmation in NGC 2808
Authors: Brown, Thomas M.; Lanz, Thierry; Sweigart, Allen V.; Cracraft,
Misty; Hubeny, Ivan; Landsman, Wayne B.
2012ApJ...748...85B Altcode: 2012arXiv1201.4070B
We present new Hubble Space Telescope far-UV spectroscopy of two
dozen hot evolved stars in NGC 2808, a massive globular cluster with
a large population of "blue-hook" (BHk) stars. The BHk stars are
found in ultraviolet color-magnitude diagrams of the most massive
globular clusters, where they fall at luminosities immediately below
the hot end of the horizontal branch (HB), in a region of the H-R
diagram unexplained by canonical stellar evolution theory. Using
new theoretical evolutionary and atmospheric models, we have shown
that these subluminous HB stars are very likely the progeny of stars
that undergo extensive internal mixing during a late He-core flash
on the white dwarf cooling curve. This flash mixing leads to hotter
temperatures and an enormous enhancement of the surface He and C
abundances; these hotter temperatures, together with the decrease in H
opacity shortward of the Lyman limit, make the BHk stars brighter in the
extreme UV while appearing subluminous in the UV and optical. Our far-UV
spectroscopy demonstrates that, relative to normal HB stars at the
same color, the BHk stars of NGC 2808 are hotter and greatly enhanced
in He and C, thus providing unambiguous evidence of flash mixing in
the subluminous population. Although the C abundance in the BHk stars
is orders of magnitude larger than that in the normal HB stars, the
atmospheric C abundance in both the BHk and normal HB stars appears to
be affected by gravitational settling. The abundance variations seen in
Si and the Fe-peak elements also indicate that atmospheric diffusion is
at play in our sample, with all of our hot subdwarfs at 25,000-50,000
K exhibiting large enhancements of the iron-peak elements. The hottest
subdwarfs in our BHk sample may be pulsators, given that they fall in
the temperature range of newly discovered pulsating subdwarfs in ω
Cen. In addition to the normal hot HB and BHk stars, we also obtain
spectra of five blue HB stars, a post-HB star, and three unclassified
stars with unusually blue UV colors. Based on observations made
with the NASA/ESA Hubble Space Telescope, obtained at STScI, which is
operated by AURA, Inc., under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: Panel Discussion II
Authors: Allard, F.; Batten, A.; Budding, E.; Devinney, E.; Eggleton,
P.; Hatzes, A.; Hubeny, I.; Kley, W.; Lammer, H.; Linnell, A.; Trimble,
V.; Wilson, R. E.
2012IAUS..282..341A Altcode:
I. Hubeny Today, the discussion will be open to the general audience. In
Sessions C, D, and E, we have talked about models and modelling
techniques so I expect the discussion will focus on these topics.

---------------------------------------------------------
Title: Panel Discussion IV
Authors: Allard, F.; Batten, A.; Budding, E.; Devinney, E.; Eggleton,
P.; Hatzes, A.; Hubeny, I.; Kley, W.; Lammer, H.; Linnell, A.; Trimble,
V.; Wilson, R. E.
2012IAUS..282..551A Altcode:
I. Hubeny Welcome to the last panel meeting. We invite general comments
either from the audience or from the panelists. V. Trimble Well,
Mercedes started us with a vocabulary item and I think I would like to
end with a vocabulary item. When they were first discovered, we called
them `extra solar system planets' which was descriptive and fine,
but it's just rather cumbersome. At some point they became `extra
solar planets.' Now I have never seen a planet inside the Sun. And
therefore `extrasolar' is not a good descriptor. `Exoplanets' is OK,
but now that there are so many of them that perhaps they are simply
`the planets.' When you want to specialize to ours, you could say
`solar system planets.' Think how much ink it would save.

---------------------------------------------------------
Title: Panel Discussion I
Authors: Allard, F.; Batten, A.; Budding, E.; Devinney, E.; Eggleton,
P.; Hatzes, A.; Hubeny, I.; Kley, W.; Lammer, H.; Linnell, A.; Trimble,
V.; Wilson, R. E.
2012IAUS..282..145A Altcode:
M. Richards: Several talks today have expressed fuzzy boundaries to
describe the objects called “stars.” Is the following classification
correct? Are stars restricted to objects that have masses greater than
0.089 solar masses and begin making energy with hydrogen burning? Do we
include the stellar remnants: the white dwarfs and neturon stars? Do we
include the brown dwarfs because they burn lithium or deuterium. We know
that planets are not in this group since they have no energy production.

---------------------------------------------------------
Title: Basic Tools for Modeling Stellar and Planetary Atmospheres
Authors: Hubeny, Ivan
2012IAUS..282..221H Altcode:
Most popular computer codes for calculating model stellar and planetary
atmospheres are briefly reviewed. A particular emphasis is devoted to
our universal computer program Tlusty (model stellar atmospheres and
accretion disks), CoolTlusty (a variant of Tlusty for computing model
atmospheres of substellar-mass objects such as giant planets and brown
dwarfs), and Synspec (an associated spectrum synthesis code). We show
the highlights of actual applications of these codes which include
extensive grids of fully line-blanketed non-LTE model atmospheres
of O and B stars, and grids of model atmospheres of extrasolar giant
planets and L and T dwarfs.

---------------------------------------------------------
Title: Day-Night Side Cooling of a Strongly Irradiated Giant Planet
Authors: Budaj, Ján; Burrows, Adam; Hubeny, Ivan
2012IAUS..282..486B Altcode:
The internal heat loss or cooling of a planet determines its structure
and evolution. We address in a consistent fashion the coupling between
the day and the night sides by means of model atmosphere calculations
with heat redistribution. We assume that a strong convection leads to
the same entropy on the day and night side and that the gravity is the
same on both hemispheres. We argue that the core cooling rate from the
two hemispheres of a strongly irradiated planet may not be the same
and that the difference depends on several important parameters. If the
day-night heat redistribution is very effective, or if it takes place
at a large optical depth, then the day-side and the night-side cooling
may be comparable. However, if the day-night heat transport is not
effective, or if it takes place at a shallow optical depth, then there
can be a big difference between the day-side and the night-side cooling
and the night side may cool more effectively. If the stellar irradiation
gets stronger e.g. due to the stellar evolution or migration, this will
reduce both the day and the night side cooling. Enhanced metallicity
in the atmosphere acts as a “blanket” and reduces both the day- and
the night-side cooling. However, the stratosphere on the day side of
the planet can enhance the day-side cooling since its opacity acts as a
“shield” which screens the stellar irradiation. These results might
affect the well known gravity darkening and bolometric albedo effects in
interacting binaries, especially for strongly irradiated cold objects.

---------------------------------------------------------
Title: New Observational Evidence of Flash Mixing on the White Dwarf
Cooling Curve
Authors: Brown, T. M.; Lanz, T.; Sweigart, A. V.; Cracraft, M.;
Hubeny, I.; Landsman, W. B.
2012ASPC..452...23B Altcode: 2012arXiv1201.4204B
Blue hook stars are a class of subluminous extreme horizontal branch
stars that were discovered in UV images of the massive globular clusters
ω Cen and NGC 2808. These stars occupy a region of the HR diagram
that is unexplained by canonical stellar evolution theory. Using
new theoretical evolutionary and atmospheric models, we have shown
that the blue hook stars are very likely the progeny of stars that
undergo extensive internal mixing during a late helium-core flash
on the white dwarf cooling curve. This “flash mixing” produces
hotter-than-normal EHB stars with atmospheres significantly enhanced
in helium and carbon. The larger bolometric correction, combined with
the decrease in hydrogen opacity, makes these stars appear subluminous
in the optical and UV. Flash mixing is more likely to occur in stars
born with a high helium abundance, due to their lower mass at the
main sequence turnoff. For this reason, the phenomenon is more common
in those massive globular clusters that show evidence for secondary
populations enhanced in helium. However, a high helium abundance does
not, by itself, explain the presence of blue hook stars in massive
globular clusters. Here, we present new observational evidence for flash
mixing, using recent HST observations. These include UV color-magnitude
diagrams of six massive globular clusters and far-UV spectroscopy of
hot subdwarfs in one of these clusters (NGC 2808).

---------------------------------------------------------
Title: 3D modeling of accretion shocks in young stellar objects :
Simulation of laboratory experiments
Authors: Ibgui, L.; González, M.; Stehlé, C.; Hubeny, I.; Lanz, T.
2012EAS....58..149I Altcode:
In order to improve the understanding of the physics of accretion shocks
around young stellar objects, we have performed a three dimensional
simulation of a radiative shock generated in a laser installation. We
depict the 3D structure of such a shock. Radiation hydrodynamics is
modeled with the HERACLES code; then, radiative transfer post-processing
is performed with the IRIS code.

---------------------------------------------------------
Title: Chemical homogeneity in the Orion Association: Oxygen
abundances of B stars
Authors: Cunha, K.; Hubeny, I.; Lanz, T.
2012EPJWC..1908005C Altcode: 2011arXiv1110.5879C
We present non-LTE oxygen abundances for a sample of B stars in the
Orion association. The abundance calculations included non-LTE line
formation and used fully blanketed non-LTE model atmospheres. The
stellar parameters were the same as adopted in the previous study by
Cunha &amp; Lambert (1994). We find that the young Orion stars in
this sample of 10 stars are described by a single oxygen abundance
with an average value of A(O) = 8.78 and a small dispersion of ±0.05,
dex which is of the order of the uncertainties in the analysis. This
average oxygen abundance compares well with the average oxygen
abundance obtained previously in Cunha &amp; Lambert (1994): A(O) =
8.72 ± 0.13 although this earlier study, based upon non-blanketed
model atmospheres in LTE, displayed larger scatter. Small scatter
of chemical abundances in Orion B stars had also been found in our
previous studies for neon and argon; all based on the same effective
temperature scale. The derived oxygen abundance distribution for the
Orion association compares well with other results for the oxygen
abundance in the solar neighborhood.

---------------------------------------------------------
Title: Day and night side core cooling of a strongly irradiated
giant planet
Authors: Budaj, J.; Hubeny, I.; Burrows, A.
2012A&A...537A.115B Altcode: 2011arXiv1111.5478B
Context. The internal heat loss, or cooling, of a planet determines
its structure and evolution. Aims: We study the effects of
irradiation, metallicity of the atmosphere, heat redistribution,
stratospheres, and the depth where the heat redistribution takes place
on the atmospheric structure, the core entropy, and subsequently on
the cooling of the interior of the planet. Methods: We address
in a consistent fashion the coupling between the day and the night
sides of a planet by means of model atmosphere calculations with heat
redistribution. We assume that strong convection leads to the same
entropy on the day and night sides and that gravity is the same on both
hemispheres. Results: We argue that the core cooling rates from
the two hemispheres of a strongly irradiated planet may not be the same
and that the difference depends on several important parameters. If the
day-night heat redistribution is very efficient or if it takes place at
the large optical depth, then the day-side and the night-side cooling
may be comparable. However, if the day-night heat transport is not
efficient or if it takes place at a shallow optical depth then there
can be a large difference between the day- and the night-side cooling
and the night side will cool more efficiently. If stellar irradiation
becomes stronger, e.g. owing to stellar evolution or migration, cooling
from both the day and the night sides is reduced. Enhanced metallicity
of the atmosphere would act as an added "blanket" and reduces both
the day- and the night-side cooling. However, a stratosphere on the
planetary day side can enhance day-side cooling since its opacity acts
as a "sunshade" that screens the stellar irradiation. These effects may
also influence the well-known gravity darkening and bolometric albedo
effects in interacting binaries, especially for strongly irradiated
cold components.

---------------------------------------------------------
Title: SPEC3D: a three-dimensional radiative transfer code for
astrophysical and laboratory applications
Authors: Ibgui, L.; Hubeny, I.; Lanz, T.; Stehlé, C.
2011sf2a.conf..485I Altcode:
We have developed a generic three-dimensional radiative transfer code,
SPEC3D, aimed at post-processing 3D radiation magnetohydrodynamics
simulations. SPEC3D solves the monochromatic 3D radiative transfer
equation. The numerical approach and the major features of the
code are presented. The wide range of applications includes the
modeling of a number of astrophysical objects and structures,
such as accretion shocks around young stellar objects, stellar and
exoplanets atmospheres, cosmological structures, but also the modeling
of laboratory astrophysics experiments such as magnetohydrodynamics
jets and radiative shocks.

---------------------------------------------------------
Title: Synspec: General Spectrum Synthesis Program
Authors: Hubeny, Ivan; Lanz, Thierry
2011ascl.soft09022H Altcode:
Synspec is a user-oriented package written in FORTRAN for modeling
stellar atmospheres and for stellar spectroscopic diagnostics. It
assumes an existing model atmosphere, calculated previously with Tlusty
or taken from the literature (for instance, from the Kurucz grid of
models). The opacity sources (continua, atomic and molecular lines)
are fully specified by the user. An arbitrary stellar rotation and
instrumental profile can be applied to the synthetic spectrum.

---------------------------------------------------------
Title: TLUSTY: Stellar Atmospheres, Accretion Disks, and Spectroscopic
Diagnostics
Authors: Hubeny, Ivan; Lanz, Thierry
2011ascl.soft09021H Altcode:
TLUSTY is a user-oriented package written in FORTRAN77 for modeling
stellar atmospheres and accretion disks and wide range of spectroscopic
diagnostics. In the program's maximum configuration, the user may start
from scratch and calculate a model atmosphere of a chosen degree of
complexity, and end with a synthetic spectrum in a wavelength region of
interest for an arbitrary stellar rotation and an arbitrary instrumental
profile. The user may also model the vertical structure of annuli of
an accretion disk.

---------------------------------------------------------
Title: A Method for the Study of Accretion Disk Emission in
Cataclysmic Variables. I. The Model
Authors: Puebla, Raúl E.; Diaz, Marcos P.; Hillier, D. John;
Hubeny, Ivan
2011ApJ...736...17P Altcode: 2011arXiv1105.0851P
We have developed a spectrum synthesis method for modeling the
ultraviolet (UV) emission from the accretion disk from cataclysmic
variables (CVs). The disk is separated into concentric rings, with
an internal structure from the Wade &amp; Hubeny disk-atmosphere
models. For each ring, a wind atmosphere is calculated in the comoving
frame with a vertical velocity structure obtained from a solution of
the Euler equation. Using simple assumptions, regarding rotation and
the wind streamlines, these one-dimensional models are combined into a
single 2.5-dimensional model for which we compute synthetic spectra. We
find that the resulting line and continuum behavior as a function
of the orbital inclination is consistent with the observations, and
verify that the accretion rate affects the wind temperature, leading
to corresponding trends in the intensity of UV lines. In general,
we also find that the primary mass has a strong effect on the P Cygni
absorption profiles, the synthetic emission line profiles are strongly
sensitive to the wind temperature structure, and an increase in the
mass-loss rate enhances the resonance line intensities. Synthetic
spectra were compared with UV data for two high orbital inclination
nova-like CVs—RW Tri and V347 Pup. We needed to include disk regions
with arbitrary enhanced mass loss to reproduce reasonably well widths
and line profiles. This fact and a lack of flux in some high ionization
lines may be the signature of the presence of density-enhanced regions
in the wind, or alternatively, may result from inadequacies in some
of our simplifying assumptions.

---------------------------------------------------------
Title: An Improved Tlusty Model Of Cataclysmic Variable Accretion
Disk Annuli.
Authors: Linnell, Albert P.; Hubeny, I.
2011AAS...21821905L Altcode: 2011BAAS..43G21905L
The Hubeny TLUSTY and SYNSPEC programs have been used in several
studies of Nova-like Cataclysmic Variable stars, especially models of
their accretion disks. Tabulations of column masses for the individual
annuli (which are always optically thick) listed values in disagreement
with theoretical models in the literature (e.g., Lasota, 2001, NAR,
45, 449, Fig. 1). The discrepancy arose from an inadequate treatment
of convection in TLUSTY (which had been developed with particular
attention to radiation-dominated accretion disks). A new version of
TLUSTY, developed by Hubeny, corrects the discrepancy and produces
an insignificant change in our published conclusions. We illustrate
the results for our study of RW Sextantis (Linnell et al., 2010,
ApJ,719,271).

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Title: The Hot Components of AM CVn Helium Cataclysmics
Authors: Sion, Edward M.; Linnell, A. P.; Godon, P.; Hubeny, I.
2011AAS...21733805S Altcode: 2011BAAS...4333805S
The AM CVn helium-transfer cataclysmic variables are fundametally
important because they are the progeny of double common envelope
evolution, they are a principal source of low frequency gravitational
wave emission, a laboratory for accretion and interior physics under
extreme conditions and candidates for Type Ia supernovae. The hot
component, consisting of a helium-rich white dwarf and disk, is
the focus of our synthetic spectral analysis using the code BINSYN
(Linnell&amp; Hubeny 1996) which takes into account the donor companion
star, the shock front which forms at the disk edge and the FUV and
NUV energy distribution. We report the first results from our
multi-component study of helium accretion and white dwarf heating. This research was supported by NASA ADP grant NN09AC94G and in part
by NSF grant AST0807892, both to Villanova University.

---------------------------------------------------------
Title: Discovery of a New AM CVn System with the Kepler Satellite
Authors: Fontaine, G.; Brassard, P.; Green, E. M.; Charpinet, S.;
Dufour, P.; Hubeny, I.; Steeghs, D.; Aerts, C.; Randall, S. K.;
Bergeron, P.; Guvenen, B.; O'Malley, C. J.; Van Grootel, V.; Østensen,
R. H.; Bloemen, S.; Silvotti, R.; Howell, S. B.; Baran, A.; Kepler,
S. O.; Marsh, T. R.; Montgomery, M. H.; Oreiro, R.; Provencal, J.;
Telting, J.; Winget, D. E.; Zima, W.; Christensen-Dalsgaard, J.;
Kjeldsen, H.
2011ApJ...726...92F Altcode:
We report the discovery of a new AM CVn system on the basis of
broadband photometry obtained with the Kepler satellite supplemented
by ground-based optical spectroscopy. Initially retained on Kepler
target lists as a potential compact pulsator, the blue object SDSS
J190817.07+394036.4 (KIC 004547333) has turned out to be a high-state
AM CVn star showing the He-dominated spectrum of its accretion disk
significantly reddened by interstellar absorption. We constructed
new grids of NLTE synthetic spectra for accretion disks in order to
analyze our spectroscopic observations. From this analysis, we infer
preliminary estimates of the rate of mass transfer, the inclination
angle of the disk, and the distance to the system. The AM CVn nature
of the system is also evident in the Kepler light curve, from which
we extracted 11 secure periodicities. The luminosity variations are
dominated by a basic periodicity of 938.507 s, likely to correspond to a
superhump modulation. The light curve folded on the period of 938.507 s
exhibits a pulse shape that is very similar to the superhump wavefront
seen in AM CVn itself, which is a high-state system and the prototype
of the class. Our Fourier analysis also suggests the likely presence
of a quasi-periodic oscillation similar to those already observed in
some high-state AM CVn systems. Furthermore, some very low-frequency,
low-amplitude aperiodic photometric activity is likely present, which
is in line with what is expected in accreting binary systems. Inspired
by previous work, we further looked for and found some intriguing
numerical relationships between the 11 secure detected frequencies,
in the sense that we can account for all of them in terms of only
three basic clocks. This is further evidence in favor of the AM CVn
nature of the system.

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Title: Hot, Massive Stars in I Zw 18
Authors: Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry; Landsman, Wayne;
Lindler, Don; Malumuth, Eliot
2011BSRSL..80..149H Altcode:
We present the far-ultraviolet spectrum of the northwest component of I
Zw 18, a blue compact galaxy having a very low metallicity. The spectrum
is compatible with continuous star-formation over the past ∼ 15 Myr
(CSF age), and a very low metallicity, log Z/Z_⊙ ∼ -1.7, although
the stellar surface may be enhanced in carbon. Stellar winds are very
weak, and the edge velocity of wind lines is very low (∼ 250 km/s).

---------------------------------------------------------
Title: The Anomalous Accretion Disk of the Cataclysmic Variable
RW Sextantis
Authors: Linnell, Albert P.; Godon, P.; Hubeny, I.; Sion, E. M.;
Szkody, P.
2011AAS...21733804L Altcode: 2011BAAS...4333804L
The standard model for stable Cataclysmic Variable (CV) accretion disks
(Frank, King and Raine 1992) derives an explicit analytic expression for
the disk effective temperature as function of radial distance from the
white dwarf (WD). That model specifies that the effective temperature,
Teff(R), varies with R as ()0.25, where () represents a
combination of parameters including R, the mass transfer rate M(dot),
and other parameters. It is well known that fits of standard model
synthetic spectra to observed CV spectra find almost no instances of
agreement. We have derived a generalized expression for the radial
temperature gradient, which preserves the total disk luminosity as
function of M(dot) but permits a different exponent from the theoretical
value of 0.25, and have applied it to RW Sex (Linnell et al.,2010,ApJ,
719,271). We find an excellent fit to observed FUSE and IUE spectra for
an exponent of 0.125, curiously close to 1/2 the theoretical value. Our
annulus synthetic spectra, combined to represent the accretion disk,
were produced with program TLUSTY, were non-LTE and included H, He, C,
Mg, Al, Si, and Fe as explicit ions. We illustrate our results with a
plot showing the failure to fit RW Sex for a range of M(dot) values,
our model fit to the observations, and a chi2 plot showing
the selection of the exponent 0.125 as the best fit for the M(dot)
range shown. (For the final model parameters see the paper cited.)

---------------------------------------------------------
Title: HST/WFC3 Photometry of NGC 2808 and Its Multiple Main Sequences
Authors: Jeffery, Elizabeth; Brown, T.; Dotter, A.; Hubeny, I.;
Landsman, W. B.; Lanz, T.; Sweigart, A. V.
2011AAS...21715212J Altcode: 2011BAAS...4315212J
Recent high precision photometric observations of several globular
clusters have revealed that these simple stellar populations may not
be as simple as we once believed. Some globular clusters have shown
interesting features in their color-magnitude diagrams indicating
the presence of multiple populations. Recent HST observations of the
massive globular cluster NGC 2808 have revealed the presence of three
main sequences. We present new observations of NGC 2808 using the
new WFC3 camera on HST. Observations are in five photometric bands
covering a large wavelength range from the UVIS to the IR channels
of WFC3. Moreover, these observations extend below the "turn down"
in the lower main sequence (as seen in the IR CMD) and therefore reach
deeper than past data sets on this cluster. We discuss various model
fits and compare with other observed CMDs of other globular clusters
to explore the origin of these multiple sequences.

---------------------------------------------------------
Title: The Stratification of Metals in Hot White Dwarf Atmospheres
Authors: Dickinson, N. J.; Barstow, M. A.; Hubeny, I.
2010AIPC.1273..400D Altcode:
The atmospheres of many hot white dwarfs contain significant quantities
of metals. Various studies have reached different conclusions on the
distribution of nitrogen in the atmosphere of REJ1032+532, a 44,350 K
DA. Here, we examine this object again to gain a better understanding
of its nitrogen distribution, and attempt to model the C IV features
seen in the DA PG0948+534 (110,0000 K).

---------------------------------------------------------
Title: Non-LTE Models and Theoretical Spectra of Accretion Disks in
Active Galactic Nuclei. III. Integrated Spectra for Hydrogen-Helium
Disks
Authors: Hubeny, Ivan; Agol, Eric; Blaes, Omer; Krolik, Julian
2010ascl.soft11016H Altcode:
We have constructed a grid of non-LTE disk models for a wide range
of black hole mass and mass accretion rate, for several values of
viscosity parameter alpha, and for two extreme values of the black
hole spin: the maximum-rotation Kerr black hole, and the Schwarzschild
(non-rotating) black hole. Our procedure calculates self-consistently
the vertical structure of all disk annuli together with the radiation
field, without any approximations imposed on the optical thickness
of the disk, and without any ad hoc approximations to the behavior of
the radiation intensity. The total spectrum of a disk is computed by
summing the spectra of the individual annuli, taking into account the
general relativistic transfer function. The grid covers nine values of
the black hole mass between M = 1/8 and 32 billion solar masses with
a two-fold increase of mass for each subsequent value; and eleven
values of the mass accretion rate, each a power of 2 times 1 solar
mass/year. The highest value of the accretion rate corresponds to 0.3
Eddington. We show the vertical structure of individual annuli within
the set of accretion disk models, along with their local emergent flux,
and discuss the internal physical self-consistency of the models. We
then present the full disk-integrated spectra, and discuss a number
of observationally interesting properties of the models, such as
optical/ultraviolet colors, the behavior of the hydrogen Lyman limit
region, polarization, and number of ionizing photons. Our calculations
are far from definitive in terms of the input physics, but generally
we find that our models exhibit rather red optical/UV colors. Flux
discontinuities in the region of the hydrogen Lyman limit are only
present in cool, low luminosity models, while hotter models exhibit
blueshifted changes in spectral slope.

---------------------------------------------------------
Title: Theory and modeling of stellar atmospheres
Authors: Hubeny, Ivan
2010AIPC.1268...73H Altcode:
I will briefly outline basic concepts of the stellar atmospheres
theory. After summarizing basic structural equations describing a
stellar atmospheres, an emphasis is given to describing efficient
numerical methods developed to deal with the stellar atmosphere problem,
namely the method of complete linearization ant its recent variants,
and the whole class of methods known by name Accelerated Lambda
Iteration. In the next part of the lectures I will briefly summarize
existing computer codes, with an emphasis on our code TLUSTY, and list
some of the most useful grids of model atmospheres that are publicly
available. Next, I will show how the model atmospheres and synthetic
spectra are used in quantitative stellar spectroscopy in order to
determine basic stellar parameters and chemical abundances. Finally,
I will briefly describe an application of model atmosphere theory
and models to related objects, such as accretion disks around various
accretors, and atmospheres of substellar-mass objects-extrasolar giant
planets and brown dwarfs.

---------------------------------------------------------
Title: The Anomalous Accretion Disk of the Cataclysmic Variable
RW Sextantis
Authors: Linnell, Albert P.; Godon, Patrick; Hubeny, Ivan; Sion,
Edward M.; Szkody, Paula
2010ApJ...719..271L Altcode: 2010arXiv1006.2832L
Synthetic spectra covering the wavelength range 900-3000 Å provide an
accurate fit, established by a χ2 ν analysis, to
a combined observed spectrum of RW Sextantis. Two separately calibrated
distances to the system establish the synthetic spectrum comparison
on an absolute flux basis but with two alternative scaling factors,
requiring alternative values of \dot{M} for final models. Based on
comparisons for a range of \dot{M} values, the observed spectrum
does not follow the standard model. Rather than the exponent 0.25
in the expression for the radial temperature profile, a value close
to 0.125 produces a synthetic spectrum with an accurate fit to the
combined spectrum. A study of time-series Far Ultraviolet Spectroscopic
Explorer spectra shows that a proposed warped or tilted disk is not
supported by the data; an alternative proposal is that an observed
non-axisymmetric wind results from an interaction with the mass transfer
stream debris. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc. under NASA contract NAS5-26555, and the NASA-CNES-CSA
Far Ultraviolet Explorer, which is operated for NASA by the Johns
Hopkins University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: The Blue Hook Populations of Massive Globular Clusters
Authors: Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Smith,
Ed; Landsman, Wayne B.; Hubeny, Ivan
2010ApJ...718.1332B Altcode: 2010arXiv1006.1591B
We present new Hubble Space Telescope ultraviolet color-magnitude
diagrams of five massive Galactic globular clusters: NGC 2419, NGC 6273,
NGC 6715, NGC 6388, and NGC 6441. These observations were obtained to
investigate the "blue hook" (BH) phenomenon previously observed in UV
images of the globular clusters ω Cen and NGC 2808. Blue hook stars are
a class of hot (approximately 35,000 K) subluminous horizontal branch
stars that occupy a region of the HR diagram that is unexplained by
canonical stellar evolution theory. By coupling new stellar evolution
models to appropriate non-LTE synthetic spectra, we investigate various
theoretical explanations for these stars. Specifically, we compare our
photometry to canonical models at standard cluster abundances, canonical
models with enhanced helium (consistent with cluster self-enrichment
at early times), and flash-mixed models formed via a late helium-core
flash on the white dwarf cooling curve. We find that flash-mixed models
are required to explain the faint luminosity of the BH stars, although
neither the canonical models nor the flash-mixed models can explain the
range of color observed in such stars, especially those in the most
metal-rich clusters. Aside from the variation in the color range, no
clear trends emerge in the morphology of the BH population with respect
to metallicity. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at STScI, and associated with proposal 10815.

---------------------------------------------------------
Title: Commission 36: Theory of Stellar Atmospheres
Authors: Asplund, Martin; Puls, Joachim; Landstreet, John; Allende
Prieto, Carlos; Ayres, Thomas; Berdyugina, Svetlana; Gustafsson,
Bengt; Hubeny, Ivan; Ludwig, Hans Günter; Mashonkina, Lyudmila;
Randich, Sofia
2010IAUTB..27..197A Altcode:
The members of the Commission 36 Organizing Committee attending the IAU
General Assembly in Rio de Janeiro met for a business session on August
7. Both members from the previous (2006-2009) and the new (2009-2012)
Organizing Committee partook in the discussions. Past president John
Landstreet described the work he had done over the past three years
in terms of supporting proposed conferences on the topic. He has
also spent significant amount of time establishing an updated mailing
list of all &gt;350 members of the commission, which is unfortunately
not provided automatically by the IAU. Such a list is critical for a
rapid dissemination of information to the commission members and for
a correct and smooth running of elections of IAU officials. Everyone
present thanked John effusively for all of his hard work over the past
three years to stimulate a high level of activity within the discipline.

---------------------------------------------------------
Title: The Determination of the Abundances of the Fe Group Elements
in Early B Stars from High Resolution FUV Spectra
Authors: Peters, Geraldine J.; Adelman, Saul J.; Hubeny, Ivan;
Lanz, Thierry
2010IAUS..265..213P Altcode:
We present selected results from an investigation that is currently
underway to determine the abundances of the Fe group elements in early
B stars and assess the extent to which contemporary NLTE and LTE models
represent their atmospheres. High resolution UV and optical spectra of
B stars that display ultrasharp lines are compared with computations
from TLUSTY/SYNSPEC and SYNTHE. Some results from our analysis of the
abundance standard ι Her (B3V) are presented here.

---------------------------------------------------------
Title: The Blue Hook Populations of Massive Globular Clusters
Authors: Brown, Thomas M.; Sweigart, A. V.; Lanz, T.; Smith, E.;
Landsman, W. B.; Hubeny, I.
2010AAS...21542516B Altcode: 2010BAAS...42..338B
We present new HST ultraviolet color-magnitude diagrams of 5 massive
Galactic globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388,
and NGC 6441. &amp;nbspThese observations were obtained in order
to investigate the "blue hook" phenomenon previously observed in
HST ultraviolet images of the globular clusters omega Cen and NGC
2808. &amp;nbspBlue hook stars are a class of hot (approximately
35,000 K) subluminous horizontal branch stars that occupy a
region of the HR diagram that is unexplained by canonical stellar
evolution theory. &amp;nbspBy coupling new stellar evolution models
to appropriate non-LTE synthetic spectra, we investigate various
theoretical explanations for these stars. &amp;nbspSpecifically, we
compare the color-magnitude diagrams to stellar models that assume
normal evolution at cluster abundances, models with enhanced helium
resulting from cluster self-enrichment, and flash-mixed models formed
via late helium core flash on the white dwarf cooling curve.

---------------------------------------------------------
Title: Models of Neptune-Mass Exoplanets: Emergent Fluxes and Albedos
Authors: Spiegel, David S.; Burrows, Adam; Ibgui, Laurent; Hubeny,
Ivan; Milsom, John A.
2010ApJ...709..149S Altcode: 2009arXiv0909.2043S
There are now many known exoplanets with Msin i within a factor
of 2 of Neptune's, including the transiting planets GJ 436b and
HAT-P-11b. Planets in this mass range are different from their more
massive cousins in several ways that are relevant to their radiative
properties and thermal structures. By analogy with Neptune and Uranus,
they are likely to have metal abundances that are an order of magnitude
or more greater than those of larger, more massive planets. This
increases their opacity, decreases Rayleigh scattering, and changes
their equation of state. Furthermore, their smaller radii mean that
fluxes from these planets are roughly an order of magnitude lower than
those of otherwise identical gas giant planets. Here, we compute a range
of plausible radiative equilibrium models of GJ 436b and HAT-P-11b. In
addition, we explore the dependence of generic Neptune-mass planets on
a range of physical properties, including their distance from their
host stars, their metallicity, the spectral type of their stars, the
redistribution of heat in their atmospheres, and the possible presence
of additional optical opacity in their upper atmospheres.

---------------------------------------------------------
Title: Quantitative Analysis of the Spectra of Early B Stars with
Ultrasharp Lines
Authors: Peters, Geraldine J.; Adelman, S.; Hubeny, I.; Lanz, T.
2009AAS...21460512P Altcode:
The extent to which contemporary NLTE and LTE models represent
the atmospheres of the early B stars with the sharpest lines is
discussed. Due to the low rotational broadening in these stars, line
blending presents minimal problems and the intrinsic line profiles are
revealed. Spectral data from the HST and FUSE spacecraft and the KPNO
Coude Feed Telescope are compared with computations from the Hubeny/Lanz
codes TLUSTY/SYNSPEC and the Kurucz code SYNTHE. The B stars include
ι Her (B3V), HR 1886 (B1V), HR 1887 (B0.5V), and AV 304 (B0.5V) in the
SMC, all of which are considered to be abundance standards. Contemporary
NLTE model atmospheres represent the photospheres of early B stars
remarkably well. Some lines are fit much better in NLTE even for the B3
stars, but widespread discrepancies seem to emerge for stars hotter than
B0.5, which also show dynamic atmospheres. Future work need to be done
on the following: 1) The determination and meaning of microturbulence
and its effect on the atmosphere, 2) Further attention to the strong
Si II lines, especially Multiplet 4, 3) Treatment of Ti, V, Cr, Mn,
Co, and Ni in NLTE, 4) Wind models for B0-B0.5 stars. The authors
appreciate support from NASA grants NAG5-11802, NAG5-12239, NAG5-13212,
and STScI grants GO-09848 &amp; GO-06709.

---------------------------------------------------------
Title: V3885 Sagittarius: A Comparison with a Range of Standard
Model Accretion Disks
Authors: Linnell, Albert P.; Godon, Patrick; Hubeny, Ivan; Sion,
Edward M.; Szkody, Paula; Barrett, Paul E.
2009ApJ...703.1839L Altcode: 2009arXiv0908.1525L
A \widetilde{χ}^2 analysis of standard model accretion disk synthetic
spectrum fits to combined Far Ultraviolet Spectroscopic Explorer and
Space Telescope Imaging Spectrograph spectra of V3885 Sagittarius, on
an absolute flux basis, selects a model that accurately represents the
observed spectral energy distribution. Calculation of the synthetic
spectrum requires the following system parameters. The cataclysmic
variable secondary star period-mass relation calibrated by Knigge in
2006 and 2007 sets the secondary component mass. A mean white dwarf (WD)
mass from the same study, which is consistent with an observationally
determined mass ratio, sets the adopted WD mass of 0.7 M sun,
and the WD radius follows from standard theoretical models. The adopted
inclination, i = 65°, is a literature consensus, and is subsequently
supported by \widetilde{χ}^2 analysis. The mass transfer rate is the
remaining parameter to set the accretion disk T eff profile,
and the Hipparcos parallax constrains that parameter to \dot{M}=(5.0±
2.0)× 10^{-9} M_{⊙} yr^{-1} by a comparison with observed
spectra. The fit to the observed spectra adopts the contribution of
a 57, 000 ± 5000 K WD. The model thus provides realistic constraints
on \dot{M} and T eff for a large \dot{M} system above the
period gap. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc. under NASA contract NAS5-26555, and the NASA-CNES-CSA
Far Ultraviolet Explorer, which is operated for NASA by the Johns
Hopkins University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: From Complete Linearization to ALI and Beyond
Authors: Hubeny, Ivan
2009AIPC.1171....3H Altcode:
Starting with the ground-breaking work of Dimitri Mihalas and Larry
Auer in the late 1960's and early 1970's, the progress in our ability
to model and understand stellar atmospheres has been enormous. Here
I briefly describe some efficient algorithms that were developed and
applied to numerical simulations. An emphasis is given to efficient
variants of the original Complete Linearization scheme, and to the
methods that use the Accelerated Lambda Iteration and related methods.

---------------------------------------------------------
Title: RECENT DIRECTIONS IN ASTROPHYSICAL QUANTITATIVE SPECTROSCOPY
AND RADIATION HYDRODYNAMICS: Proceedings of the International
Conference in Honor of Dimitri Mihalas for His Lifetime Scientific
Contributions on the Occasion of His 70th Birthday
Authors: Hubeny, Ivan; Stone, James M.; MacGregor, Keith; Werner, Klaus
2009AIPC.1171.....H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Quantitative Analysis of the Spectra of Early B Stars with
Ultrasharp Lines
Authors: Peters, Geraldine J.; Adelman, Saul J.; Hubeny, Ivan;
Lanz, Thierry
2009AIPC.1171..345P Altcode:
We present selected results from an investigation that is currently
underway to determine the abundances of light and Fe group elements
in early B stars and assess the extent to which contemporary NLTE and
LTE models represent their atmospheres. Spectral data of B stars that
display ultrasharp lines, obtained with HST, FUSE, and the KPNO Coudé
Feed Telescope, are compared with computations from TLUSTY/SYNSPEC
and SYNTHE. The B stars include the abundance standards ι Her (B3V),
HR 1886 (B1V), and HR 1887 (B0.5V).

---------------------------------------------------------
Title: Beyond the classical paradigm of stellar winds: Investigating
clumping, rotation and the weak wind problem in SMC O stars
Authors: Hubeny, Ivan
2009hst..prop11625H Altcode:
SMC O stars provide an unrivaled opportunity to probe star formation,
evolution, and the feedback of massive stars in an environment similar
to the epoch of the peak in star formation history. Two recent
breakthroughs in the study of hot, massive stars have important
consequences for understanding the chemical enrichment and buildup
of stellar mass in the Universe. The first is the realization that
rotation plays a major role in influencing the evolution of massive
stars and their feedback on the surrounding environment. The second
is a drastic downward revision of the mass loss rates of massive stars
coming from an improved description of their winds. STIS spectroscopy
of SMC O stars combined with state-of-the-art NLTE analyses has
shed new light on these two topics. A majority of SMC O stars reveal
CNO-cycle processed material brought at their surface by rotational
mixing. Secondly, the FUV wind lines of early O stars provide strong
indications of the clumped nature of their wind. Moreover, we first
drew attention to some late-O dwarfs showing extremely weak wind
signatures. Consequently, we have derived mass loss rates from STIS
spectroscopy that are significantly lower than the current theoretical
predictions used in evolutionary models. Because of the limited size of
the current sample {and some clear bias toward stars with sharp-lined
spectra}, these results must however be viewed as tentative. Thanks to
the high efficiency of COS in the FUV range, we propose now to obtain
high-resolution FUV spectra with COS of a larger sample of SMC O stars
to study systematically rotation and wind properties of massive stars
at low metallicity. The analysis of the FUV wind lines will be based
on our 2D extension of CMFGEN to model axi-symmetric rotating winds.

---------------------------------------------------------
Title: A new detailed examination of white dwarfs in NGC 3532 and
NGC 2287
Authors: Dobbie, P. D.; Napiwotzki, R.; Burleigh, M. R.; Williams,
K. A.; Sharp, R.; Barstow, M. A.; Casewell, S. L.; Hubeny, I.
2009MNRAS.395.2248D Altcode: 2009arXiv0902.4259D; 2009MNRAS.tmp..480D
We present the results of a photometric and spectroscopic study of the
white dwarf candidate members of the intermediate age open clusters NGC
3532 and NGC 2287. Of the nine objects investigated, it is determined
that six are probable members of the clusters, four in NGC 3532 and
two in NGC 2287. For these six white dwarfs, we use our estimates of
their cooling times together with the cluster ages to constrain the
lifetimes and masses of their progenitor stars. We examine the location
of these objects in initial mass-final mass space and find that they
now provide no evidence for substantial scatter in initial mass-final
mass relation (IFMR) as suggested by previous investigations. Instead,
we demonstrate that, when combined with current data from other solar
metallicity open clusters and the Sirius binary system, they hint at
an IFMR that is steeper in the initial mass range 3 Msolar
&lt;~ Minit &lt;~ 4 Msolar than at progenitor
masses immediately lower and higher than this. This form is generally
consistent with the predictions of stellar evolutionary models and can
aid population synthesis models in reproducing the relatively sharp drop
observed at the high mass end of the main peak in the mass distribution
of white dwarfs. * Based on observations made with ESO telescopes
at the La Silla Paranal Observatory under programme IDs 079.D-0490(A)
and 080.D-0654(A). E-mail: pdd@aao.gov.au

---------------------------------------------------------
Title: New faint optical spectrophotometric standards: hot white
dwarfs from the Sloan Digital Sky Survey
Authors: Allende Prieto, Carlos; Hubeny, Ivan; Smith, J. Allyn
2009MNRAS.396..759A Altcode: 2009arXiv0903.2420A; 2009MNRAS.tmp..646A
The spectral energy distributions for pure-hydrogen (DA) hot white
dwarfs can be accurately predicted by model atmospheres. This makes
it possible to define spectrophotometric calibrators by scaling the
theoretical spectral shapes with broad-band photometric observations -
a strategy successfully exploited for the spectrographs onboard the
Hubble Space Telescope (HST) using three primary DA standards. Absolute
fluxes for non-DA secondary standards, introduced to increase the
density of calibrators in the sky, need to be referred to the primary
standards, but a far better solution would be to employ a network
of DA stars scattered throughout the sky. We search for blue objects
in the sixth data release of the Sloan Digital Sky Survey (SDSS) and
fit DA model fluxes to identify suitable candidates. Reddening needs
to be considered in the analysis of many of these stars. We propose
a list of nine pure-hydrogen white dwarfs with absolute fluxes with
estimated uncertainties below 3 per cent, including four objects with
estimated errors &lt;2 per cent, as candidates for spectrophotometric
standards in the range 14 &lt; g &lt; 18, and provide model-based
fluxes scaled to match the SDSS broad-band fluxes for each. We apply
the same method to the three HST DA standards, linking the zero point
of their absolute fluxes to ugr magnitudes transformed from photometry
obtained with the US Naval Observatory 1-m telescope. For these stars,
we estimate uncertainties of &lt;1 per cent in the optical, finding
good consistency with the fluxes adopted for HST calibration.

---------------------------------------------------------
Title: Effective area calibration of the reflection grating
spectrometers of XMM-Newton. II. X-ray spectroscopy of DA white dwarfs
Authors: Kaastra, J. S.; Lanz, T.; Hubeny, I.; Paerels, F. B. S.
2009A&A...497..311K Altcode: 2009arXiv0902.2282K
Context: White dwarf spectra have been widely used as a calibration
source for X-ray and EUV instruments. The in-flight effective area
calibration of the reflection grating spectrometers (RGS) of XMM-Newton
depend upon the availability of reliable calibration sources. Aims: We investigate how well these white dwarf spectra can be used
as standard candles at the lowest X-ray energies in order to gauge the
absolute effective area scale of X-ray instruments. Methods:
We calculate a grid of model atmospheres for Sirius B and HZ 43A,
and adjust the parameters using several constraints until the ratio of
the spectra of both stars agrees with the ratio as observed by the low
energy transmission grating spectrometer (LETGS) of Chandra. This ratio
is independent of any errors in the effective area of the LETGS. Results: We find that we can constrain the absolute X-ray spectrum
of both stars with better than 5% accuracy. The best-fit model for
both stars is close to a pure hydrogen atmosphere, and we put tight
limits to the amount of helium or the thickness of a hydrogen layer
in both stars. Our upper limit to the helium abundance in Sirius B
is 4 times below the previous detection based on EUVE data. We also
find that our results are sensitive to the adopted cut-off in the Lyman
pseudo-continuum opacity in Sirius B. We get best agreement with a long
wavelength cut-off. Conclusions: White dwarf model atmospheres
can be used to derive the effective area of X-ray spectrometers in the
lowest energy band. An accuracy of 3-4% in the absolute effective area
can be achieved.

---------------------------------------------------------
Title: Recent Progress in Modeling Stellar Atmospheres
Authors: Hubeny, I.; Lanz, T.
2009ASSP....7..211H Altcode: 2009nqsa.conf..211H
We briefly review recent progress in modeling stellar atmospheres. We
first summarize various levels of approximations being used in
constructing the models, and present a brief overview of existing
model atmosphere codes. We then concentrate on a description of our
universal computer program Tlusty, and our recent grids of NLTE metal
line blanketed model photospheres of O-stars - OSTAR2002 - and early
B-stars - BSTAR2006.

---------------------------------------------------------
Title: Spectrum and atmosphere models of irradiated transiting
giant planets
Authors: Hubeny, Ivan; Burrows, Adam
2009IAUS..253..239H Altcode: 2008arXiv0807.3588H
We show that a consistent fit to observed secondary eclipse data for
several strongly irradiated transiting planets demands a temperature
inversion (stratosphere) at altitude. Such a thermal inversion
significantly influences the planet/star contrast ratios at the
secondary eclipse, their wavelength dependences, and, importantly,
the day-night flux contrast during a planetary orbit. The presence of
the thermal inversion/stratosphere seems to roughly correlate with the
stellar flux at the planet. Such temperature inversions might be caused
by an upper-atmosphere absorber whose exact nature is still uncertain.

---------------------------------------------------------
Title: Commission 29: Stellar Spectra
Authors: Parthasarathy, Mudumba; Piskunov, Nikolai E.; Sneden,
Christopher; Carpenter, Kenneth G.; Castelli, Fiorella; Cunha, Katia;
Eenens, Phillippe R. J.; Hubeny, Ivan; Rossi, Silvia; Takada-Hidai,
Masahide; Wahlgren, Glenn M.; Weiss, Werner W.
2009IAUTA..27..209P Altcode:
The members of IAU Commission 29 Stellar Spectra are actively
engaged in the quantitative analysis of spectra of various types
of stars. With large and medium size telescopes equipped with high
resolution spectrographs LTE and Non-LTE analysis of spectra of all
types stars are being carried out. Spectra of stars in our Galaxy,
in globular and open clusters, stars in LMC and SMC and in nearby
galaxies are being studied. Accurate chemical composition analysis
of various types of stars has been carried out during the past three
years. Now the analysis of stellar spectra covers the wavelength range
from X-ray region to IR and sub-millimeter range. Recently stellar
spectra are being analysed using time-dependent, 3D, hydrodynamical
model atmospheres to derive accurate stellar abundances.

---------------------------------------------------------
Title: NLTE model calculations for the solar atmosphere with an
iterative treatment of opacity distribution functions
Authors: Haberreiter, M.; Schmutz, W.; Hubeny, I.
2008A&A...492..833H Altcode: 2008arXiv0810.3471H
Context: Modeling the variability of the solar spectral irradiance is
a key factor in understanding the Sun's influence on the climate of
the Earth. Aims: As a first step toward calculating the solar
spectral irradiance variations, we reproduce the solar spectrum
for the quiet Sun over a broad wavelength range with an emphasis
on the UV. Methods: We introduce the radiative transfer code
COSI, which calculates solar synthetic spectra under conditions
of non-local thermodynamic equilibrium (NLTE). A self-consistent
simultaneous solution of the radiative transfer and the statistical
equation for the level populations guarantees that the correct physics
is considered for wavelength regions where the assumption of local
thermodynamic equilibrium (LTE) breaks down. The new concept of
iterated opacity distribution functions (NLTE-ODFs) is presented,
through which all line opacities are included in the NLTE radiative
transfer calculation. Results: We show that it is essential to
include the line opacities in the radiative transfer to reproduce the
solar spectrum in the UV. Conclusions: Through the implemented
scheme of NLTE-ODFs, the COSI code is successful in reproducing the
spectral energy distribution of the quiet Sun. Table 2 is only
available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: Modeling UX Ursae Majoris: An Abundance of Challenges
Authors: Linnell, Albert P.; Godon, Patrick; Hubeny, Ivan; Sion,
Edward M.; Szkody, Paula
2008ApJ...688..568L Altcode: 2008arXiv0807.3920L
We present a system model for optical and far-UV spectra of the
nova-like variable UX UMa involving a white dwarf, secondary star, gas
stream, hot spot, and accretion disk using our code BINSYN and based
on an initially adopted system distance. Calculated SED intensity
data successfully fit successive tomographically extracted annuli
longward of the Balmer limit but require a postulated "iron curtain"
shortward of the Balmer limit that is applied to the annulus section
closest to the secondary star, while postulated recombination emission
fills in the model SED shortward of the Balmer limit and is applied
to the annulus section more remote from the secondary star. The same
model fits UBV 1954 light curves by Walker and Herbig. Fits to HST FOS
spectra are approximate but require assumed time-variable changes in the
SED. Comparable effects, possibly involving variable absorption, afflict
FUSE spectra. Fits to IUE spectra by the model show time-dependent
residuals that indicate changes in the accretion disk temperature
profile, possibly indicative of a slightly variable \dot{M} from the
secondary star. Using model-based component light contributions and the
improvement on the Bailey relation by Knigge we determine the system
distance and mass transfer rate. Based on observations made with
the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy, Inc., under NASA contract NAS5-26555, and
the NASA-CNES-CSA Far Ultraviolet Explorer, which is operated for NASA
by the Johns Hopkins University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: Optical Albedo Theory of Strongly Irradiated Giant Planets:
The Case of HD 209458b
Authors: Burrows, A.; Ibgui, L.; Hubeny, I.
2008ApJ...682.1277B Altcode: 2008arXiv0803.2523B
We calculate a new suite of albedo models for close-in extrasolar
giant planets and compare with the recent stringent upper limit
for HD 209458b of Rowe et al. using MOST. We find that all models
without scattering clouds are consistent with this optical limit. We
explore the dependence on wavelength and wave band, metallicity, the
degree of heat redistribution, and the possible presence of thermal
inversions and find a rich diversity of behaviors. Measurements of
transiting extrasolar giant planets (EGPs) at short wavelengths by
MOST, Kepler, and CoRoT, as well as by proposed dedicated multiband
missions, can complement measurements in the near- and mid-IR using
Spitzer and JWST. Collectively, such measurements can help determine
metallicity, compositions, atmospheric temperatures, and the cause
of thermal inversions (when they arise) for EGPs with a broad range
of radii, masses, degrees of stellar insolation, and ages. With this
paper we reappraise and highlight the diagnostic potential of albedo
measurements of hot EGPs shortward of ~1.3 μm.

---------------------------------------------------------
Title: Hubble Space Telescope STIS Spectroscopy of Long-Period Dwarf
Novae in Quiescence
Authors: Sion, Edward M.; Gänsicke, Boris T.; Long, Knox S.; Szkody,
Paula; Knigge, Christian; Hubeny, Ivan; deMartino, Domitilla; Godon,
Patrick
2008ApJ...681..543S Altcode: 2008arXiv0801.4703S
We present the results of a synthetic spectral analysis of HST STIS
spectra of five long-period dwarf novae obtained during their quiescence
to determine the properties of their white dwarfs, which are little
known for systems above the CV period gap. The five systems, TU Men,
BD Pav, SS Aur, TT Crt, and V442 Cen, were observed as part of an HST
Snapshot project. The spectra are described and fitted with combinations
of white dwarf photospheres and accretion disks. We provide evidence
that the white dwarfs in all five systems are at least partially
exposed. We discuss the evolutionary implications of our model-fitting
results and compare these dwarf novae to previously analyzed FUV
spectra of other dwarf novae above the period gap. The dispersion
in CV WD temperatures above the period gap is substantially greater
than one finds below the period gap, where there is a surprisingly
narrow dispersion in temperatures around 15,000 K. There appears to
be a larger spread of surface temperatures in dwarf novae above the
period than is seen below the gap.

---------------------------------------------------------
Title: A Far Ultraviolet Archival Study of Cataclysmic
Variables. I. FUSE and HST STIS Spectra of the Exposed White Dwarf
in Dwarf Nova Systems
Authors: Godon, Patrick; Sion, Edward M.; Barrett, Paul E.; Hubeny,
Ivan; Linnell, Albert P.; Szkody, Paula
2008ApJ...679.1447G Altcode: 2008arXiv0802.2022G
We present a synthetic spectral analysis of Far Ultraviolet
Spectroscopic Explorer (FUSE) and Hubble Space Telescope Space
Telescope Imaging Spectrograph (HST STIS) spectra of five dwarf
novae above and below the period gap during quiescence. We use our
synthetic spectral code, including options for the treatment of the
hydrogen quasi-molecular satellite lines (for low-temperature stellar
atmospheres), non-LTE (NLTE) approximation (for high-temperature stellar
atmospheres), and for one system (RU Peg) we model the interstellar
medium (ISM) molecular and atomic hydrogen lines. In all the systems
presented here the FUV flux continuum is due to the white dwarf
(WD). These spectra also exhibit some broad emission lines. In this
work we confirm some of the previous FUV analysis results, but we also
present new results. For four systems we combine the FUSE and STIS
spectra to cover a larger wavelength range and to improve the spectral
fit. This work is part of our broader HST archival research program,
in which we aim to provide accurate system parameters for cataclysmic
variables above and below the period gap by combining FUSE and HST
FUV spectra. Based on observations made with the NASA-CNES-CSA
Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by
the Johns Hopkins University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: Argon Abundances in the Solar Neighborhood: Non-LTE Analysis
of Orion Association B-Type Stars
Authors: Lanz, Thierry; Cunha, Katia; Holtzman, Jon; Hubeny, Ivan
2008ApJ...678.1342L Altcode: 2007arXiv0709.2147L
Argon abundances have been derived for a sample of B main-sequence
stars in the Orion association. The abundance calculations are based on
non-LTE (NLTE) metal line-blanketed model atmospheres calculated with
the NLTE code TLUSTY and an updated and complete argon model atom. We
derive an average argon abundance for this young population of A(Ar)
= 6.66 +/- 0.06. While our result is in excellent agreement with a
recent analysis of the Orion Nebula, it is significantly higher than
the currently recommended solar value, which is based on abundance
measurements in the solar corona. Moreover, the derived argon abundances
in the Orion B stars agree very well with a measurement from a solar
impulsive flare during which unmodified solar photospheric material
was brought to flare conditions. We therefore argue that the argon
abundances obtained independently for both the Orion B stars and the
Orion Nebula are representative of the disk abundance value in the
solar neighborhood. The lower coronal abundance may reflect a depletion
related to the first ionization potential (FIP) effect. We propose a new
reference value for the abundance of argon in the solar neighborhood,
A(Ar) = 6.63 +/- 0.10, corresponding to Ar/O = 0.009 +/- 0.002. Based on observations obtained with the Apache Point Observatory
3.5 m telescope, which is owned and operated by the Astrophysical
Research Consortium.

---------------------------------------------------------
Title: Theoretical Spectra and Light Curves of Close-in Extrasolar
Giant Planets and Comparison with Data
Authors: Burrows, A.; Budaj, J.; Hubeny, I.
2008ApJ...678.1436B Altcode: 2007arXiv0709.4080B
We present theoretical atmosphere, spectral, and light-curve models for
extrasolar giant planets (EGPs) undergoing strong irradiation for which
Spitzer planet/star contrast ratios or light curves have been published
(circa 2007 June). These include HD 209458b, HD 189733b, TrES-1, HD
149026b, HD 179949b, and υ And b. By comparing models with data, we
find that a number of EGP atmospheres experience thermal inversions
and have stratospheres. This is particularly true for HD 209458b,
HD 149026b, and υ And b. This finding translates into qualitative
changes in the planet/star contrast ratios at secondary eclipse and in
close-in EGP orbital light curves. Moreover, the presence of atmospheric
water in abundance is fully consistent with all the Spitzer data for
the measured planets. For planets with stratospheres, water absorption
features invert into emission features and mid-infrared fluxes can be
enhanced by a factor of 2. In addition, the character of near-infrared
planetary spectra can be radically altered. We derive a correlation
between the importance of such stratospheres and the stellar flux on
the planet, suggesting that close-in EGPs bifurcate into two groups:
those with and without stratospheres. From the finding that TrES-1
shows no signs of a stratosphere, while HD 209458b does, we estimate
the magnitude of this stellar flux breakpoint. We find that the heat
redistribution parameter, Pn, for the family of close-in
EGPs assumes values from ~0.1 to ~0.4. This paper provides a broad
theoretical context for the future direct characterization of EGPs in
tight orbits around their illuminating stars.

---------------------------------------------------------
Title: Synthetic Spectrum Constraints on a Model of the Cataclysmic
Variable QU Carinae
Authors: Linnell, Albert P.; Godon, Patrick; Hubeny, Ivan; Sion,
Edward M.; Szkody, Paula; Barrett, Paul E.
2008ApJ...676.1226L Altcode: 2008arXiv0801.0704L
Neither standard model SEDs nor truncated standard model SEDs fit
observed spectra of QU Carinae with acceptable accuracy over the
range 900-3000 Å. Nonstandard model SEDs fit the observation set
accurately. The nonstandard accretion disk models have a hot region
extending from the white dwarf to R = 1.36RWD, a narrow
intermediate-temperature annulus, and an isothermal remainder to
the tidal cutoff boundary. The models include a range of \dot{M}
values between 1.0 × 10-7 and 1.0 × 10-6
M⊙ yr-1 and limiting values of MWD
between 0.6 and 1.2 M⊙. A solution with MWD
= 1.2 M⊙ is consistent with an empirical mass-period
relation. The set of models agree on a limited range of possible
isothermal region Teff values between 14,000 and 18,000
K. The model-to-model residuals are so similar that it is not
possible to choose a best model. The Hipparcos distance, 610 pc, is
representative of the model results. The orbital inclination is between
40° and 60°. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5-26555, and the NASA-CNES-CSA
Far Ultraviolet Explorer, which is operated for NASA by the Johns
Hopkins University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: White Dwarf Spectra and Calibration of X-ray Grating
Spectrometers
Authors: Kaastra, Jelle S.; Lanz, T.; Hubeny, I.; Paerels, F.
2008HEAD...10.0406K Altcode:
White dwarf spectra have been widely used as calibration sources for
X-ray and EUV instruments. The in-flight effective area calibration
of the RGS of XMM-Newton and of the LETGS of Chandra depend upon the
availability of reliable calibration sources. We calculate a grid of
model atmospheres for Sirius B and HZ 43A, and adjust the parameters
using several constraints until the ratio of the spectra of both stars
agrees with the ratio observed with the LETGS of Chandra. This ratio
is independent of any errors in the effective area of the LETGS. We
determine how accurately the effective area of the LETGS is determined
using our method, and find interesting constraints on the parameters
for both stars. We discuss the role of the Lyman pseudo-continuum
in the calculation of the spectrum of Sirius B. The treatment of
that pseudo-continuum appears to play a fundamental role in the
ultimate accuracy that can be reached. With the proper treatment of
the pseudo-continuum, the soft X-ray flux of both stars and thereby
the absolute effective area of the LETGS can be determined with an
uncertainty of less than 5 %.

---------------------------------------------------------
Title: A Systematic Study of Departures from Chemical Equilibrium
in the Atmospheres of Substellar Mass Objects
Authors: Hubeny, Ivan; Burrows, Adam
2007ApJ...669.1248H Altcode: 2007arXiv0705.3922H
We present a systematic study of the spectral consequences of departures
from chemical equilibrium in the atmospheres of L and T dwarfs,
and for even cooler dwarfs. The temperature/pressure profiles of the
nonequilibrium models are fully consistent with the nonequilibrium
chemistry. Our grid of nonequilibrium models includes spectra for
effective temperatures from 200 to 1800 K, three surface gravities,
four possible values of the coefficient of eddy diffusion in the
radiative zone, and three different CO/CH 4 chemical reaction
prescriptions. We also provide clear and cloudy model variants. We find,
in keeping with previous studies, that there are essentially only
two spectral regions where the effects of departures from chemical
equilibrium can influence the predicted spectrum. These are in the
M (~4-5 μm) and N (8-14 μm) bands due to CO and NH 3,
respectively. The overabundance of CO translates into flux suppressions
of at most ~40% between effective temperatures of 600 and 1800 K. The
effect is largest around Teff~1100 K. The underabundance
of ammonia translates into flux enhancements of no more than ~20%
for the Teff range from 300 to 1800 K, with the largest
effects at the lowest values of Teff. The magnitude of the
departure from chemical equilibrium increases with decreasing gravity,
with increasing eddy diffusion coefficient, and with decreasing speed
of the CO/CH4 reaction. Although these effects are modest,
they lead to better fits with the measured T dwarf spectra. Furthermore,
the suppression in the M band due to nonequilibrium enhancements in the
CO abundance disappears below ~500 K, and is only partial above ~500 K,
preserving the M-band flux as a useful diagnostic of cool atmospheres
and maintaining its importance for searches for the cooler brown dwarfs
beyond the T dwarfs.

---------------------------------------------------------
Title: A Statistical Study of Accretion Disk Model Spectra for
Cataclysmic Variables
Authors: Puebla, Raúl E.; Diaz, Marcos P.; Hubeny, Ivan
2007AJ....134.1923P Altcode:
We have performed a statistical test of the currently used accretion
disk models for cataclysmic variables (CVs) using a set of 33 CVs
with steady disks (10 old novae and 23 nova-like systems). The mass
transfer rate () for each system was also calculated. Ultraviolet (UV)
data were fitted by model spectra using a multiparametric optimization
method, aiming to constrain the values. It was verified that these
accretion disk models fail to fit both color and flux simultaneously,
as previously noted when composite stellar atmosphere models were
fitted to the UV spectra of CVs by Wade. By applying such models to a
sample of novae and nova-like CVs, we confirm that the limb-darkening
effect must be taken into account when estimating mass transfer rates,
especially for high-inclination systems. Important fitting degeneracies
of the basic disk parameters are analyzed. Our simulations suggest that
to reproduce the observations a revision of the temperature profile,
at least in the innermost parts of the disk, seems to be required,
and possibly the vertical distribution of the viscosity should be
revised. In addition, an optically thin layer or an extended disk
component should be considered. This component may be physically
represented by a disk wind and/or a chromosphere. A physical description
of the emission-line profiles may help to break the degeneracies that
appear when only the continuum is analyzed. The average value of found
for nova-like systems is ~9.3 × 10-9 Modot
yr-1, while ~1.3 × 10-8 Modot
yr-1 is found for old classical novae. No clear evidence
is found for either the presence or absence of a correlation between
and the orbital period. Such correlation analysis was performed for
high accretion rate systems (15 nova-like systems and 10 old novae),
but we were not able to find a well-defined correlation as found by
Patterson. By measuring the equivalent width of the emission lines
(C IV λ1550 and He II λ1640) we found a lack of systems with low and
strong UV emission lines. A correlation between the equivalent width
of such lines and the orbital inclination (i) was also confirmed.

---------------------------------------------------------
Title: Theoretical Spectral Models of the Planet HD 209458b with a
Thermal Inversion and Water Emission Bands
Authors: Burrows, A.; Hubeny, I.; Budaj, J.; Knutson, H. A.;
Charbonneau, D.
2007ApJ...668L.171B Altcode: 2007arXiv0709.3980B
We find that a theoretical fit to all the HD 209458b data at secondary
eclipse requires that the day-side atmosphere of HD 209458b have
a thermal inversion and a stratosphere. This inversion is caused
by the capture of optical stellar flux by an absorber of uncertain
origin that resides at altitude. One consequence of stratospheric
heating and temperature inversion is the flipping of water absorption
features into emission features from the near- to the mid-infrared,
and we see evidence of such a water emission feature in the recent HD
209458b IRAC data of Knutson et al. In addition, an upper-atmosphere
optical absorber may help explain both the weaker-than-expected Na
D feature seen in transit and the fact that the transit radius at 24
μm is smaller than the corresponding radius in the optical. Moreover,
it may be a factor in why HD 209458b's optical transit radius is as
large as it is. We speculate on the nature of this absorber and the
planets whose atmospheres may, or may not, be affected by its presence.

---------------------------------------------------------
Title: Comparative study of dust cloud modelling for substellar
atmospheres
Authors: Helling, Ch.; Ackerman, A. S.; Allard, F.; Dehen, M.;
Hauschildt, P.; Hubeny, I.; Homeier, D.; Lodders, K.; Marley, M.;
Tsuji, T.; Woitke, P.
2007AN....328..655H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Professor Mirek J. Plavec
Authors: Harmanec, Petr; Grygar, Jiří; Batten, Alan H.; Peters,
Geraldine J.; Linnell, Albert P.; Hubeny, Ivan; Guinan, Edward F.
2007IAUS..240...17H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Synthetic Spectrum and Light-Curve Analysis of the
Cataclysmic Variable IX Velorum
Authors: Linnell, Albert P.; Godon, Patrick; Hubeny, Ivan; Sion,
Edward M.; Szkody, Paula
2007ApJ...662.1204L Altcode: 2006astro.ph.12657L
Spectrum synthesis analysis of FUSE and STIS spectra for the cataclysmic
variable IX Vel shows that it is possible to achieve a close synthetic
spectrum fit with a mass transfer rate of M˙=5×10-9
Msolar yr-1 and a modified standard model
temperature profile. The innermost four annuli of the accretion disk
model, extending to r/rWD~4, are isothermal; beyond that
limit the temperatures follow the standard model. A light synthesis
fit to K-band photometry requires shallow eclipses of the accretion
disk rim and secondary star limb. The geometry constrains the orbital
inclination to i=57deg+/-2deg. The synthetic
light curve requires a vertically extended accretion disk rim, beyond
that predicted by gravitational equilibrium, to shadow the secondary
star. The enhanced vertical extension is consistent with recent MHD
predictions for CV accretion disks. Matching differing observed heights
of alternate K-band light-curve maxima requires a warm rim region
downstream of the intersection point of the mass transfer stream
with the accretion disk rim. The temperature of the warm region is
inconsistent with expectation for a bright spot associated with a
rim shock. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5-26555, and the NASA-CNES-CSA
Far Ultraviolet Spectroscopic Explorer, which is operated for NASA by
Johns Hopkins University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: VizieR Online Data Catalog: VLT-FLAMES survey of massive stars
(Hunter+, 2007)
Authors: Hunter, I.; Dufton, P. L.; Smartt, S. J.; Ryans, R. S. I.;
Evans, C. J.; Lennon, D. J.; Trundle, C.; Hubeny, I.; Lanz, T.
2007yCat..34660277H Altcode:
Files table3.dat, table4.dat and table5.dat contain the equivalent
widths and abundances estimates of the observed metal lines for
B-type stars in NGC 6611, N11 and NGC 346 respectively. The equivalent
widths of the spectral lines have been measured using the line fitting
program ELF in the spectral analysis package DIPSO (Howarth et al.,
1994, Starlink User Note, No. 50.15). The nitrogen equivalent widths
and abundances are upper limits for objects N11-037, N11-047, N11-101,
NGC 346-029, NGC 346-039, NGC 346-040, NGC 346-043, NGC 346-044 and
NGC 346-075. table6.dat contains details of the adopted atomic
data for the absorption lines considered in this analysis. Model atoms
are discussed in Lanz &amp; Hubeny (2003ApJS..146..417L) and Allende
Prieto et al. (2003ApJS..147..363A), see Dufton et al. (2005, Cat. )
for further details. Note, the loggf values of the Si II lines at 4128
and 4131 angstroms have been updated to include improved atomic data
(A. Korn, private communication). (5 data files).

---------------------------------------------------------
Title: Possible Solutions to the Radius Anomalies of Transiting
Giant Planets
Authors: Burrows, A.; Hubeny, I.; Budaj, J.; Hubbard, W. B.
2007ApJ...661..502B Altcode: 2006astro.ph.12703B
We calculate the theoretical evolution of the radii of all 14 of
the known transiting extrasolar giant planets (EGPs) for a variety of
assumptions concerning atmospheric opacity, dense inner core masses, and
possible internal power sources. We incorporate the effects of stellar
irradiation and customize such effects for each EGP and star. Looking
collectively at the family as a whole, we find that there are in
fact two radius anomalies to be explained. Not only are the radii
of a subset of the known transiting EGPs larger than expected from
previous theory, but many of the other objects are smaller than the
default theory would allow. We suggest that the larger EGPs can be
explained by invoking enhanced atmospheric opacities that naturally
retain internal heat. This explanation might obviate the necessity for
an extra internal power source. We explain the smaller radii by the
presence in perhaps all the known transiting EGPs of dense cores, such
as have been inferred for Saturn and Jupiter. Importantly, we derive a
rough correlation between the masses of our “best-fit” cores and the
stellar metallicity that seems to buttress the core-accretion model
of their formation. Although many caveats and uncertainties remain,
the resulting comprehensive theory that incorporates enhanced-opacity
atmospheres and dense cores is in reasonable accord with all the
current structural data for the known transiting giant planets.

---------------------------------------------------------
Title: Effects of mass loss for highly-irradiated giant planets
Authors: Hubbard, W. B.; Hattori, M. F.; Burrows, A.; Hubeny, I.;
Sudarsky, D.
2007Icar..187..358H Altcode:
We present calculations for the evolution and surviving mass of
highly-irradiated extrasolar giant planets (EGPs) at orbital semimajor
axes ranging from 0.023 to 0.057 AU using a generalized scaled theory
for mass loss, together with new surface-condition grids for hot EGPs
and a consistent treatment of tidal truncation. Theoretical estimates
for the rate of energy-limited hydrogen escape from giant-planet
atmospheres differ by two orders of magnitude, when one holds planetary
mass, composition, and irradiation constant. Baraffe et al. [Baraffe,
I., Selsis, F., Chabrier, G., Barman, T.S., Allard, F., Hauschildt,
P.H., Lammer, H., 2004. Astron. Astrophys. 419, L13-L16] predict
the highest rate, based on the theory of Lammer et al. [Lammer,
H., Selsis, F., Ribas, I., Guinan, E.F., Bauer, S.J., Weiss, W.W.,
2003. Astrophys. J. 598, L121-L124]. Scaling the theory of Watson et
al. [Watson, A.J., Donahue, T.M., Walker, J.C.G., 1981. Icarus 48,
150-166] to parameters for a highly-irradiated exoplanet, we find
an escape rate ∼10 lower than Baraffe's. With the scaled Watson
theory we find modest mass loss, occurring early in the history of a
hot EGP. In this theory, mass loss including the effect of Roche-lobe
overflow becomes significant primarily for masses below a Saturn mass,
for semimajor axes ⩾0.023 AU. This contrasts with the Baraffe model,
where hot EGPs are claimed to be remnants of much more massive bodies,
originally several times Jupiter and still losing substantial mass
fractions at present.

---------------------------------------------------------
Title: The VLT-FLAMES survey of massive stars: surface chemical
compositions of B-type stars in the Magellanic Clouds
Authors: Hunter, I.; Dufton, P. L.; Smartt, S. J.; Ryans, R. S. I.;
Evans, C. J.; Lennon, D. J.; Trundle, C.; Hubeny, I.; Lanz, T.
2007A&A...466..277H Altcode: 2006astro.ph..9710H
We present an analysis of high-resolution FLAMES spectra of
approximately 50 early B-type stars in three young clusters at different
metallicities, NGC 6611 in the Galaxy, N 11 in the Large Magellanic
Cloud (LMC) and NGC 346 in the Small Magellanic Cloud (SMC). Using
the tlusty non-LTE model atmospheres code, atmospheric parameters and
photospheric abundances (C, N, O, Mg and Si) of each star have been
determined. These results represent a significant improvement on the
number of Magellanic Cloud B-type stars with detailed and homogeneous
estimates of their atmospheric parameters and chemical compositions. The
relationships between effective temperature and spectral type are
discussed for all three metallicity regimes, with the effective
temperature for a given spectral type increasing as one moves to a
lower metallicity regime. Additionally the difficulties in estimating
the microturbulent velocity and the anomalous values obtained,
particularly in the lowest metallicity regime, are discussed. Our
chemical composition estimates are compared with previous studies,
both stellar and interstellar with, in general, encouraging agreement
being found. Abundances in the Magellanic Clouds relative to the
Galaxy are discussed and we also present our best estimates of the
base-line chemical composition of the LMC and SMC as derived from B-type
stars. Additionally we discuss the use of nitrogen as a probe of the
evolutionary history of stars, investigating the roles of rotational
mixing, mass-loss, blue loops and binarity on the observed nitrogen
abundances and making comparisons with stellar evolutionary models
where possible. Based on observations at the European Southern
Observatory in programmes 171.0237 and 073.0234. Tables 3-6 and
Figs. 7-31 are only available in electronic form at http://www.aanda.org

---------------------------------------------------------
Title: A New Algorithm for Two-Dimensional Transport for Astrophysical
Simulations. I. General Formulation and Tests for the One-Dimensional
Spherical Case
Authors: Hubeny, Ivan; Burrows, Adam
2007ApJ...659.1458H Altcode: 2006astro.ph..9049H
We derive new equations using the mixed-frame approach for one- and
two-dimensional (axisymmetric) time-dependent radiation transport and
the associated couplings with matter. Our formulation is multigroup
and multiangle and includes anisotropic scattering, frequency
(energy)-dependent scattering and absorption, complete velocity
dependence to order v/c, rotation, and energy redistribution due to
inelastic scattering. Hence, the “2D” realization is actually “6
1/2”-dimensional. The effects of radiation viscosity are automatically
incorporated. Moreover, we develop accelerated lambda iteration,
Krylov subspace (GMRES), discontinuous finite element, and Feautrier
numerical methods for solving the equations and present the results
of one-dimensional numerical tests of the new formalism. The virtues
of the mixed-frame approach include simple velocity dependence with
no velocity derivatives, straight characteristics, simple physical
interpretation, and clear generalization to higher dimensions. Our
treatment can be used for both photon and neutrino transport, but we
focus on neutrino transport and applications to core-collapse supernova
theory in the discussions and examples.

---------------------------------------------------------
Title: A Grid of NLTE Line-blanketed Model Atmospheres of Early
B-Type Stars
Authors: Lanz, Thierry; Hubeny, Ivan
2007ApJS..169...83L Altcode: 2006astro.ph.11891L
We have constructed a comprehensive grid of 1540 metal line-blanketed,
NLTE, plane-parallel, hydrostatic model atmospheres for the basic
parameters appropriate to early B-type stars. The BSTAR2006
grid considers 16 values of effective temperatures, 15,000
K&lt;=Teff&lt;=30,000 K with 1000 K steps, 13 surface
gravities, 1.75&lt;=logg&lt;=4.75 with 0.25 dex steps, six chemical
compositions, and a microturbulent velocity of 2 km s-1. The
lower limit of logg for a given effective temperature is set by an
approximate location of the Eddington limit. The selected chemical
compositions range from twice to one-tenth of the solar metallicity
and metal-free. Additional model atmospheres for B supergiants
(logg&lt;=3.0) have been calculated with a higher microturbulent
velocity (10 km s-1) and a surface composition that is
enriched in helium and nitrogen and depleted in carbon. This new grid
complements our earlier OSTAR2002 grid of O-type stars (our Paper
I). The paper contains a description of the BSTAR2006 grid and some
illustrative examples and comparisons. NLTE ionization fractions,
bolometric corrections, radiative accelerations, and effective
gravities are obtained over the parameter range covered by the grid. By
extrapolating radiative accelerations, we have determined an improved
estimate of the Eddington limit in absence of rotation between 55,000
and 15,000 K. The complete BSTAR2006 grid is available at the TLUSTY
Web site.

---------------------------------------------------------
Title: Commission 29: Stellar Spectra
Authors: Sneden, Christopher; Parthasarathy, M.; Castelli, Fiorella;
Cunha, Katia; Eenens, Philippe; Friel, Eileen; Gratton, Raffaele;
Hubeny, Ivan; Landstreet, John D.; Mathys, Gautier; Piskunov, Nikolai;
Primas, Francesca; Takada-Hidai, Masahide; Weiss, Werner
2007IAUTA..26..203S Altcode:
Participants in Commission 29 study various aspects of stellar spectra
and the information that can be extracted from spectra. The list
of fields of interest of the Organizing Committee members suggest
some major current research topics in this area are stellar chemical
compositions and surface/envelope phenomena. Some of the topics of this
commission have overlap with other commissions, such as Commission
14 (Atomic and Molecular Data), 26 (Double and Multiple Stars),
27 (Variable Stars), 30 (Radial Velocities), 36 (Theory of Stellar
Atmospheres), and 37 (Star Clusters and Associations). Many Commission
29 members are also members of these other commissions.

---------------------------------------------------------
Title: A Mass Function Constraint on Extrasolar Giant Planet
Evaporation Rates
Authors: Hubbard, W. B.; Hattori, M. F.; Burrows, A.; Hubeny, I.
2007ApJ...658L..59H Altcode: 2007astro.ph..2276H
The observed mass function for all known extrasolar giant planets
(EGPs) varies approximately as M-1 for mass M between ~0.2
Jupiter masses (MJ) and ~5 MJ. In order to study
evaporation effects for highly irradiated EGPs in this mass range, we
have constructed an observational mass function for a subset of EGPs in
the same mass range but with orbital radii &lt;0.07 AU. Surprisingly,
the mass function for such highly irradiated EGPs agrees quantitatively
with the M-1 law, implying that the mass function for EGPs
is preserved despite migration to small orbital radii. Unless there
is a remarkable compensation of mass-dependent orbital migration
for mass-dependent evaporation, this result places a constraint on
orbital migration models and rules out the most extreme mass-loss
rates in the literature. A theory that predicts more moderate mass
loss gives a mass function that is closer to observed statistics but
still disagrees for M&lt;1 MJ.

---------------------------------------------------------
Title: An Illustration of Modeling Cataclysmic Variables: HST, FUSE,
and SDSS Spectra of SDSS J080908.39+381406.2
Authors: Linnell, Albert P.; Hoard, D. W.; Szkody, Paula; Long,
Knox S.; Hubeny, Ivan; Gänsicke, Boris; Sion, Edward M.
2007ApJ...654.1036L Altcode: 2006astro.ph.12692L
FUSE, HST, and SDSS spectra of the cataclysmic variable SDSS
J080908.39+381406.2 provide a spectral flux distribution from 900 to
9200 Å. This data set is used to illustrate procedures for calculating
and testing system models. The spectra are not contemporaneous; it
is necessary to assume that the combined spectra are representative
of the system. The illustrations are based on a system with a 1.0
Msolar white dwarf, a 0.30 Msolar, 3500 K,
Roche lobe-filling secondary star, and an accretion disk extending
to the tidal cutoff radius. Assuming a similar accretion state for
the nonsimultaneous spectra, the best standard model fit is with
a mass transfer rate of 3.0 × 10-9Msolar
yr-1. Extensive simulations demonstrate that the accretion
disk must be truncated at its inner edge if the temperature profile
follows the standard model, but truncated models face severe
objections, which we address. Following additional simulation
tests, we obtain a model accretion disk with a temperature profile
comparable to the profile for SW Sex as determined from tomographic
image reconstruction. This model fits the discovery SDSS spectrum
well but has a flux deficit in the UV and FUV. Emission from a white
dwarf is a plausible source of additional flux. Adding this source
to the disk synthetic spectrum produces FUV flux that can explain
the observed flux. An additional (archival) SDSS spectrum is fainter
by about 0.3 mag in the optical. Additional analysis showed that UV
residuals from a model fitting the archival optical wavelength spectrum
are unacceptably large. Contemporaneous spectra from all wavelength
regions would be necessary for a reliable system model. Our discussion
illustrates how this conclusion follows from the system models. Based on observations made with the NASA/ESA Hubble Space Telescope,
obtained at the Space Telescope Science Institute, which is operated by
the Association of Universities for Research in Astronomy, Inc. under
NASA contract NAS5-26555, the NASA-CNES-CSA Far Ultraviolet Explorer,
which is operated for NASA by the Johns Hopkins University under
NASA contract NAS5-32985, and with the Apache Point Observatory 3.5 m
telescope, which is operated by the Astrophysical Research Corporation.

---------------------------------------------------------
Title: Theory for the Secondary Eclipse Fluxes, Spectra, Atmospheres,
and Light Curves of Transiting Extrasolar Giant Planets
Authors: Burrows, A.; Sudarsky, D.; Hubeny, I.
2006ApJ...650.1140B Altcode: 2006astro.ph..7014B
We have created a general methodology for calculating the
wavelength-dependent light curves of close-in extrasolar giant planets
(EGPs) as they traverse their orbits. Focusing on the transiting EGPs
HD 189733b, TrES-1, and HD 209458b, we calculate planet/star flux
ratios during secondary eclipse and compare them with the Spitzer
data points obtained so far in the mid-infrared. We introduce a
simple parameterization for the redistribution of heat to the planet's
night side, derive constraints on this parameter (Pn), and
provide a general set of predictions for planet/star contrast ratios
as a function of wavelength, model, and phase. Moreover, we calculate
average dayside and nightside atmospheric temperature/pressure profiles
for each transiting planet/Pn pair with which existing and
anticipated Spitzer data can be used to probe the atmospheric thermal
structure of severely irradiated EGPs. We find that the baseline models
do a good job of fitting the current secondary eclipse data set, but
that the Spitzer error bars are not yet small enough to discriminate
cleanly among all the various possibilities.

---------------------------------------------------------
Title: Neon Abundances in B Stars of the Orion Association: Solving
the Solar Model Problem?
Authors: Cunha, Katia; Hubeny, Ivan; Lanz, Thierry
2006ApJ...647L.143C Altcode: 2006astro.ph..6738C
We report on non-LTE Ne abundances for a sample of B-type stellar
members of the Orion association. The abundances were derived by means
of non-LTE fully metal-blanketed model atmospheres and extensive model
atoms with updated atomic data. We find that these young stars have
a very homogeneous abundance of A(Ne)=8.11+/-0.04. This abundance
is higher by ~0.3 dex than the currently adopted solar value,
A(Ne)=7.84, which is derived from lines produced in the corona and
active regions. The general agreement between the abundances of C, N,
and O derived for B stars with the solar abundances of these elements
derived from three-dimensional hydrodynamical models atmospheres
strongly suggests that the abundance patterns of the light elements in
the Sun and B stars are broadly similar. If this hypothesis is true,
then the Ne abundance derived here will help to reconcile solar models
with helioseismological observations.

---------------------------------------------------------
Title: VizieR Online Data Catalog: SMC NGC 346-11 and AV 304 spectra
(Hunter+, 2005)
Authors: Hunter, I.; Dufton, P. L.; Ryans, R. S. I.; Lennon, D. J.;
Rolleston, W. R. J.; Hubeny, I.; Lanz, T.
2006yCat..34360687H Altcode:
High-resolution spectra have been obtained for both NGC 346-11 and AV
304 using the Ultraviolet and Visual Echelle Spectrograph, UVES, on
the UT2 (Kueyen) telescope at the European Southern Observatory. NGC
346-11 was observed during a three night run in November 2001,
whilst the observations of AV 304, taken during a two night run
in January 2001, have been previously discussed by Rolleston et
al. (2003A&amp;A...400...21R). (1 data file).

---------------------------------------------------------
Title: A FUSE Legacy Program of DA White Dwarf Observations
Authors: Holberg, J. B.; Sing, D. K.; Hubeny, I.; Barstow, M. A.;
Burleigh, M. R.
2006ASPC..348..230H Altcode:
We are currently conducting a Cycle 5 Legacy program designed to fill
critical gaps in the FUSE coverage of hot DA white dwarf stars. In
particular this program will systematically sample stars in the
temperature region above 50,000 K as well as cover the widest possible
range of surface gravities. The specific scientific objective of this
program is to resolve the recently discovered discrepancy between
Lyman line and Balmer line temperature determinations that is evident
for DA white dwarfs above 50,000 K. This program is now underway,
with three of our 24 targets having been observed.

---------------------------------------------------------
Title: FUSE Spectroscopy of Hybrid White Dwarfs
Authors: Lanz, T.; Holberg, J. B.; Hubeny, I.
2006ASPC..348..197L Altcode:
We have obtained FUSE spectra of three white dwarfs showing hybrid
helium and hydrogen spectra. We present initial results of our
NLTE model atmosphere analysis, and discuss their implication for
understanding the origin of DAB white dwarfs and the physical processes
in white dwarf atmospheres.

---------------------------------------------------------
Title: The RXTE, ROSAT, IUE, EUVE, Optical Campaign Covering the
45-Day Supercycle of V1159 Orionis
Authors: Szkody, P.; Honeycutt, K.; Robertson, J.; Silber, A.; Hoard,
D. W.; Pastwick, L.; Hubeny, I.; Cannizzo, J.; Liller, W.; Zissell,
R.; Walker, G.; La Dous, C.; Drew, J.
2006JAVSO..35..135S Altcode:
A comprehensive data set covering the superoutburst and 8 outbursts
of the 45-day supercycle of the dwarf nova V1159 Ori was obtained in
Feb-Mar 1996. The use of RXTE, ROSAT, IUE, and EUVE satellites, combined
with ground-based optical photometry and spectroscopy, provided a broad
picture of the accretion disk at different states. Results include
an inverse correlation of the X-ray with the UV/optical fluxes, an
outflowing wind during all outbursts, and large changes in the disk
spectrum on time scales of less than a day. These results are compared
to other dwarf novae and general theories of dwarf nova outbursts.

---------------------------------------------------------
Title: A Grid of Relativistic, Non-LTE Accretion Disk Models for
Spectral Fitting of Black Hole Binaries
Authors: Davis, Shane W.; Hubeny, Ivan
2006ApJS..164..530D Altcode: 2006astro.ph..2499D
Self-consistent vertical structure models together with non-LTE
radiative transfer should produce spectra from accretion disks around
black holes, which differ from multitemperature blackbodies at levels
that may be observed. High-resolution, high signal-to-noise observations
warrant spectral modeling that both accounts for relativistic effects
and treats the physics of radiative transfer in detail. In Davis et
al. we presented spectral models that accounted for non-LTE effects,
Compton scattering, and the opacities due to ions of abundant
metals. Using a modification of this method, we have tabulated
spectra for black hole masses typical of Galactic binaries. We make
them publicly available for spectral fitting as an XSPEC model. These
models represent the most complete realization of standard accretion
disk theory to date. Thus, they are well suited both for testing
the theory's applicability to observed systems and for constraining
properties of the black holes, including their spins.

---------------------------------------------------------
Title: New Praesepe white dwarfs and the initial mass-final mass
relation
Authors: Dobbie, P. D.; Napiwotzki, R.; Burleigh, M. R.; Barstow,
M. A.; Boyce, D. D.; Casewell, S. L.; Jameson, R. F.; Hubeny, I.;
Fontaine, G.
2006MNRAS.369..383D Altcode: 2006astro.ph..3314D
We report the spectroscopic confirmation of four further white dwarf
members of Praesepe. This brings the total number of confirmed white
dwarf members to 11, making this the second largest collection of
these objects in an open cluster identified to date. This number is
consistent with the high-mass end of the initial mass function of
Praesepe being Salpeter in form. Furthermore, it suggests that the
bulk of Praesepe white dwarfs did not gain a substantial recoil kick
velocity from possible asymmetries in their loss of mass during the
asymptotic giant branch phase of evolution. By comparing our estimates
of the effective temperatures and the surface gravities of WD0833+194,
WD0840+190, WD0840+205 and WD0843+184 to modern theoretical evolutionary
tracks, we have derived their masses to be in the range 0.72-0.76
Msolar and their cooling ages ~300 Myr. For an assumed
cluster age of 625 +/- 50 Myr, the inferred progenitor masses are
between 3.3 and 3.5 Msolar. Examining these new data in the
context of the initial mass-final mass relation, we find that it can be
adequately represented by a linear function (a0 = 0.289 +/-
0.051,a1 = 0.133 +/- 0.015) over the initial mass range 2.7-6
Msolar. Assuming an extrapolation of this relation to larger
initial masses is valid and adopting a maximum white dwarf mass of 1.3
Msolar, our results support a minimum mass for core-collapse
supernovae progenitors in the range ~6.8-8.6 Msolar.

---------------------------------------------------------
Title: L and T Dwarf Models and the L to T Transition
Authors: Burrows, Adam; Sudarsky, David; Hubeny, Ivan
2006ApJ...640.1063B Altcode: 2005astro.ph..9066B
Using a model for refractory clouds, a novel algorithm for handling
them, and the latest gas-phase molecular opacities, we have produced a
new series of L and T dwarf spectral and atmosphere models as a function
of gravity and metallicity, spanning the Teff range from 2200
to 700 K. The correspondence with observed spectra and infrared colors
for early and mid-L dwarfs and for mid- to late T dwarfs is good. We
find that the width in infrared color-magnitude diagrams of both the T
and L dwarf branches is naturally explained by reasonable variations
in gravity and therefore that gravity is the “second parameter” of
the L-T dwarf sequence. We investigate the dependence of theoretical
dwarf spectra and color-magnitude diagrams on various cloud properties,
such as particle size and cloud spatial distribution. In the region
of the L--&gt;T transition, we find that no single combination of
cloud particle size and gravity can be made to fit all the observed
data. Our results suggest that current ignorance of detailed cloud
meteorology renders ambiguous the extraction of various physical
quantities such as Teff and gravity for mid-L to early T
dwarfs. Nevertheless, for decreasing Teff, we capture with
some accuracy the major spectral features and signatures observed. We
speculate that the subdwarf branch of the L dwarfs would be narrower in
effective temperature and that for low enough metallicity the L dwarfs
would disappear altogether as a spectroscopic class. Furthermore, we
note that the new, lower solar oxygen abundances of Allende-Prieto
and coworkers produce better fits to brown dwarf data than do the
older values. Finally, we discuss various issues in cloud physics and
modeling and speculate on how a better correspondence between theory
and observation in the problematic L--&gt;T transition region could
be achieved.

---------------------------------------------------------
Title: FUSE and HST STIS Far-Ultraviolet Observations of AM Herculis
in an Extended Low State
Authors: Gänsicke, Boris T.; Long, Knox S.; Barstow, Martin A.;
Hubeny, Ivan
2006ApJ...639.1039G Altcode: 2005astro.ph.11100G
We have obtained FUSE and HST STIS time-resolved spectroscopy of the
Polar AM Herculis during a deep low state. The spectra are entirely
dominated by the emission of the white dwarf. Both the far-ultraviolet
(FUV) flux and the spectral shape vary substantially over the orbital
period, with maximum flux occurring at the same phase as during the
high state. The variations are due to the presence of a hot spot
on the white dwarf, which we model quantitatively. The white dwarf
parameters can be determined from a spectral fit to the faint-phase
data, when the hot spot is self-eclipsed. Adopting the distance
of 79+8-6 pc determined by Thorstensen,
we find an effective temperature of 19,800+/-700 K and a mass of
MWD=0.78+0.12-0.17 Msolar. The hot spot has
a lower temperature than during the high state, ~34,000-40,000 K,
but covers a similar area, ~10% of the white dwarf surface. Low-state
FUSE and STIS spectra taken during four different epochs in 2002-2003
show no variation of the FUV flux level or spectral shape, implying
that the white dwarf temperature and the hot spot temperature, size,
and location do not depend on the amount of time the system has spent
in the low state. Possible explanations are ongoing accretion at a low
level or deep heating; both alternatives have some weaknesses, which we
discuss. No photospheric metal absorption lines are detected in the FUSE
and STIS spectra, suggesting that the average metal abundances in the
white dwarf atmosphere are lower than ~10-3 times their solar
values. Based on observations made with the NASA/ESA Hubble Space
Telescope, obtained at the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy,
Inc., under NASA contract NAS 5-26555, and on observations made with the
NASA-CNES-CSA Far-Ultraviolet Spectroscopic Explorer. FUSE is operated
for NASA by the Johns Hopkins University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: Fundamental Properties of O-Type Stars
Authors: Heap, Sara R.; Lanz, Thierry; Hubeny, Ivan
2006ApJ...638..409H Altcode: 2004astro.ph.12345H
We present a comprehensive analysis of high-resolution, far-ultraviolet
HST STIS, FUSE, and optical spectra of 18 O stars in the Small
Magellanic Cloud. Our analysis is based on the OSTAR2002 grid of
NLTE metal-line-blanketed model atmospheres calculated with our
code TLUSTY. We systematically explore and present the sensitivity
of various UV and optical lines to different stellar parameters. We
have obtained consistent fits of the UV and the optical spectrum to
derive the effective temperature, surface gravity, surface composition,
and microturbulent velocity of each star. Stellar radii, masses, and
luminosities follow directly. For stars of the same spectral subtype,
we find a general good agreement between effective temperature
determinations obtained with TLUSTY, CMFGEN, and FASTWIND models,
which are all lower than the standard Teff calibration
of O stars. We propose a new calibration between the spectral type
and effective temperature based on our results from UV metal lines,
as well as optical hydrogen and helium lines. The lower effective
temperatures translate into ionizing luminosities that are smaller
by a factor of 3 compared to luminosities inferred from previous
standard calibrations. The chemical composition analysis reveals
that the surface of about 80% of the program stars is moderately to
strongly enriched in nitrogen, while showing the original helium,
carbon, and oxygen abundances. Our results support the new stellar
evolution models that predict that the surface of fast rotating
stars becomes nitrogen-rich during the main-sequence phase because of
rotationally induced mixing. Enrichment factors are, however, larger
than predicted by stellar evolution models. Most stars exhibit the
“mass discrepancy” problem, which we interpret as a result of fast
rotation that lowers the measured effective gravity. Nitrogen enrichment
and low spectroscopic masses are therefore two manifestations of fast
rotation. Our study thus emphasizes the importance of rotation in
our understanding of the properties of massive stars and provides a
framework for investigating populations of low-metallicity massive
stars at low and high redshifts. Based on observations with
the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy, Inc., under NASA contract NAS5-2655. Also
based on observations made with the NASA-CNES-CSA Far Ultraviolet
Spectroscopic Explorer, which is operated for NASA by Johns Hopkins
University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: Radiative Transfer in Astrophysical Applications
Authors: Hubeny, I.
2006cmt..conf...15H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Non-LTE Line-Blanketed Model Atmospheres of B-type Stars
Authors: Lanz, T.; Hubeny, I.
2005AAS...20718221L Altcode: 2005BAAS...37.1467L
We present an extension of our OSTAR2002 grid of NLTE model atmospheres
to B-type stars. We have calculated over 1,300 metal line-blanketed,
NLTE, plane-parallel, hydrostatic model atmospheres for the basic
parameters appropriate to B stars. The grid covers 16 effective
temperatures from 15,000 to 30,000 K, with 1000 K steps, 13 surface
gravities, log g≤ 4.75 down to the Eddington limit, and 5 compositions
(2, 1, 0.5, 0.2, and 0.1 times solar). We have adopted a microturbulent
velocity of 2 km/s for all models. In the lower surface gravity range
(log g≤ 3.0), we supplemented the main grid with additional model
atmospheres accounting for higher microtutbulent velocity (10 km/s)
and for alterated surface composition (He and N-rich, C-deficient),
as observed in B supergiants. The models incorporate basically all
known atomic levels of 46 ions of H, He, C, N, O, Ne, Mg, Al, Si, S,
and Fe, which are grouped into 1127 superlevels. Models and spectra
will be available at our Web site, http://nova.astro.umd.edu.

---------------------------------------------------------
Title: Photospheric phosphorus in the FUSE spectra of GD71 and two
similar DA white dwarfs
Authors: Dobbie, P. D.; Barstow, M. A.; Hubeny, I.; Holberg, J. B.;
Burleigh, M. R.; Forbes, A. E.
2005MNRAS.363..763D Altcode: 2005MNRAS.tmp..867D; 2005astro.ph..7638D
We report the detection, from the Far Ultraviolet Spectroscopic
Explorer (FUSE) data, of phosphorus in the atmospheres of GD71 and
two similar DA white dwarfs. This is the first detection of a trace
metal in the photosphere of the spectrophotometric standard star
GD71. Collectively, these objects represent the coolest DA white dwarfs
in which photospheric phosphorus has been observed. We use a grid of
homogeneous non-local thermodynamic equilibrium synthetic spectra to
measure abundances of [P/H]=-8.57+0.09-0.13,
-8.70+0.23-0.37 and
-8.36+0.14-0.19 in GD71, RE J1918+595 and RE
J0605-482 respectively. At the observed level we find that phosphorus
has no significant impact on the overall energy distribution of
GD71. We explore possible mechanisms responsible for the presence of
this element in these stars, concluding that the most likely is an
interplay between radiative levitation and gravitational settling,
possibly modified by weak mass loss.

---------------------------------------------------------
Title: Heavy element abundances in DAO white dwarfs measured from
FUSE data
Authors: Good, S. A.; Barstow, M. A.; Burleigh, M. R.; Dobbie, P. D.;
Holberg, J. B.; Hubeny, I.
2005MNRAS.363..183G Altcode: 2005MNRAS.tmp..762G; 2005astro.ph..7341G
We present heavy element abundance measurements for 16 DAO white
dwarfs, determined from Far-Ultraviolet Spectroscopic Explorer (FUSE)
spectra. Evidence of absorption by heavy elements was found in the
spectra of all the objects. Measurements were made using models that
adopted the temperatures, gravities and helium abundances determined
from both optical and FUSE data by Good et al. It was found that,
when using the values for the parameters measured from optical data,
the carbon abundance measurements follow and extend a similar trend of
increasing abundance with temperature for DA white dwarfs, as discovered
by Barstow et al. However, when the FUSE measurements are used, the DAO
abundances no longer join this trend since the temperatures are higher
than the optical measures. Silicon abundances were found to increase
with temperature, but no similar trend was identified in the nitrogen,
oxygen, iron or nickel abundances, and no dependence on gravity or
helium abundances was noted. However, the models were not able to
reproduce the observed silicon and iron line strengths satisfactorily in
the spectra of half of the objects, and the oxygen features of all but
three. Despite the different evolutionary paths that the types of DAO
white dwarfs are thought to evolve through, their abundances were not
found to vary significantly, apart from for the silicon abundances. Abundances measured when the FUSE-derived values of temperature,
gravity and helium abundance were adopted were, in general, a factor
1-10 higher than those determined when the optical measure of those
parameters was used. Satisfactory fits to the absorption lines were
achieved in an approximately equal number. The models that used the
FUSE-determined parameters seemed better at reproducing the strength of
the nitrogen and iron lines, while for oxygen the optical parameters
were better. For the three objects whose temperature measured from
FUSE data exceeds 120000 K, the carbon, nitrogen and oxygen lines
were too weak in the models that used the FUSE parameters. However,
the model that used the optical parameters also did not reproduce the
strength of all the lines accurately.

---------------------------------------------------------
Title: Hubble Space Telescope spectroscopy of the Balmer lines in
Sirius B*
Authors: Barstow, M. A.; Bond, Howard E.; Holberg, J. B.; Burleigh,
M. R.; Hubeny, I.; Koester, D.
2005MNRAS.362.1134B Altcode: 2005astro.ph..6600B; 2005MNRAS.tmp..739B
Sirius B is the nearest and brightest of all white dwarfs, but it is
very difficult to observe at visible wavelengths due to the overwhelming
scattered light contribution from Sirius A. However, from space we can
take advantage of the superb spatial resolution of the Hubble Space
Telescope (HST) to resolve the A and B components. Since the closest
approach in 1993, the separation between the two stars has become
increasingly favourable and we have recently been able to obtain a
spectrum of the complete Balmer line series for Sirius B using the
HST Space Telescope Imaging Spectrograph (STIS). The quality of the
STIS spectra greatly exceeds that of previous ground-based spectra,
and can be used to provide an important determination of the stellar
temperature (Teff= 25193 K) and gravity (logg= 8.556). In
addition, we have obtained a new, more accurate, gravitational
redshift of 80.42 +/- 4.83 km s-1 for Sirius B. Combining
these results with the photometric data and the Hipparcos parallax,
we obtain new determinations of the stellar mass for comparison
with the theoretical mass-radius relation. However, there are some
disparities between the results obtained independently from logg and
the gravitational redshift which may arise from flux losses in the
narrow 50 × 0.2 arcsec2 slit. Combining our measurements
of Teff and logg with the Wood evolutionary mass-radius
relation, we obtain a best estimate for the white dwarf mass of
0.978 Msolar. Within the overall uncertainties, this is in
agreement with a mass of 1.02 Msolar obtained by matching
our new gravitational redshift to the theoretical mass-radius relation.

---------------------------------------------------------
Title: High-resolution extreme ultraviolet spectroscopy of G191-B2B:
structure of the stellar photosphere and the surrounding interstellar
medium
Authors: Barstow, M. A.; Cruddace, R. G.; Kowalski, M. P.; Bannister,
N. P.; Yentis, D.; Lapington, J. S.; Tandy, J. A.; Hubeny, I.; Schuh,
S.; Dreizler, S.; Barbee, T. W.
2005MNRAS.362.1273B Altcode: 2005MNRAS.tmp..749B
We have continued our detailed analysis of the high-resolution (R=
4000) spectroscopic observation of the DA white dwarf G191-B2B,
obtained by the Joint Astrophysical Plasmadynamic Experiment (J-PEX)
normal incidence sounding rocket-borne telescope, comparing the
observed data with theoretical predictions for both homogeneous and
stratified atmosphere structures. We find that the former models
give the best agreement over the narrow waveband covered by J-PEX,
in conflict with what is expected from previous studies of the lower
resolution but broader wavelength coverage Extreme Ultraviolet Explorer
spectra. We discuss the possible limitations of the atomic data and
our understanding of the stellar atmospheres that might give rise to
this inconsistency. In our earlier study, we obtained an unusually
high ionization fraction for the ionized HeII present along the line
of sight to the star. In the present paper, we obtain a better fit
when we assume, as suggested by Space Telescope Imaging Spectrograph
results, that this HeII resides in two separate components. When one
of these is assigned to the local interstellar cloud, the implied He
ionization fraction is consistent with measurements along other lines
of sight. However, the resolving power and signal-to-noise available
from the instrument configuration used in this first successful J-PEX
flight are not sufficient to clearly identify and prove the existence
of the two components.

---------------------------------------------------------
Title: A near-IR spectrum of the DO white dwarf <ASTROBJ>RE
J0503-285</ASTROBJ>
Authors: Dobbie, P. D.; Burleigh, M. R.; Levan, A. J.; Barstow, M. A.;
Napiwotzki, R.; Hubeny, I.
2005A&A...439.1159D Altcode: 2005astro.ph..6384D
We present a near-IR spectroscopic analysis of the intriguing DO white
dwarf RE J0503-285. The IR spectrum fails to reveal evidence of the
presence of a spatially unresolved, cool, late-type companion. Hence
we have placed an approximate limit on the spectral-type and mass of a
putative companion (later than M8, M&lt;0.085~M⊙). This
result rules out ongoing interaction between the white dwarf and a
close companion with M≥0.085~M⊙ as responsible for
the discrepancies between the observed photospheric abundances and
model predictions. As the possibility remains that there is a cooler
companion lying beyond the detection threshold of this study we use
our modelling to estimate the improvement in sensitivity offered by
a Spitzer observation.

---------------------------------------------------------
Title: X-ray Heated Black Hole Accretion Discs
Authors: Wickramasinghe, D.; Hubeny, I.
2005ASPC..330..219W Altcode:
The response of the disc to external irradiation depends on the
importance of the irradiation flux relative to the viscous flux, and
on the details of the incident spectrum; the higher energy photons will
preferentially Compton scatter and heat/cool the disc, while the lower
energy photons will be mainly photo absorbed and re-emitted. Irradiation
with a black body spectrum at temperatures appropriate to the inner
regions of the disc generally results in a temperature inversion in
the upper disc atmosphere at all disc radii. A general characteristic
of all models is that as the irradiation flux increases relative to the
viscously generated flux, and the effects of heating penetrates to the
disc mid plane, the temperature profile at large optical depths becomes
flatter. In addition, irradiation reduces the continuum jumps and makes
the spectrum more black body like. For moderate irradiation, a Compton
reflection peak develops at frequencies corresponding approximately
to the peak of the irradiating flux. This is seen as a flux excess
over the thermal emission that would normally be expected from the
heated disc if the effects of scattering were ignored. In the limit of
strong irradiation, a saturated inverse Compton spectrum develops with
a high energy Wien tail. The detection of a Compton reflection peak,
and a study of its spectral characteristics, will provide a strong
diagnostic on the nature of the irradiating source. Hα
line profiles are also presented.

---------------------------------------------------------
Title: Mass Loss for Highly-Irradiated Giant Planets
Authors: Hubbard, W. B.; Burrows, A.; Hubeny, I.; Sudarsky, D.;
Hattori, M. F.
2005DPS....37.3105H Altcode: 2005BAAS...37..684H
We present calculations for the surviving mass of highly-irradiated
extrasolar giant planets (EGPs) at orbital semimajor axes ranging
from 0.023 to 0.057 AU using a generalized scaled theory for mass
loss, together with new surface-condition grids for hot EGPs and a
consistent treatment of tidal truncation. Available theoretical
estimates for the rate of energy-limited hydrogen escape from
giant-planet atmospheres range over four orders of magnitude, when
one holds planetary mass, composition, and irradiation constant. Yelle
(Icarus 170, 167-179, 2004) predicts the lowest escape rate. Baraffe
et al. (A&amp;A 419, L13-L16, 2004) predict the highest rate, based on
the theory of Lammer et al. (ApJ 598, L121-L124, 2003). Scaling the
theory of Watson et al. (Icarus 48, 150-166, 1981) to parameters for
a highly-irradiated exoplanet, we find an intermediate escape rate,
∼ 102 higher than Yelle's but ∼ 102 lower
than Baraffe's. With the scaled Watson theory and the scaled
Yelle theory we find modest mass loss, occurring early in the history
of a hot EGP. Particularly for the Yelle theory, the effect of tidal
truncation sets the minimum mass limit, well below a Saturn mass for
the distances investigated. This contrasts with the Baraffe model,
where hot EGPs are claimed to be remnants of much more massive bodies,
originally several times Jupiter and still losing substantial mass
fractions at present. Supported by NASA Grant NAG5-13775 (PGG)
and NASA Grant NNG04GL22G (ATP).

---------------------------------------------------------
Title: Effects of mass loss for highly-irradiated giant planets
Authors: Hubbard, W. B.; Hattori, M. F.; Burrows, A.; Hubeny, I.;
Sudarsky, D.
2005astro.ph..8591H Altcode:
We present calculations for the evolution and surviving mass of
highly-irradiated extrasolar giant planets (EGPs) at orbital semimajor
axes ranging from 0.023 to 0.057 AU using a generalized scaled theory
for mass loss, together with new surface-condition grids for hot EGPs
and a consistent treatment of tidal truncation. Theoretical estimates
for the rate of energy-limited hydrogen escape from giant-planet
atmospheres differ by two orders of magnitude, when one holds planetary
mass, composition, and irradiation constant. Baraffe et al. (2004,
A&amp;A 419, L13-L16) predict the highest rate, based on the theory
of Lammer et al. (2003, Astrophys. J. 598, L121-L124). Scaling the
theory of Watson et al. (1981, Icarus 48, 150-166) to parameters for
a highly-irradiated exoplanet, we find an escape rate ~102 lower than
Baraffe's. With the scaled Watson theory we find modest mass loss,
occurring early in the history of a hot EGP. In this theory, mass
loss including the effect of Roche-lobe overflow becomes significant
primarily for masses below a Saturn mass, for semimajor axes = 0.023
AU. This contrasts with the Baraffe model, where hot EGPs are claimed
to be remnants of much more massive bodies, originally several times
Jupiter and still losing substantial mass fractions at present.

---------------------------------------------------------
Title: HST Observations of the Sirius B Balmer Lines
Authors: Barstow, M. A.; Burleigh, M. R.; Holberg, J. B.; Hubeny,
I.; Bond, H. E.; Koester, D.
2005ASPC..334..175B Altcode:
For most of the lifetime of the Hubble Space Telescope (HST), it has
not been possible to observe Sirius B spectroscopically at visible
wavelengths due to the overwhelming scattered light contribution
from Sirius A. However, as the separation between the two stars is
becoming larger we have been able to obtain a spectrum of the complete
Balmer line series for Sirius B. This is the first such spectrum to be
obtained, apart from old ground-based photographic spectra, and can be
used to provide an important determination of the stellar temperature
and gravity.

---------------------------------------------------------
Title: Phase Functions and Light Curves of Wide-Separation Extrasolar
Giant Planets
Authors: Sudarsky, David; Burrows, Adam; Hubeny, Ivan; Li, Aigen
2005ApJ...627..520S Altcode: 2005astro.ph..1109S
We calculate self-consistent extrasolar giant planet (EGP) phase
functions and light curves for orbital distances ranging from 0.2 to
15 AU. We explore the dependence on wavelength, cloud condensation,
and Keplerian orbital elements. We find that the light curves of
EGPs depend strongly on wavelength, the presence of clouds, and
cloud particle sizes. Furthermore, the optical and infrared colors
of most EGPs are phase-dependent, tending to be reddest at crescent
phases in V-R and R-I. Assuming circular orbits, we find that at
optical wavelengths most EGPs are 3-4 times brighter near full phase
than near greatest elongation for highly inclined (i.e., close to
edge-on) orbits. Furthermore, we show that the planet/star flux ratios
depend strongly on the Keplerian elements of the orbit, particularly
inclination and eccentricity. Given a sufficiently eccentric orbit,
an EGP's atmosphere may make periodic transitions from cloudy to
cloud-free, an effect that may be reflected in the shape and magnitude
of the planet's light curve. Such elliptical orbits also introduce an
offset between the time of the planet's light-curve maximum and the
time of full planetary phase, and for some sets of orbital parameters,
this light-curve maximum can be a steeply increasing function of
eccentricity. We investigate the detectability of EGPs by proposed
space-based direct-imaging instruments.

---------------------------------------------------------
Title: Searching for the Donor Star in Close Binary Supersoft Sources
Authors: Lanz, Thierry; Audard, Marc; Hubeny, Ivan; Paerels, Frits
2005sptz.prop20130L Altcode:
The identification of Type Ia Supernova progenitors still remains
elusive. The most promising channel to form SNe Ia involves accreting
white dwarfs (WDs) in close binary systems. The hot WDs sustain
steady nuclear burning and are observed as ultrasoft X-ray sources. The
companions have remained until now undetected because the UV and optical
spectrum is dominated by the emission from the accretion disk around
the WDs. At present, these close binary supersoft sources (CBSS) have
not been observed in the infrared, with the single exception of near-IR
photometry of the Galactic CBSS QR And that provides the first hint
of the secondary. Because of the general lack of a direct detection,
the nature of the donor stars in these systems is still debated. By
combining ultraviolet and optical SEDs with Spitzer observations of 4
CBSS in the mid-infrared, we will be able to definitively identify the
nature of the donor stars in CBSS systems, as well as determining the
size of the accretion disks. We propose to observe eclipsing systems
at different phases to disentangle the contribution of the companions
from the emission of the disks. Spitzer observations of CBSS will
therefore represent a significant step forward in characterizing the
binary systems that are best candidates for becoming SNe Ia.

---------------------------------------------------------
Title: Non-LTE Spectra of Accretion Disks around Intermediate-Mass
Black Holes
Authors: Hui, Yawei; Krolik, Julian H.; Hubeny, Ivan
2005ApJ...625..913H Altcode: 2005astro.ph..2355H
We have calculated the structures and the emergent spectra of
stationary, geometrically thin accretion disks around 100 and 1000
Msolar black holes in both the Schwarzschild and extreme
Kerr metrics. Equations of radiative transfer, hydrostatic equilibrium,
energy balance, ionization equilibrium, and statistical equilibrium are
solved simultaneously and consistently. The six most astrophysically
abundant elements (H, He, C, N, O, and Fe) are included, as well
as energy transfer by Comptonization. The observed spectrum as a
function of viewing angle is computed, incorporating all general
relativistic effects. We find that, in contrast with the predictions
of the commonly used multicolor disk (MCD) model, opacity associated
with photoionization of heavy elements can significantly alter the
spectrum near its peak. These ionization edges can create spectral
breaks visible in the spectra of slowly spinning black holes viewed
from almost all angles and in the spectra of rapidly spinning black
holes seen approximately pole-on. For fixed mass and accretion rate
relative to Eddington, both the black hole spin and the viewing angle
can significantly shift the observed peak energy of the spectrum,
particularly for rapid spin viewed obliquely or edge-on. We present
a detailed test of the approximations made in various forms of the
MCD model. Linear limb-darkening is confirmed to be a reasonable
approximation for the integrated flux but not for many specific
frequencies of interest.

---------------------------------------------------------
Title: A non-LTE analysis of the spectra of two narrow lined main
sequence stars in the SMC
Authors: Hunter, I.; Dufton, P. L.; Ryans, R. S. I.; Lennon, D. J.;
Rolleston, W. R. J.; Hubeny, I.; Lanz, T.
2005A&A...436..687H Altcode: 2005astro.ph..3662H
An analysis of high-resolution VLT/UVES spectra of two B-type main
sequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud
(SMC), has been undertaken, using the non-LTE tlusty model atmospheres
to derive the stellar parameters and chemical compositions of each
star. The chemical compositions of the two stars are in reasonable
agreement. Moreover, our stellar analysis agrees well with earlier
analyses of H II regions. The results derived here should be
representative of the current base-line chemical composition of the
SMC interstellar medium as derived from B-type stars.

---------------------------------------------------------
Title: A Theoretical Interpretation of the Measurements of the
Secondary Eclipses of TrES-1 and HD 209458b
Authors: Burrows, A.; Hubeny, I.; Sudarsky, D.
2005ApJ...625L.135B Altcode: 2005astro.ph..3522B
We calculate the planet-to-star flux density ratios as a function
of wavelength from 0.5 to 25 μm for the transiting extrasolar
giant planets TrES-1 and HD 209458b and compare them with the recent
Spitzer/IRAC-MIPS secondary eclipse data in the 4.5, 8.0, and 24 μm
bands. With only three data points and generic calibration issues,
detailed conclusions are difficult, but inferences regarding atmospheric
composition, temperature, and global circulation can be made. Our
models reproduce the observations reasonably well, but not perfectly,
and we speculate on the theoretical consequences of variations around
our baseline models. One preliminary conclusion is that we may be seeing
in the data indications that the day side of a close-in extrasolar
giant planet is brighter in the mid-infrared than its night side,
unlike Jupiter and Saturn. This correspondence will be further tested
when the data anticipated in other Spitzer bands are acquired, and we
make predictions for what those data may show.

---------------------------------------------------------
Title: MV Lyrae in Low, Intermediate, and High States
Authors: Linnell, Albert P.; Szkody, Paula; Gänsicke, Boris; Long,
Knox S.; Sion, Edward M.; Hoard, D. W.; Hubeny, Ivan
2005ApJ...624..923L Altcode: 2006astro.ph.12696L
Archival IUE spectra of the VY Sculptoris system MV Lyrae, taken during
an intermediate state, can be best fit by an isothermal accretion disk
extending half-way to the tidal cutoff radius. In contrast, a recent
HST spectrum, while MV Lyr was in a high state, can be best fit with
a standard T(R) profile for an accretion disk extending from an inner
truncation radius to an intermediate radius with an isothermal accretion
disk beyond. These fits use component-star parameters determined from
a study of MV Lyr in a low state. Model systems containing accretion
disks with standard T(R) profiles have continua that are too blue. The
observed high-state absorption-line spectrum exhibits excitation
higher than provided by the T(R) profile, indicating likely line
formation in a high-temperature region extending vertically above
the accretion disk. The absorption lines show a blueshift and line
broadening corresponding to formation in a low-velocity wind apparently
coextensive with the high-temperature region. Lines of N V, Si IV, C IV,
and He II are anomalously strong relative to our synthetic spectra,
indicating possible composition effects, but unmodeled excitation
effects could also produce the anomalies. An analysis of a low state
of MV Lyr, considered in an earlier study and extended in this paper,
sets a limit of 2500 K for the Teff of an accretion disk
that may be present in the low state. This limit is in conflict with
two recent models of the VY Sculptoris phenomenon. Based on
observations made with the NASA/ESA Hubble Space Telescope, obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under
NASA contract NAS5-26555, and with the Apache Point Observatory 3.5 m
telescope, which is operated by the Astrophysical Research Corporation.

---------------------------------------------------------
Title: B-type supergiants in the SMC: Chemical compositions and
comparison of static and unified models
Authors: Dufton, P. L.; Ryans, R. S. I.; Trundle, C.; Lennon, D. J.;
Hubeny, I.; Lanz, T.; Allende Prieto, C.
2005A&A...434.1125D Altcode: 2004astro.ph.12367D
High-resolution UCLES/AAT spectra are presented for nine B-type
supergiants in the SMC, chosen on the basis that they may show
varying amounts of nuclear-synthetically processed material mixed to
their surface. These spectra have been analysed using a new grid of
approximately 12 000 non-LTE line blanketed tlusty model atmospheres
to estimate atmospheric parameters and chemical composition. The
abundance estimates for O, Mg and Si are in excellent agreement with
those deduced from other studies, whilst the low estimate for C may
reflect the use of the C II doublet at 4267 Å. The N estimates
are approximately an order of magnitude greater than those found
in unevolved B-type stars or H II regions but are consistent with
the other estimates in AB-type supergiants. These results have been
combined with results from a unified model atmosphere analysis of
UVES/VLT spectra of B-type supergiants (Trundle et al. 2004, A&amp;A,
417, 217) to discuss the evolutionary status of these objects. For
two stars that are in common with those discussed by Trundle et al.,
we have undertaken a careful comparison in order to try to understand
the relative importance of the different uncertainties present in
such analyses, including observational errors and the use of static
or unified models. We find that even for these relatively luminous
supergiants, tlusty models yield atmospheric parameters and chemical
compositions similar to those deduced from the unified code fastwind.

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Title: A Study of the Near-Ultraviolet Spectrum of Vega
Authors: García-Gil, Alejandro; García López, Ramón J.; Allende
Prieto, Carlos; Hubeny, Ivan
2005ApJ...623..460G Altcode: 2005astro.ph..1213G
UV, optical, and near-IR spectra of Vega have been combined to test
our understanding of stellar atmospheric opacities and to examine the
possibility of constraining chemical abundances from low-resolution
UV fluxes. We have carried out a detailed analysis assuming local
thermodynamic equilibrium (LTE) to identify the most important
contributors to the UV continuous opacity: H, H-, C I, and
Si II. Our analysis also assumes that Vega is spherically symmetric
and that its atmosphere is well described with the plane-parallel
approximation. By comparing observations and computed fluxes, we have
been able to discriminate between two different flux scales that have
been proposed, the IUE-INES and the HST scales; we favor the latter. The
effective temperature and angular diameter derived from the analysis
of observed optical and near-UV spectra are in very good agreement
with previous determinations based on different techniques. The silicon
abundance is poorly constrained by the UV observations of the continuum
and strong lines, but the situation is more favorable for carbon, and
the abundances inferred from the UV continuum and optical absorption
lines are in good agreement. Some spectral intervals in the UV spectrum
of Vega poorly reproduced by the calculations are likely affected
by deviations from LTE, but we conclude that our understanding of UV
atmospheric opacities is fairly complete for early A-type stars.

---------------------------------------------------------
Title: Relativistic Accretion Disk Models of High-State Black Hole
X-Ray Binary Spectra
Authors: Davis, Shane W.; Blaes, Omer M.; Hubeny, Ivan; Turner, Neal J.
2005ApJ...621..372D Altcode: 2004astro.ph..8590D
We present calculations of non-LTE, relativistic accretion disk models
applicable to the high/soft state of black hole X-ray binaries. We
include the effects of thermal Comptonization and bound-free and
free-free opacities of all abundant ion species. Taking into account
the relativistic propagation of photons from the local disk surface to
an observer at infinity, we present spectra calculated for a variety
of accretion rates, black hole spin parameters, disk inclinations, and
stress prescriptions. We also consider nonzero inner torques on the
disk and explore different vertical dissipation profiles, including
some that are motivated by recent radiation magnetohydrodynamic
(MHD) simulations of magnetorotational turbulence. Bound-free metal
opacity generally produces significantly less spectral hardening than
previous models that only considered Compton scattering and free-free
opacity. We find that the resulting effective photosphere usually lies
at a small fraction of the total column depth, producing spectra that
are remarkably independent of the stress prescription and vertical
structure assumptions. We provide detailed comparisons between our
models and the widely used multicolor disk model. Frequency-dependent
discrepancies exist that may affect the parameters of other spectral
components when this simpler disk model is used to fit modern X-ray
data. For a given source, our models predict that the luminosity in the
high/soft state should approximately scale with the fourth power of the
empirically inferred maximum temperature, but with a slight hardening
at high luminosities. This is in good agreement with observations.

---------------------------------------------------------
Title: A near-infrared spectroscopic search for very-low-mass cool
companions to notable DA white dwarfs
Authors: Dobbie, P. D.; Burleigh, M. R.; Levan, A. J.; Barstow, M. A.;
Napiwotzki, R.; Holberg, J. B.; Hubeny, I.; Howell, S. B.
2005MNRAS.357.1049D Altcode: 2004astro.ph.12222D; 2005MNRAS.tmp...83D
We have undertaken a detailed near-infrared spectroscopic
analysis of eight notable white dwarfs, predominantly of southern
declination. In each case the spectrum failed to reveal compelling
evidence for the presence of a spatially unresolved, cool, late-type
companion. Therefore, we have placed an approximate limit on the
spectral type of a putative companion to each degenerate. From these
limits we conclude that if GD659, GD50, GD71 or WD2359-434 possesses
an unresolved companion then most probably it is substellar in nature
(M &lt; 0.072 Msolar). Furthermore, any spatially unresolved
late-type companion to RE J0457-280, RE J0623-374, RE J0723-274 or RE
J2214-491 most likely has M &lt; 0.082 Msolar. These results
imply that if weak accretion from a nearby late-type companion is the
cause of the unusual photospheric composition observed in a number of
these degenerates then the companions are of very low mass, beyond
the detection thresholds of this study. Furthermore, these results
do not contradict a previously noted deficit of very-low-mass stellar
and brown dwarf companions to main sequence F, G, K and early-M type
primaries (a&lt;~ 1000 au).

---------------------------------------------------------
Title: VizieR Online Data Catalog: B-type Supergiants in the SMC
(Dufton+, 2005)
Authors: Dufton, P. L.; Ryans, R. S. I.; Trundle, C.; Lennon, D. J.;
Hubeny, I.; Lanz, T.; Allende Prieto, C.
2005yCat..34341125D Altcode:
Table 4 contains the adopted atomic data, equivalent widths and
abundance estimates for all the metal lines observed in the SMC
supergiants. It also contains data for the SMC near main sequence
star AzV 304, which has been used in a differential analysis. (2
data files).

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Title: Quasars and the Big Blue Bump
Authors: Shang, Zhaohui; Brotherton, Michael S.; Green, Richard F.;
Kriss, Gerard A.; Scott, Jennifer; Quijano, Jessica Kim; Blaes, Omer;
Hubeny, Ivan; Hutchings, John; Kaiser, Mary Elizabeth; Koratkar,
Anuradha; Oegerle, William; Zheng, Wei
2005ApJ...619...41S Altcode: 2004astro.ph..9697S
We investigate the ultraviolet-to-optical spectral energy distributions
of 17 active galactic nuclei (AGNs) using quasi-simultaneous
spectrophotometry spanning 900-9000 Å (rest frame). We employ
data from the Far Ultraviolet Spectroscopic Explorer, the Hubble
Space Telescope, and the 2.1 m telescope at Kitt Peak National
Observatory. Taking advantage of the short-wavelength coverage, we
are able to study the so-called big blue bump, the region in which
the energy output peaks, in detail. Most objects exhibit a spectral
break around 1100 Å. Although this result is formally associated
with large uncertainty for some objects, there is strong evidence
in the data that the far-ultraviolet spectral region is below the
extrapolation of the near-ultraviolet-optical slope, indicating a
spectral break around 1100 Å. We compare the behavior of our sample
to those of non-LTE thin-disk models covering a range in black hole
mass, Eddington ratio, disk inclination, and other parameters. The
distribution of ultraviolet-optical spectral indices redward of the
break and far-ultraviolet indices shortward of the break are in rough
agreement with the models. However, we do not see a correlation between
the far-ultraviolet spectral index and the black hole mass, as seen
in some accretion disk models. We argue that the observed spectral
break is intrinsic to AGNs, although intrinsic reddening as well
as Comptonization can strongly affect the far-ultraviolet spectral
index. We make our data available online in digital format.

---------------------------------------------------------
Title: Preliminary analysis of an extreme helium sdO star: BD+25 4655
Authors: Budaj, J.; Elkin, V.; Hubeny, I.
2005astro.ph..1082B Altcode:
Preliminary analysis of CCD spectra obtained by the 6m SAO telescope
is presented. We have used simple H-He NLTE model atmospheres computed
by TLUSTY to derive the basic parameters of the star.

---------------------------------------------------------
Title: Constraining Galaxy Formation Epoch
Authors: Yi, S.; Brown, T.; Heap, S.; Hubeny, I.; Landsman, W.; Lanz,
T.; Sweigart, A.
2005IAUS..201..536Y Altcode: 2000IAUS..201E.108Y
The galaxy formation epoch (zf) is an important and unique
product of cosmological models, and thus much effort has been made to
constrain it. The most direct way of constraining zf is to
estimate the ages of bright elliptical galaxies at high redshifts
arrucately, but it has not been an easy task. We have finally
reached the stage where the stellar ages can be determined within
15% uncertainty. Using this advantage, the recent spectral analysis
of LBDS 53W091 by Spinrad and his collaborators (1997) has suggested
that this red galaxy at z=1.552 is at least 3.5 Gyr old. This provides
a significant constraint to cosmology, i.e., zf &gt; 6.5,
as LBDS 53W091 is the most distant passiviely evolving galaxy whose UV
spectrum is available. The results, based on our population synthesis
models, however, suggest a much smaller age for this galaxy, i.e.,
1.5 - 2 Gyr. The large discrepancy between their age estimate and ours
originates from the difference in the model integrated spectra: their
model spectra are significantly bluer than ours. Such a large difference
in the models for an apparently simple population is unreasonable. A
simple test seems to favor our models. If we adopt our age estimate,
the continuum of this galaxy suggests zf ≈ 2 - 3. This
anaysis, when performed to a larger sample, would efficiently constrain
the global galaxy formation epoch.

---------------------------------------------------------
Title: Non-LTE Model Atmosphere Analysis of the Large Magellanic
Cloud Supersoft X-Ray Source CAL 83
Authors: Lanz, Thierry; Telis, Gisela A.; Audard, Marc; Paerels,
Frits; Rasmussen, Andrew P.; Hubeny, Ivan
2005ApJ...619..517L Altcode: 2004astro.ph.10093L
We present a non-LTE (NLTE) model atmosphere analysis of Chandra High
Resolution Camera (HRC-S) and Low Energy Transmission Grating (LETG)
and XMM-Newton Reflection Grating Spectrometer (RGS) spectroscopy
of the prototypical supersoft source CAL 83 in the Large Magellanic
Cloud. Taken with a 16 month interval, the Chandra and XMM-Newton
spectra are very similar. They reveal a very rich absorption-line
spectrum from the hot white dwarf photosphere but no spectral signatures
of a wind. We also report a third X-ray off-state during a later Chandra
observation, demonstrating the recurrent nature of CAL 83. Moreover,
we found evidence of short-timescale variability in the soft X-ray
spectrum. We completed the analysis of the LETG and RGS spectra of
CAL 83 with new NLTE line-blanketed model atmospheres that explicitly
include 74 ions of the 11 most abundant species. We successfully matched
the Chandra and XMM-Newton spectra assuming a model composition with
LMC metallicity. We derived the basic stellar parameters of the hot
white dwarf, but the current state of atomic data in the soft X-ray
domain precludes a detailed chemical analysis. We have obtained the
first direct spectroscopic evidence that the white dwarf is massive
(MWD&gt;~1 Msolar). The short timescale of
the X-ray off-states is consistent with a high white dwarf mass. Our
analysis thus provides direct support for supersoft sources as likely
progenitors of Type Ia supernovae (SNe Ia).

---------------------------------------------------------
Title: Optical Imaging of Extrasolar Giant Planets From Space
Authors: Heap, S. R.; Burrows, A.; Hubeny, I.; Sudarsky, D.
2004ASPC..321...31H Altcode:
We describe initial plans for detecting extrasolar giant planets with
Eclipse, a 1.8-m space telescope + coronagraph that we will propose
to NASA in the coming year. Our plans take both theoretical and
observational constraints into account.

---------------------------------------------------------
Title: FUSE Observations of Sirius B
Authors: Holberg, J. B.; Barstow, M. A.; Burleigh, M. R.; Kruk, J. W.;
Hubeny, I.; Koester, D.
2004AAS...20510303H Altcode: 2004BAAS...36.1514H
Observations of the white dwarf Sirius B, obtained in the far
ultraviolet with the Far Ultraviolet Spectroscopic Explorer Satellite
(FUSE), reveal the Lyman line region of Sirius B free from significant
photometric contamination from Sirius A. The FUSE spectra provide a
new estimate of the white dwarf gravitational redshift based on the
Lyman lines, as well as Lyman line-based determination of the surface
gravity, and an estimate of the absolute flux from the white dwarf
in the FUV. Sirius B is seen to possess strong quasi-molecular Lyman
line features associated with the Lyman Beta and Lyman Gamma lines. No
spectroscopic features due to any elements other than hydrogen are
found in the Sirius B photosphere.

---------------------------------------------------------
Title: Praesepe and the seven white dwarfs
Authors: Dobbie, P. D.; Pinfield, D. J.; Napiwotzki, R.; Hambly,
N. C.; Burleigh, M. R.; Barstow, M. A.; Jameson, R. F.; Hubeny, I.
2004MNRAS.355L..39D Altcode: 2004astro.ph.10529D; 2004MNRAS.tmp..657D
We report the discovery, from our preliminary survey of the
Praesepe open cluster, of two new spectroscopically confirmed
white dwarf candidate members. We derive the effective
temperatures and surface gravities of WD0837+218 and WD0837+185
(LB5959) to be 17845+555-565K
and log g = 8.48+0.07-0.08
and 14170+1380-1590K and log g =
8.46+0.15-0.16, respectively. Using
theoretical evolutionary tracks we estimate the masses and cooling
ages of these white dwarfs to be 0.92 +/- 0.05Msolar
and 280+40-30 Myr and 0.90 +/- 0.10
Msolar and 500+170-100 Myr,
respectively. Adopting reasonable values for the cluster age we
infer the progenitors of WD0837+218 and WD0837+185 had masses of 2.6
&lt;=M&lt;=McritMsolar and 2.4 &lt;=M&lt;=
3.5Msolar, respectively, where Mcrit is the
maximum mass of a white dwarf progenitor. We briefly discuss these
findings in the context of the observed deficit of white dwarfs in
open clusters and the initial mass-final mass relationship.

---------------------------------------------------------
Title: NLTE Analysis of Chandra and XMM-Newton Spectroscopy of CAL 83
Authors: Lanz, T.; Telis, G. A.; Audard, M.; Paerels, F.; Hubeny, I.
2004HEAD....8.1703L Altcode: 2004BAAS...36..936L
We present a new NLTE model atmosphere analysis of Chandra HRC-S/LETG
and XMM-Newton RGS spectroscopy of the supersoft source CAL 83 in
the LMC. The two spectra are very similar, and reveal a very rich
absorption line spectrum from the hot white dwarf photosphere. We also
report a third X-ray off-state during a later Chandra observation,
showing the recurrent nature of CAL 83. Moreover, we found evidence
of short-timescale variability in the soft X-ray spectrum. The Chandra
and XMM-Newton spectra have been successfully matched with our new NLTE
line-blanketed model atmospheres that explicitly include 74 ions of the
11 most abundant species. We derived the basic stellar parameters of the
hot white dwarf, obtaining the first direct spectroscopic evidence that
the white dwarf is massive. The short timescale of the X-ray off-states
is consistent with a high WD mass. Our analysis, therefore, provides
direct support for supersoft sources as likely progenitors of SN Ia. This work was supported by a grant from the NASA Astrophysics Theory
Program. The Columbia group acknowledges support from NASA to Columbia
University for XMM-Newton mission support and data analysis

---------------------------------------------------------
Title: Relativistic, non-LTE Accretion Disk Models of High State
Black Hole X-ray Binaries
Authors: Davis, S. W.; Blaes, O. M.; Hubeny, I.
2004HEAD....8.1503D Altcode: 2004BAAS...36..926D
We present the first self-consistent calculations of non-LTE,
relativistic accretion disk models applicable to the high/soft state
of black hole X-ray binaries. We include the effects of thermal
Comptonization and bound-free and free-free opacities of abundant ion
species. Taking into account the relativistic propagation of photons
from the local disk surface to an observer at infinity, we present
spectra calculated for a variety of accretion rates, black hole spin
parameters, disk inclinations, and assumptions of the microphysics of
the disk. The last include a variety of viscosity prescriptions and
vertical dissipation profiles, including those recently derived from
radiation MHD simulations of magnetorotational turbulence. The
computed spectra are remarkably robust and independent of many of the
microphysical assumptions. However, the improvement in the physics
results in discrepancies with previous, more simplified models,
that are easily discernible with modern X-ray observatories. Fitting
our spectra with simple, isotropic color-corrected multi-temperature
blackbodies results in frequency dependent, 10 percent discrepancies
that may affect the fitted parameters of other spectral components,
and their interpretation. We compare our models to existing spectral
data of the high/soft state, and recover the observed relation between
luminosity and inner disk temperature. We provide an XSPEC table
model of our spectral calculations for use by the community. This
work was supported by NASA grant NAG5-13228.

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Title: X-ray Heated Accretion Discs around Stellar Mass Black Holes
Authors: Hubeny, I.; Wickramasinghe, D. T.
2004RMxAC..20..200H Altcode: 2004IAUCo.194..200H
We investigate the effects of irradiation on the vertical structure
of accretion discs around black holes and its impact on the emergent
energy distribution. Models are presented for a 10 Solar mass black
hole in a low mass X-ray binary assuming a black body spectrum for
the incident radiation. We show that for a disc annulus at a given
radius, the spectra become increasingly distorted as the incident flux
increases relative to the viscously generated heating flux in the
disc. Significant effects are apparent for rings even at distances
of ~ 10,000 Schwarzschild radii from the black hole for realistic
dilution factors.

---------------------------------------------------------
Title: Theoretical Radii of Transiting Giant Planets: The Case
of OGLE-TR-56b
Authors: Burrows, A.; Hubeny, I.; Hubbard, W. B.; Sudarsky, D.;
Fortney, J. J.
2004ApJ...610L..53B Altcode: 2004astro.ph..5264B
We calculate radius versus age trajectories for the photometrically
selected transiting extrasolar giant planet, OGLE-TR-56b, and find
agreement between theory and observation, without introducing an ad hoc
extra source of heat in its core. The fact that the radius of HD 209458b
seems larger than the radii of the recently discovered OGLE family
of extremely close-in transiting planets suggests that HD 209458b is
anomalous. Nevertheless, our good fit to OGLE-TR-56b bolsters the notion
that the generic dependence of transit radii on stellar irradiation,
mass, and age is, to within error bars, now quantitatively understood.

---------------------------------------------------------
Title: Spectra and Diagnostics for the Direct Detection of
Wide-Separation Extrasolar Giant Planets
Authors: Burrows, Adam; Sudarsky, David; Hubeny, Ivan
2004ApJ...609..407B Altcode: 2004astro.ph..1522B
We calculate as a function of orbital distance, mass, and age the
theoretical spectra and orbit-averaged planet/star flux ratios for
representative wide-separation extrasolar giant planets (EGPs) in
the optical, near-infrared, and mid-infrared. Stellar irradiation of
the planet's atmosphere and the effects of water and ammonia clouds
are incorporated and handled in a consistent fashion. We include
predictions for 12 specific known EGPs. In the process, we derive
physical diagnostics that can inform the direct EGP detection and
remote sensing programs now being planned or proposed. Furthermore, we
calculate the effects of irradiation on the spectra of a representative
companion brown dwarf as a function of orbital distance.

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Title: Evidence for Flash Mixing in He-rich sdB Stars
Authors: Sweigart, Allen V.; Lanz, Thierry; Brown, Thomas M.; Hubeny,
Ivan; Landsman, Wayne B.
2004Ap&SS.291..367S Altcode: 2003astro.ph..8517S
We present FUSE spectra of three He-rich sdB stars. Two of these stars,
PG1544+488 and JL87, reveal extremely strong C III lines, suggesting
that they have mixed triple-α carbon from the deep interior out to
their surfaces. Using TLUSTY NLTE line-blanketed model atmospheres, we
find that PG1544+488 has a surface composition of 96% He, 2% C, and 1%
N. JL87 shows a similar surface enrichment of C and N but still retains
a significant amount of hydrogen. In contrast, the third star, LB1766,
is devoid of hydrogen and strongly depleted of carbon, indicating that
its surface material has undergone CN-cycle processing.

---------------------------------------------------------
Title: Thoughts on the Theory of Irradiated Giant Planets
Authors: Burrows, Adam; Sudarsky, David; Hubeny, Ivan
2004AIPC..713..143B Altcode:
We have derived physical diagnostics that can inform the direct
detection and remote sensing programs of extrasolar giant planets
(EGPs) now being planned or proposed. Stellar irradiation of the
planet's atmosphere and the effects of water and ammonia clouds are
incorporated in a consistent fashion. Whether an EGP is at wide or
close-in separations from its parent star, direct detection will soon
be possible and will yield centrally important physical and chemical
constraints. Our theory of irradiated EGPs is being developed to meet
this challenge.

---------------------------------------------------------
Title: AGN SEDs and the Big Blue Bump
Authors: Shang, Z.; Brotherton, M.; Green, R.; Kriss, G.; Scott, J.;
Kim, J.; Blaes, O.; Hubeny, I.; Zheng, W.; Kaiser, M.; Oegerle, W.;
Hutchings, J.
2004AAS...204.9206S Altcode: 2004BAAS...36Q.824S
We have constructed SEDs of 17 AGNs with quasi-simultaneous
spectrophotometry from 900-9000Å (rest frame), using FUSE, HST and
KPNO. Taking advantage of the short-wavelength coverage, we are able
to study the big blue bump in detail. A wavelength break around
1000Å is seen for most objects, but we do not see a correlation
between the far ultraviolet spectral index and the black hole mass,
as expected by some accretion disk models. We have also compared our
sample with the disk models of different black hole mass, Eddington
ratio, and disk inclination etc., and argue that intrinsic reddening
as well as Comptonization can strongly bias the observed FUV spectral
index. This work is based on data obtained for the Guaranteed Time
Team by the NASA-CNES-CSA FUSE mission operated by the Johns Hopkins
University. Financial support to U. S. participants has been provided
by NASA contract NAS5-32985 and NAS5-26555.

---------------------------------------------------------
Title: The Hot White Dwarf in the Cataclysmic Variable MV Lyrae
Authors: Hoard, D. W.; Linnell, A. P.; Szkody, Paula; Fried, Robert
E.; Sion, Edward M.; Hubeny, Ivan; Wolfe, M. A.
2004ApJ...604..346H Altcode:
We have obtained the first far-ultraviolet spectrum of the nova-like
cataclysmic variable MV Lyrae using the Far Ultraviolet Spectroscopic
Explorer (FUSE). We also obtained contemporaneous optical light
curves and spectra. All data are from a deep faint accretion state
of MV Lyr. We constructed a model for the system using the BINSYN
software package; results from this model include the following: (1)
The white dwarf has Teff=47,000K, photospheric logg=8.25,
and metallicity of Z~0.3Zsolar. (2) The secondary star is
cooler than 3500 K; it contributes nothing to the far-ultraviolet
flux and a varying amount to the optical flux (from 10% at 5200
Å to 60% at 7800 Å). (3) The accretion disk, if present at all,
contributes negligibly to the spectrum of MV Lyr. Irradiation
considerations imply that the mass transfer rate is no more than
3×10-13Msolaryr-1. (4) Assuming
no disk is present, the model optical light curve has an amplitude
approximately 50% larger than that of the sinusoidal modulation (on
the orbital period) in the observed optical light curve, suggesting
that the secondary star might be shaded by a nascent disk and/or have
starspots near the L1 point. (5) The scaling of the model spectrum to
the observed data leads to a distance of d=505+/-50pc to MV Lyr. This research is based on observations with the NASA-CNES-CSA Far
Ultraviolet Spectroscopic Explorer, which is operated for NASA by
the Johns Hopkins University under NASA contract NAS5-32985, and on
observations obtained with the Apache Point Observatory 3.5 m telescope,
which is owned and operated by the Astrophysical Research Consortium.

---------------------------------------------------------
Title: Far Ultraviolet Spectroscopic Explorer Observations of G226-29:
First Detection of the H2 Quasi-molecular Satellite at
1150 Å
Authors: Allard, N. F.; Hébrard, G.; Dupuis, J.; Chayer, P.; Kruk,
J. W.; Kielkopf, J.; Hubeny, I.
2004ApJ...601L.183A Altcode: 2003astro.ph.12494A
We present new far-ultraviolet observations of the pulsating DA white
dwarf G226-29 obtained with the Far Ultraviolet Spectroscopic Explorer
(FUSE). This ZZ Ceti star is the brightest one of its class and the
coolest white dwarf observed by FUSE. We report the first detection of
the broad quasi-molecular collision-induced satellite of Lyβ at 1150
Å, an absorption feature that is due to transitions that take place
during close collisions of hydrogen atoms. The physical interpretation
of this feature is based on recent progress of the line-broadening
theory of the far wing of Lyβ. This predicted feature had never been
observed before, even in laboratory spectra.

---------------------------------------------------------
Title: Flash Mixing on the White Dwarf Cooling Curve: Far Ultraviolet
Spectroscopic Explorer Observations of Three He-rich sdB Stars
Authors: Lanz, Thierry; Brown, Thomas M.; Sweigart, Allen V.; Hubeny,
Ivan; Landsman, Wayne B.
2004ApJ...602..342L Altcode: 2003astro.ph..8440L
We present Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of
three He-rich sdB stars. Two of these stars, PG 1544+488 and JL 87,
reveal extremely strong C III lines at 977 and 1176 Å, while the
carbon lines are quite weak in the third star, LB 1766. We have
analyzed the FUSE data using TLUSTY non-LTE line-blanketed model
atmospheres and find that PG 1544+488 has a surface composition of
96% He, 2% C, and 1% N. JL 87 shows a similar surface enrichment of
carbon and nitrogen, but some significant fraction of hydrogen still
remains in its atmosphere. Finally, LB 1766 has a surface composition
devoid of hydrogen and strongly depleted of carbon, indicating that
its surface material has undergone CN-cycle processing. We interpret
these observations with new evolutionary calculations which suggest
that He-rich sdB stars with C-rich compositions are the progeny of
stars which underwent a delayed helium-core flash on the white-dwarf
cooling curve. During such a flash the interior convection zone will
penetrate into the hydrogen envelope, thereby mixing the envelope
with the He- and C-rich core. Such “flash-mixed” stars will arrive
on the extreme horizontal branch (EHB) with He- and C-rich surface
compositions and will be hotter than the hottest canonical (i.e.,
unmixed) EHB stars. Two types of flash mixing are possible: “deep” and
“shallow,” depending on whether the hydrogen envelope is mixed deeply
into the site of the helium flash or only with the outer layers of the
core. Based on both their stellar parameters and surface compositions,
we suggest that PG 1544+488 and JL 87 are examples of “deep” and
“shallow” flash mixing, respectively. Flash mixing may therefore
represent a new evolutionary channel for producing the hottest EHB
stars. However, flash mixing cannot explain the abundance pattern in
LB 1766, which remains a challenge to current evolutionary models. Based on observations made with the NASA-CNES-CSA Far Ultraviolet
Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins
University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: Quantitative Spectroscopy of O Stars at Low Metallicity:
O Dwarfs in NGC 346
Authors: Bouret, J. -C.; Lanz, T.; Hillier, D. J.; Heap, S. R.;
Hubeny, I.; Lennon, D. J.; Smith, L. J.; Evans, C. J.
2003ApJ...595.1182B Altcode: 2003astro.ph..1454B
We present the results of a detailed analysis of the properties of dwarf
O-type stars in a metal-poor environment. High-resolution, high-quality
ultraviolet and optical spectra of six O-type stars in the H II region
NGC 346 have been obtained from a spectroscopic survey of O stars in the
SMC. Stellar parameters and chemical abundances have been determined
using non-LTE (NLTE) line-blanketed photospheric models calculated
with TLUSTY. Additionally, we have modeled the spectra with the NLTE
line-blanketed wind code CMFGEN to derive wind parameters. Stellar
parameters, chemical abundances, and in particular iron abundances
obtained with the two NLTE codes compare quite favorably. This
consistency demonstrates that basic photospheric parameters of
main-sequence O stars can be reliably determined using NLTE static
model atmospheres. With the two NLTE codes, we need to introduce a
microturbulent velocity to match the observed spectra. Our results
hint at a decrease of the required microturbulent velocity from a value
close to the sonic velocity in early O stars to a low value in late O
stars. As in several recent studies of Galactic, LMC, and SMC stars, we
derive effective temperatures lower than predicted from the widely used
relation between spectral type and Teff, resulting in lower
stellar luminosities and lower ionizing fluxes. From evolutionary tracks
in the H-R diagram, we find the age 3×106 yr for NGC 346. A
majority of the stars in our sample reveal CNO cycle-processed material
at their surface during the main-sequence stage, thus indicating fast
stellar rotation and/or very efficient mixing processes. We obtain an
overall metallicity Z=0.2Zsolar, in good agreement with other
recent analyses of SMC stars. We study the dependence of the mass-loss
rate on the stellar metallicity and find a satisfactory agreement with
recent theoretical predictions for the three most luminous stars of
the sample. The wind momentum-luminosity relation for our sample stars
derived for these stars agrees with previous studies. However, the three
other stars of our sample reveal very weak signatures of mass loss. We
obtain mass-loss rates that are significantly lower than 10-8
Msolar yr-1, below the predictions of radiative
line-driven wind theory by an order of magnitude or more. Furthermore,
evidence of clumping in the wind of main-sequence O stars is provided
by O V λ1371. As in previous studies of O star winds, we are unable to
reproduce this line with homogeneous-wind models, but we have achieved
very good fits with clumped models. Clumped-wind models systematically
yield lower mass-loss rates than theoretical predictions. Based
on observations with the NASA/ESA Hubble Space Telescope, obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under
NASA contract NAS 5-2655. Also based on observations obtained at the
European Southern Observatory (La Silla) and at the Anglo-Australian
Observatory (Siding Spring).

---------------------------------------------------------
Title: Search for forced oscillations in binaries. IV. The eclipsing
binary V436 Per revisited
Authors: Janík, J.; Harmanec, P.; Lehmann, H.; Yang, S.; Božić,
H.; Ak, H.; Hadrava, P.; Eenens, P.; Ruždjak, D.; Sudar, D.; Hubeny,
I.; Linnell, A. P.
2003A&A...408..611J Altcode:
An analysis of new spectroscopic and photoelectric U B V observations,
satisfactorily covering the whole orbital period of V436 Per, together
with existing data allowed us to improve the knowledge of the basic
physical characteristics of the binary and its components. In several
aspects, our new results differ from the findings of Paper I of this
series: in particular, we found that it is the star eclipsed in the
secondary minimum which is slightly more massive and larger than
the optical primary. We also conclude that the apsidal advance -
if present at all - is much slower than that estimated in a previous
study. The orbital period might be increasing by 0.28 s per year but
also this finding is very uncertain and needs verification by future
observations. It is encouraging to note that two completely independent
sets of programs for light-curve solutions lead to identical results. A
notable finding is that both binary components rotate with very
similar - if not identical - rotational periods of 1d.45
and 1d.40, much shorter than what would correspond to a
10d.9 spin-orbit synchronization period at periastron. Rapid
line-profile changes reported earlier could not be confirmed from new,
dedicated series of high-resolution and S/N spectra. This research
is based on spectra from the Dominion Astrophysical Observatory (DAO),
Ondřejov, and Thuringer Landessternwarte (TLS) Tautenburg.

---------------------------------------------------------
Title: VizieR Online Data Catalog: V436 Persei UBV photometry
(Janik+, 2003)
Authors: Janik, J.; Harmanec, P.; Lehmann, H.; Yang, S.; Bozic, H.;
Ak, H.; Hadrava, P.; Eenens, P.; Ruzdjak, D.; Sudar, D.; Hubeny, I.;
Linnell, A. P.
2003yCat..34080611J Altcode:
An analysis of new spectroscopic and photoelectric UBV observations,
satisfactorily covering the whole orbital period of V436 Per, together
with existing data allowed us to improve the knowledge of the basic
physical characteristics of the binary and its components. In several
aspects, our new results differ from the findings of Paper I (Harmanec
et al., 1997, Cat. &lt;J/A+A/319/867&gt;) of this series: In particular,
we found that it is the star eclipsed in the secondary minimum which
is slightly more massive and larger than the optical primary. We also
conclude that the apsidal advance -- if present at all -- is much
slower than that estimated in a previous study. The orbital period
might be increasing by 0.28 s per year but also this finding is
very uncertain and needs verification by future observations. It is
encouraging to note that two completely independent sets of programs
for light-curve solutions lead to identical results. A notable finding
is that both binary components rotate with very similar -- if not
identical -- rotational periods of 1.45d and 1.40d, much shorter than
what would correspond to a 10.9d spin-orbit synchronization period
at periastron. Rapid line-profile changes reported earlier could not
be confirmed from new, dedicated series of high-resolution and S/N
spectra. (2 data files).

---------------------------------------------------------
Title: Anomalous Ultraviolet Line Flux Ratios in the Cataclysmic
Variables 1RXS J232953.9+062814, CE 315, BZ Ursae Majoris, and EY
Cygni, Observed with the Hubble Space Telescope Space Telescope
Imaging Spectrograph
Authors: Gänsicke, Boris T.; Szkody, Paula; de Martino, Domitilla;
Beuermann, Klaus; Long, Knox S.; Sion, Edward M.; Knigge, Christian;
Marsh, Tom; Hubeny, Ivan
2003ApJ...594..443G Altcode: 2003astro.ph..5264G
Brief Hubble Space Telescope Space Telescope Imaging Spectrograph
spectroscopic snapshot exposures of the cataclysmic variables 1RXS
J232953.9+062814, CE 315, BZ UMa, and EY Cyg reveal very large N
V/C IV line flux ratios, similar to those observed in AE Aqr. Such
anomalous line flux ratios have so far been observed in 10 systems and
presumably reflect a different composition of the accreted material
compared to the majority of cataclysmic variables. We discuss the
properties of this small sample in the context of the recent proposal
by Schenker et al. that a significant fraction of the present-day
population of cataclysmic variables may have passed through a phase of
thermal timescale mass transfer. Based on observations made with
the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities
for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: A comparison of DA white dwarf temperatures and gravities
from FUSE Lyman line and ground-based Balmer line observations
Authors: Barstow, M. A.; Good, S. A.; Burleigh, M. R.; Hubeny, I.;
Holberg, J. B.; Levan, A. J.
2003MNRAS.344..562B Altcode: 2003astro.ph..6077B
The observation of the strengths and profiles of the hydrogen
Balmer absorption series is an established technique for determining
the effective temperature and surface gravity of hot H-rich white
dwarf stars. In principle, the Lyman series lines should be equally
useful but, lying in the far-ultraviolet (FUV), are only accessible
from space. Nevertheless, there are situations (for example, where
the optical white dwarf spectrum is highly contaminated by the
presence of a companion) in which use of the Lyman series may be
essential. Therefore, it is important to establish whether or not the
Lyman lines provide an equally valid means of measurement. We have
already made a first attempt to study this problem, comparing Lyman
line measurements from a variety of FUV instruments with ground-based
Balmer line studies. Within the measurement uncertainties, we found
the results from each line series to be broadly in agreement. However,
we noted a number of potential systematic effects that could bias
either measurement. With the availability of the Far Ultraviolet
Spectroscopic Explorer (FUSE) data archive and observations from our
own Guest Observer programmes, we now have an opportunity to examine
the use of the Lyman series in more detail from observations of 16
DA white dwarfs. Here we have data produced by a single instrument
and processed with a uniform data reduction pipeline, eliminating
some of the possible systematic differences between observations of
the same or different stars. We have also examined the scatter in
values derived from multiple observations of the same star, which is
significant. The new results partially reproduce the earlier study,
showing that Balmer and Lyman line determined temperatures are in
good agreement up to ~50 000 K. However, above this value there is an
increasing systematic difference between the Lyman and Balmer line
results, the former yielding the higher temperature. At the moment,
there is no clear explanation of this effect but we think that it
is most likely associated with deficiencies in the detailed physics
incorporated into the stellar model atmosphere calculations. Even so,
the data do demonstrate that, for temperatures below 50 000 K, the
Lyman lines give reliable results. Furthermore, for the hotter stars,
a useful empirical calibration of the relationship between the Lyman
and Balmer measurements has been obtained, which can be applied to
other FUSE observations.

---------------------------------------------------------
Title: A Possible Bifurcation in Atmospheres of Strongly Irradiated
Stars and Planets
Authors: Hubeny, Ivan; Burrows, Adam; Sudarsky, David
2003ApJ...594.1011H Altcode: 2003astro.ph..5349H
We show that under certain circumstances the differences between
the absorption mean and Planck mean opacities can lead to multiple
solutions for an LTE atmospheric structure. Since the absorption and
Planck mean opacities are not expected to differ significantly in the
usual case of radiative equilibrium, nonirradiated atmospheres, the
most interesting situations in which the effect may play a role are
strongly irradiated stars and planets, and also possibly structures
in which there is a significant deposition of mechanical energy, such
as stellar chromospheres and accretion disks. We have presented an
illustrative example of a strongly irradiated giant planet in which
the bifurcation effect is predicted to occur for a certain range of
distances from the star.

---------------------------------------------------------
Title: Non-LTE Model Atmospheres for Late-Type Stars. I. A Collection
of Data for Light Neutral and Singly Ionized Atoms
Authors: Allende Prieto, Carlos; Lambert, David L.; Hubeny, Ivan;
Lanz, Thierry
2003ApJS..147..363A Altcode: 2003astro.ph..3559A; 2003astro.ph..3559P
With the goal of producing a reliable set of model atoms and singly
ionized ions for use in building NLTE model atmospheres, we have
combined measured energy levels, critically compiled line transition
probabilities, and resonance-averaged calculations of photoionization
cross sections. A majority of the elements from Li to Ca are
considered, covering most of the important species in late-type
atmospheres. These include elements that contribute free electrons
and/or continuous opacity in the ultraviolet (e.g., Mg and Si), as well
as trace elements whose abundance determinations rely on ultraviolet
lines (e.g., B from B I lines). The new data complement and, for the
species in common, supersede a previous collection of model atoms
originally designed for use in studies of early-type stars.

---------------------------------------------------------
Title: Erratum: “A Grid of Non-LTE Line-blanketed Model Atmospheres
of O-Type Stars” (<A href="bib_query?2003ApJS...146..417L">ApJS,
146, 417 [2003]</A>)
Authors: Lanz, Thierry; Hubeny, Ivan
2003ApJS..147..225L Altcode:
We have constructed a comprehensive grid of 680 metal line-blanketed,
non-LTE, plane-parallel, hydrostatic model atmospheres for the basic
parameters appropriate to O-type stars. The OSTAR2002 grid considers 12
values of effective temperatures, 27,500K&lt;=Teff&lt;=55,000
K with 2500 K steps, eight surface gravities, 3.0&lt;=logg&lt;=4.75
with 0.25 dex steps, and 10 chemical compositions, from metal-rich
relative to the Sun to metal-free. The lower limit of logg for a
given effective temperature is set by an approximate location of the
Eddington limit. The selected chemical compositions have been chosen
to cover a number of typical environments of massive stars: the
Galactic center, the Magellanic Clouds, blue compact dwarf galaxies
like I Zw 18, and galaxies at high redshifts. The paper contains a
description of the OSTAR2002 grid and some illustrative examples and
comparisons. The complete OSTAR2002 grid is available at our Web site
at <A href="/abs/2003ApJ..S.146..417L">ApJS, 146, 417 [2003]</A>. Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight
Center, Code 681, Greenbelt, MD 20771.

---------------------------------------------------------
Title: Modeling of the Lyman gamma satellites in FUSE spectra of
DA white dwarfs
Authors: Hébrard, G.; Allard, N. F.; Kielkopf, J. F.; Chayer, P.;
Dupuis, J.; Kruk, J. W.; Hubeny, I.
2003A&A...405.1153H Altcode: 2003astro.ph..5356H
We present new theoretical calculations of the line profile of Lyman
gamma that include transitions in which a photon is absorbed by a
neutral hydrogen atom while it interacts with a proton. Models show
that two absorption features located near 992 Å and 996 Å are due
to H-H+ collisions. These quasi-molecular satellites are
similar to those that were identified in the wings of Lyman alpha and
Lyman beta lines of hydrogen-rich white dwarfs. We compute synthetic
spectra that take account of these new theoretical profiles and compare
them to the spectra of four DA white dwarfs that were observed with
FUSE. The models predict the absorption features that are observed in
the wing of Lyman gamma near 995 Å, and confirm that these features
are quasi-molecular satellites. This paper is dedicated in memory
of J. L. Greenstein, discoverer of the quasi-molecular lines in white
dwarfs, who passed away on October 21, 2002.

---------------------------------------------------------
Title: Non-LTE Model Atmospheres for Late-Type Stars. II. Restricted
Non-LTE Calculations for a Solar-like Atmosphere
Authors: Allende Prieto, Carlos; Hubeny, Ivan; Lambert, David L.
2003ApJ...591.1192A Altcode: 2003astro.ph..3560A; 2003astro.ph..3560P
We test our knowledge of the atomic opacity in the solar UV
spectrum. Using the atomic data compiled in the first paper in
this series from modern, publicly available databases, we perform
calculations that are compared with space-based observations of
the Sun. At wavelengths longer than about 2600 Å, LTE modeling can
reproduce quite closely the observed fluxes; uncertainties in the atomic
line data account fully for the differences between calculated and
observed fluxes. At shorter wavelengths, departures from LTE appear
to be important, since our LTE and restricted non-LTE calculations
differ. Analysis of visible/near-IR Na I and O I lines, two species
that produce a negligible absorption in the UV, shows that observed
departures from LTE for these species can be reproduced very accurately
with restricted (fixed atmospheric structure) non-LTE calculations.

---------------------------------------------------------
Title: A Grid of Non-LTE Line-blanketed Model Atmospheres of O-Type
Stars
Authors: Lanz, Thierry; Hubeny, Ivan
2003ApJS..146..417L Altcode: 2002astro.ph.10157L
We have constructed a comprehensive grid of 680 metal line-blanketed,
non-LTE, plane-parallel, hydrostatic model atmospheres for the basic
parameters appropriate to O-type stars. The OSTAR2002 grid considers 12
values of effective temperatures, 27,500K&lt;=Teff&lt;=55,000
K with 2500 K steps, eight surface gravities, 3.0&lt;=logg&lt;=4.75
with 0.25 dex steps, and 10 chemical compositions, from metal-rich
relative to the Sun to metal-free. The lower limit of logg for a given
effective temperature is set by an approximate location of the Eddington
limit. The selected chemical compositions have been chosen to cover a
number of typical environments of massive stars: the Galactic center,
the Magellanic Clouds, blue compact dwarf galaxies like I Zw 18, and
galaxies at high redshifts. The paper contains a description of the
OSTAR2002 grid and some illustrative examples and comparisons. The
complete OSTAR2002 grid is available on-line.

---------------------------------------------------------
Title: Theoretical Spectra and Atmospheres of Extrasolar Giant Planets
Authors: Sudarsky, David; Burrows, Adam; Hubeny, Ivan
2003ApJ...588.1121S Altcode: 2002astro.ph.10216S
We present a comprehensive theory of the spectra and atmospheres of
irradiated extrasolar giant planets. We explore the dependences on
stellar type, orbital distance, cloud characteristics, planet mass, and
surface gravity. Phase-averaged spectra for specific known extrasolar
giant planets that span a wide range of the relevant parameters are
calculated, plotted, and discussed. The connection between atmospheric
composition and emergent spectrum is explored in detail. Furthermore, we
calculate the effect of stellar insolation on brown dwarfs. We review
a variety of representative observational techniques and programs
for their potential for direct detection, in light of our theoretical
expectations, and we calculate planet-to-star flux ratios as a function
of wavelength. Our results suggest which spectral features are most
diagnostic of giant planet atmospheres and reveal the best bands in
which to image planets of whatever physical and orbital characteristics.

---------------------------------------------------------
Title: Heavy-element abundance patterns in hot DA white dwarfs
Authors: Barstow, M. A.; Good, S. A.; Holberg, J. B.; Hubeny, I.;
Bannister, N. P.; Bruhweiler, F. C.; Burleigh, M. R.; Napiwotzki, R.
2003MNRAS.341..870B Altcode: 2003astro.ph..1519B
We present a series of systematic abundance measurements for 25 hot
DA white dwarfs in the temperature range ~20 000-110 000 K, based
on far-ultraviolet spectroscopy with the Space Telescope Imaging
Spectrograph (STIS)/Goddard High Resolution Spectrograph (GHRS)
on-board Hubble Space Telescope, IUE and FUSE. Using our latest
heavy-element blanketed non-local thermodynamic equilibrium (non-LTE)
stellar atmosphere calculations we have addressed the heavy-element
abundance patterns, making completely objective measurements of
abundance values and upper limits using a χ2 fitting
technique to determine the uncertainties in the abundance measurements,
which can be related to the formal upper limits in those stars
where particular elements are not detected. We find that the
presence or absence of heavy elements in the hot DA white dwarfs
largely reflects what would be expected if radiative levitation is
the supporting mechanism, although the measured abundances do not
match the predicted values very well, as reported by other authors
in the past. Almost all stars hotter than ~50 000 K contain heavy
elements. For most of these the spread in element abundances is quite
narrow and similar to the abundances measured in G191-B2B. However,
there is an unexplained dichotomy at lower temperatures with some
stars having apparently pure H envelopes and others having detectable
quantities of heavy elements. The heavy elements present in these
cooler stars are often stratified, lying in the outermost layers of
the envelope. A few strong temperature/evolutionary effects are seen
in the abundance measurements. There is a decreasing Si abundance with
temperature, the N abundance pattern splits into two groups at lower
temperature and there is a sharp decline in Fe and Ni abundance to zero,
below ~50 000 K. When detected, the Fe and Ni abundances maintain an
approximately constant ratio, close to the cosmic value of ~20. For
the hottest white dwarfs observed by STIS, the strongest determinant
of abundance appears to be gravity.

---------------------------------------------------------
Title: A Tale of Two Stars: The Extreme O7 Iaf+ Supergiant AV 83
and the OC7.5 III((f)) star AV 69
Authors: Hillier, D. John; Lanz, T.; Heap, S. R.; Hubeny, I.; Smith,
L. J.; Evans, C. J.; Lennon, D. J.; Bouret, J. C.
2003ApJ...588.1039H Altcode:
We present a detailed study of AV 83, an O7 Iaf+ supergiant,
and AV 69 [OC7.5 III((f))] in the SMC. The stars have similar
effective temperatures and luminosities but show very different wind
signatures. For our study we have used the non-LTE line-blanketed
atmosphere code developed by Hillier and Miller, which explicitly
allows for line blanketing by C, N, O, S, Ar, Ne, Fe, and other
elements. Our study finds that AV 83 has an effective temperature of
approximately 33,000 K and logg~3.25. It has an extended photosphere
as a result of a “low” effective surface gravity and a much denser
wind than main-sequence O stars. We can match the spectrum only by
using a slow velocity law with β~2, a value that is much larger
than the values of around 1 predicted by standard radiation wind
theory. Further, we show that the Hα emission profile in AV 83 is
sensitive to the adopted surface gravity. To fit the spectrum of AV 83,
we have considered conventional models in which the wind is smooth
and alternate models in which the winds are highly clumped. Both
types of winds yield a satisfactory fit to the majority of lines
in the observed spectrum; however, strong UV photospheric lines
and the P V resonance transitions favor a clumped wind. If clumping
is important, it must begin at relatively low velocities (i.e., 30
km s-1, not 300 km s-1). In the smooth wind,
the line force is too small to drive the wind. In the clumped wind,
the line force is generally sufficient to drive the wind, although
there are still some discrepancies around the sonic point. In AV
83, the N abundance is substantially enhanced relative to normal
SMC abundances, while both C and O are SMC-like, consistent with
the presence of internally processed CNO material at the stellar
surface. The N III λ4640 multiplet, which is known to be produced by
dielectronic recombination, is well reproduced by the models. These
lines, and the adjacent C III λ4649 multiplet, show a significant
sensitivity to surface gravity, as well as the usual sensitivity to
abundance and effective temperature. Incoherent electron scattering,
occurring within the photosphere, can explain the broad wings seen
on these lines. We have modeled the Fe spectrum (Fe IV-Fe VI) in the
UV in both AV 83 and AV 69. For stars with an effective temperature
around 33,000 K, the Fe IV-to-Fe V line ratios form a useful effective
temperature diagnostic and give results consistent with those found
from optical and UV line diagnostics. The derived iron abundance, which
is sensitive to the adopted microturbulent velocity, is 0.2-0.4 times
the solar iron abundance in AV 83, while 0.2 solar gives a good fit
for AV 69. The wind of AV 69 is substantially less dense than that of
AV 83. Because of the lack of suitable diagnostics, it is impossible
to constrain the mass-loss rate and velocity law independently. Its
spectrum indicates that it has a similar effective temperature to
AV 83 (Teff~34,000 K), a substantially higher gravity
(logg=3.5) than AV 83, and a CNO abundance pattern that has not been
influenced by internal CNO processing. We show that the N/C abundance
ratio is substantially below solar, in agreement with SMC nebular and
stellar abundance studies. The differences between the spectra of AV
83 and AV 69, and between the derived masses and surface abundances,
are striking. We have examined possible causes, and only one seems
consistent with the observations and our current understanding of
massive star evolution. AV 83 was most likely a fast rotator that
experienced rotationally enhanced mass loss. The presence of enhanced
N but almost normal C and O abundances is a direct indication of
rotationally induced mixing. On the other hand, AV 69 is a slow
rotator. As part of our analyses, we have systematically examined
the influence of the H/He abundance ratio, the mass-loss rate, the
velocity law, the Fe abundance, microturbulence, and clumping on
the theoretical spectrum. We illustrate which lines provide useful
diagnostics and highlight some of the difficulties associated with
spectroscopic analyses of O stars. The spectrum of AV 83 shows the
presence of photospheric absorption lines, the presence of lines formed
at the base of the wind, and numerous wind lines. Since these lines
sample the photosphere and the entire wind, extreme O If supergiants,
such as AV 83, are ideal candidates to probe conditions in stellar
winds and hence further our knowledge of O star winds. Based on
observations with the NASA/ESA Hubble Space Telescope, obtained at the
Space Telescope Science Institute, which is operated by the Association
of Universities for Research in Astronomy, Inc., under NASA contract
NAS5-26555. Based in part, on observations with the NASA-CNES-CSA Far
Ultraviolet Spectroscopic Explorer. FUSE is operated by Johns Hopkins
University under NASA contract NAS5-32985.

---------------------------------------------------------
Title: On the Indirect Detection of Sodium in the Atmosphere of the
Planetary Companion to HD 209458
Authors: Fortney, J. J.; Sudarsky, D.; Hubeny, I.; Cooper, C. S.;
Hubbard, W. B.; Burrows, A.; Lunine, J. I.
2003ApJ...589..615F Altcode: 2002astro.ph..8263F
Using a self-consistent atmosphere code, we construct a new model of
the atmosphere of the transiting extrasolar giant planet HD 209458b to
investigate the disparity between the observed strength of the sodium
absorption feature at 589 nm and the predictions of previous models. For
the atmospheric temperature-pressure profile we derive, silicate and
iron clouds reside at a pressure of several millibars in the planet's
atmosphere. These clouds have significant vertical extent and optical
depth because of our slant viewing geometry and lead to increased
absorption in bands directly adjacent to the sodium line core. Using a
non-LTE sodium ionization model that includes photoionization by stellar
UV flux, collisional processes with H2, and radiative
recombination, we show that the ionization depth in the planet's
atmosphere reaches ~1/2 mbar at the day/night terminator. Ionization
leads to a slight weakening of the sodium feature. We present our
baseline model, including ionization and clouds, which falls near the
observational error bars. The sensitivity of our conclusions to the
derived atmospheric temperature-pressure profile is discussed.

---------------------------------------------------------
Title: An analysis of the optical spectra of the post-asymptotic
giant branch stars LSIV -12 111 and HD 341617
Authors: Ryans, R. S. I.; Dufton, P. L.; Mooney, C. J.; Rolleston,
W. R. J.; Keenan, F. P.; Hubeny, I.; Lanz, T.
2003A&A...401.1119R Altcode:
High spectral resolution and signal-to-noise observations of the
absorption and emission line spectra in two post-asymptotic-giant-branch
(PAGB) stellar candidates, LSIV -12 111 and HD 314617 are discussed. The
absorption line spectra have been analysed using non-LTE model
atmosphere techniques to determine stellar atmospheric parameters
and chemical compositions, both in absolute terms and relative to
a standard star, HD 13841. The atmospheric parameters differ from
previous estimates based on LTE model atmospheres, probably due to
non-LTE effects. In turn these imply stellar masses that are generally
larger than have been previously estimated. Both PAGB candidates have
relative uniform underabundances of metals with mean values of -0.35
dex for LSIV -12 111 and -0.50 dex for HD 314617. Furthermore,
their abundance patterns are remarkably similar to that observed
for optically bright, F-type PAGBs. From the emission spectra, the
plasma parameters and angular size of the circumstellar material are
constrained, and these are consistent with previous estimates and with
a PAGB evolutionary stage.

---------------------------------------------------------
Title: A New Formation Mechanism for the Hottest Horizontal-Branch
Stars
Authors: Sweigart, A. V.; Brown, T. M.; Lanz, T.; Landsman, W. B.;
Hubeny, I.
2003ASPC..296..313S Altcode: 2003nhgc.conf..313S; 2002astro.ph..7343S
Stars with very large mass loss on the red-giant branch can undergo
the helium flash while descending the white-dwarf cooling curve. Under
these conditions the flash convection zone will mix the hydrogen
envelope with the hot helium- burning core. Such “flash-mixed”
stars will arrive on the extreme horizontal branch (EHB) with helium-
and carbon-rich envelopes and will lie at higher temperatures than the
hottest canonical (i.e., unmixed) EHB stars. Flash mixing provides a
new evolutionary channel for populating the hot end of the EHB and may
explain the origin of the high gravity, helium-rich sdO and sdB stars.

---------------------------------------------------------
Title: Multidimensional ALI Radiative Transfer in Cartesian,
Cylindrical and Spherical Coordinate Systems
Authors: van Noort, M.; Hubeny, I.; Lanz, T.
2003ASPC..288..445V Altcode: 2003sam..conf..445V
A new Radiative Transfer code that can calculate the non-LTE line
transfer problem in a two-level atom formulation in Cartesian,
cylindrical and spherical coordinate systems is presented. The transfer
equation is solved using the ALI and the short characteristics methods,
while allowing for an arbitrary 3-dimensional velocity field. The
code is modularised so that changing geometry can be accomplished by
simply setting a switch, and parallelised for use on a networked PC
cluster to increase computational speed. The spatial parallelization
method is employed. It is found to be robust and efficient, while not
relying heavily on fast communication. The internal accuracy of
the code is tested extensively in all three geometries and is shown
to be in good agreement with appropriate 1-D solutions.

---------------------------------------------------------
Title: Comparisons between Observed and Computed Visible and Near-UV
Spectra of Vega
Authors: García-Gil, A.; Allende Prieto, C.; García López, R. J.;
Hubeny, I.
2003ASPC..288..145G Altcode: 2003sam..conf..145G
By using the Synspec program with different LTE and NLTE atmospheric
models of Alpha Lyrae (Vega, spectral type A0V), we obtain different
emitted fluxes. Taking into account the distance from Hipparcos, it
is obtained the spectrum that would be observed from Earth for each
model. This spectrum is compared with UV calibrations from the IUE
and UARS satellites and visible ground-based observations. Absolute
fluxes from the SOLSTICE experiment onboard UARS provide an independent
source to assess the quality of the available data. The main goal
of this work is to better understand and solve the controversy about
the missing opacity problem in the UV. This is just the first step in
that direction.

---------------------------------------------------------
Title: A Detailed View of the Photosphere of the Hot White Dwarf
G191-B2B from STIS
Authors: Holberg, J. B.; Barstow, M. A.; Hubeny, I.; Sahu, M. S.;
Bruhweiler, F. C.; Landsman, W. B.
2003ASPC..291..383H Altcode: 2003hslf.conf..383H
No abstract at ADS

---------------------------------------------------------
Title: Quantitative analysis of O-type stars properties, at low
metallicity
Authors: Bouret, Jean-Claude; Lanz, Thierry M.; Heap, Sara R.; Hubeny,
Ivan; Hillier, D. John; Lennon, Daniel J.; Evans, Christopher J.;
Smith, Linda J.
2003IAUS..212..156B Altcode:
We have investigated the properties of main-sequence O-type stars in
the SMC. Mass-loss rates, luminosities and Teff are much
smaller for these stars than for Galactic ones, resulting in a steeper
wind-momentum relation.

---------------------------------------------------------
Title: Accelerated Lambda Iteration: An Overview
Authors: Hubeny, I.
2003ASPC..288...17H Altcode: 2003sam..conf...17H
No abstract at ADS

---------------------------------------------------------
Title: A Grid of Non-LTE Line-Blanketed Model Atmospheres of O Stars
Authors: Lanz, T.; Hubeny, I.
2003ASPC..288..157L Altcode: 2003sam..conf..157L
We have constructed a grid of over 300 NLTE fully-blanketed model
atmospheres covering the parameter range of O-type stars at various
metallicities. We have assumed a plane-parallel geometry, hydrostatic
and radiative equilibria. The models incorporate about 100,000 NLTE
atomic levels of over 40 ions of H, He, C, N, O, Ne, Si, P, S, Fe,
and Ni, which are grouped into about 900 superlevels. The models will
be made publicly available in the coming months.

---------------------------------------------------------
Title: Stellar Atmosphere Modeling
Authors: Hubeny, Ivan; Mihalas, Dimitri; Werner, Klaus
2003ASPC..288.....H Altcode: 2003sam..conf.....H
No abstract at ADS

---------------------------------------------------------
Title: Quantitative Spectroscopy of O stars at low metallicity. O
Dwarfs in NGC 346
Authors: Bouret, J. -C.; Lanz, T.; Heap, S.; Hubeny, I.; Hillier,
D.; Lennon, D.; Evans, C. J.; Smith, L.
2003sf2a.conf..499B Altcode: 2003sf2a.confE.214B
Dwarf O-type stars have been observed in NGC 346, the largest HII
region in the SMC. UV and optical spectra have been analysed with
NLTE line-blanketed models. These stars reveal CNO- cycle processed
material at their surface, indicating fast stellar rotation and/or
very efficient mixing processes. We obtain an overall metallicity
Z = 0.2 Zodot. The dependence of the mass loss rate with
the stellar metallicity is in good agreement with recent theoretical
predictions for three most luminous stars of the sample. However, three
other stars reveal mass loss rates that are significantly lower than
10-8 msol/yr, which is below the predictions of radiative
line-driven wind theory by an order of magnitude or more. Clumping in
the wind is evidenced by OV at 1371. Clumped wind models systematically
yield lower mass loss rates than theoretical predictions.

---------------------------------------------------------
Title: NLTE Line Formation in Late-Type Stellar Atmospheres
Authors: Allende Prieto, C.; Hubeny, I.; Lambert, D. L.; Lanz, T.
2003IAUS..210P.A24A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Quasi-molecular lines in Lyman wings of cool DA white dwarfs;
Application to FUSE observations of G 231--40
Authors: Allard, N. F.; Hébrard, G.; Ferlet, R.; Vidal-Madjar, A.;
Hubeny, I.; Lacour, S.
2003ASIB..105..163A Altcode: 2003whsw.conf..163A; 2003whdw.conf..163A
We present new theoretical calculations of Lyman β profiles which
include perturbations by both neutral hydrogen and protons. They are
used to improve theoretical modeling of synthetic spectra for cool DA
white dwarfs.

---------------------------------------------------------
Title: Model Photospheres with Accelerated Lambda Iteration
Authors: Hubeny, I.; Lanz, T.
2003ASPC..288...51H Altcode: 2003sam..conf...51H
No abstract at ADS

---------------------------------------------------------
Title: Lyman gamma satellites due to H-H+ collisions:
Application to FUSE observations of hot white dwarfs
Authors: Allard, N. F.; Bourdreux, S.; Kielkopf, J. F.; Hébrard, G.;
Désert, J. -M.; Ferlet, R.; Vidal-Madjar, A.; Hubeny, I.; Peek, J. M.
2003ASIB..105..161A Altcode: 2003whsw.conf..161A; 2003whdw.conf..161A
New theoretical calculations of Lyman γ profiles perturbed by protons
allow us to identificate a large feature near 995Å in FUSE observations
of hot white dwarfs. This feature is due to quasi-molecular Lyman γ
satellites due to H-H+ collisions near 991 and 996Å.

---------------------------------------------------------
Title: Preliminary Analysis of an Extreme Helium sdO Star: BD+25 4655
Authors: Budaj, J.; Elkin, V.; Hubeny, I.
2003IAUS..210P.E44B Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Atomic Data in Non-LTE Model Stellar Atmospheres
Authors: Lanz, T.; Hubeny, I.
2003ASPC..288..117L Altcode: 2003sam..conf..117L
Extensive sources of atomic data are required to calculate NLTE
line-blanketed model atmospheres. I will discuss their implementation in
our NLTE model atmosphere code, TLUSTY, and in our spectrum synthesis
code, SYNSPEC, with a particular attention to the statistical methods
required to incorporate the opacity of iron-peak elements. A few
typical results and comparisons to other codes will be shown.

---------------------------------------------------------
Title: NLTE Line-Blanketed Model Stellar Atmospheres
Authors: Lanz, T.; Hubeny, I.; Heap, S. R.
2003IAUS..210...67L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: FUSE Observations of He-rich sdB Stars
Authors: Lanz, T.; Brown, T. M.; Sweigart, A. V.; Hubeny, I.; Landsman,
W. B.
2002AAS...20111308L Altcode: 2002BAAS...34.1285L
Subdwarf B stars all show significant abundance anomalies. Most are
extremely deficient in helium and selected light elements, but a
minority are helium-rich. Deficiencies in helium and heavier elements
have been attributed to gravitational settling, but the helium-enriched
members of the class present a puzzling exception, because radiative
levitation should be too weak to prevent gravitational settling. New
evolutionary calculations suggest that these helium-rich sdB stars are
the result of a delayed helium-core flash on the white dwarf cooling
curve. The convective zone produced by this flash will penetrate
the hydrogen envelope, mixing hydrogen into the hot helium-burning
interior, where it is rapidly consumed. The resulting star should show
greatly enhanced helium and carbon with respect to the other heavy
elements. This phenomenon is analogous to the born again scenario
for producing hydrogen-deficient R CrB stars following a very late
helium-shell flash. We have recently obtained FUSE spectra of two
helium-rich sdB stars, revealing huge C III lines at 977 and 1176
Å. Our preliminary analysis yields a surface composition of 97% He
and 3% C, in agreement with the new evolutionary scenario. This work
is supported in part by NASA grant NAG5-12383.

---------------------------------------------------------
Title: Erratum: “Isolating Clusters with Wolf-Rayet Stars in I Zw
18” (<A href="/abs/2002ApJ...579L..75B">ApJ, 579, L75 [2002]</A>)
Authors: Brown, Thomas M.; Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry;
Lindler, Don
2002ApJ...581L.129B Altcode:
During publication, the software used to process Figure 1 corrupted
the Y-axis label. The label should read “Flux (10-16
ergs s-1 cm-2 Å-1)” instead of the
published “Flux (1016 ergs s1 cm2
Å1).” The Press sincerely regrets this error.

---------------------------------------------------------
Title: Observations of Novalike Cataclysmic Variables with the Far
Ultraviolet Spectroscopic Explorer
Authors: Hoard, D. W.; Szkody, Paula; Linnell, Albert; Long, K.;
Sion, E. M.; Hubeny, I.; Knigge, C.
2002AAS...201.3304H Altcode: 2002BAAS...34.1155H
Observations at short wavelengths probe the innermost region of
cataclysmic variables (CVs), at the zone of interaction between the
accretion flow or disk and the boundary layer and/or white dwarf. We
present new spectra of three CVs (DW UMa, LS Peg, MV Lyr) obtained
with the Far Ultraviolet Spectroscopic Explorer satellite. These three
systems are members of the novalike class of CV, which is characterized
by high accretion rates and prominent disks. The mean far-UV spectra
of these three novalike CVs are remarkably different. In addition, our
time-resolved FUSE spectra of DW UMa may support the recent suggestion
that a weak white dwarf magnetic field is present in some or all
novalikes (e.g., the SW Sextantis stars). (Our FUSE observations of
another CV, the magnetic system YY Dra, will be presented at the AAS
meeting by A. Linnell, et al.)

---------------------------------------------------------
Title: Isolating Clusters with Wolf-Rayet Stars in I Zw 18
Authors: Brown, Thomas M.; Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry;
Lindler, Don
2002ApJ...579L..75B Altcode: 2002astro.ph.10089B
We present UV images and spectra of the starburst galaxy I Zw 18,
taken with the Space Telescope Imaging Spectrograph. The high
spatial resolution of these data allows us to isolate clusters
containing Wolf-Rayet stars of the subtype WC. Our far-UV spectra
clearly show C IV λλ1548, 1551 and He II λ1640 emission of WC
stars in two clusters: one within the bright (northwest) half of I
Zw 18 and one on the outskirts of this region. The latter spectrum
is unusual because the C IV is seen only in emission, indicating a
spectrum dominated by WC stars. These data also demonstrate that the
H I column in I Zw 18 is strongly peaked in the fainter (southeast)
half of I Zw 18, with a column depth far larger than that reported in
previous analyses. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS 5-26555. These observations
are associated with proposal 9054.

---------------------------------------------------------
Title: Quasi-molecular lines in Lyman wings of cool DA white
dwarfs. Application to FUSE observations of G 231-40
Authors: Hébrard, G.; Allard, N. F.; Hubeny, I.; Lacour, S.; Ferlet,
R.; Vidal-Madjar, A.
2002A&A...394..647H Altcode: 2002astro.ph..8372H
We present new theoretical calculations of the total line profiles
of Lyman alpha and Lyman beta which include perturbations by both
neutral hydrogen and protons and all possible quasi-molecular states of
H2 and H2+. They are used to improve
theoretical modeling of synthetic spectra for cool DA white dwarfs. We
compare them with FUSE observation of G 231-40. The appearance of the
line wings between Lyman alpha and Lyman beta is shown to be sensitive
to the relative abundance of hydrogen ions and neutral atoms, and
thereby to provide a temperature diagnostic for stellar atmospheres
and laboratory plasmas.

---------------------------------------------------------
Title: Theoretical Spectral Models of T Dwarfs at Short Wavelengths
and Their Comparison with Data
Authors: Burrows, Adam; Burgasser, Adam J.; Kirkpatrick, J. Davy;
Liebert, James; Milsom, J. A.; Sudarsky, D.; Hubeny, I.
2002ApJ...573..394B Altcode: 2001astro.ph..9227B
We have generated new, self-consistent spectral and atmosphere models
for the effective temperature range 600-1300 K thought to encompass the
known T dwarfs. For the first time, theoretical models are compared
with a family of measured T dwarf spectra at wavelengths shortward
of ~1.0 μm. By defining spectral indices and standard colors in the
optical and very near-infrared, we explore the theoretical systematics
with Teff, gravity, and metallicity. We conclude that the
short-wavelength range is rich in diagnostics that complement those in
the near-infrared now used for spectral subtyping. We also conclude that
the wings of the Na D and K I (7700 Å) resonance lines and aggressive
rainout of heavy metals (with the resulting enhancement of the sodium
and potassium abundances at altitude) are required to fit the new
data shortward of 1.0 μm. Furthermore, we find that the water bands
weaken with increasing gravity, that modest decreases in metallicity
enhance the effect in the optical of the sodium and potassium lines,
and that at low values of Teff, in a reversal of the
normal pattern, optical spectra become bluer with further decreases in
Teff. Moreover, we conclude that T dwarf subtype is not a
function of Teff alone but that it is a nontrivial function
of gravity and metallicity as well. As do Marley and coworkers in their
2002 work, we see evidence in early T dwarf atmospheres of a residual
effect of clouds. With cloudless models, we obtain spectral fits to
the two late T dwarfs with known parallaxes, but a residual effect
of clouds on the emergent spectra of even late T dwarfs cannot yet be
discounted. However, our focus is not on detailed fits to individual
objects but on the interpretation of the overall spectral and color
trends of the entire class of T dwarfs, as seen at shorter wavelengths.

---------------------------------------------------------
Title: Towards a spherical code for the evaluation of solar UV-bands
that influence the chemical composition in the stratosphere
Authors: Haberreiter, M.; Hubeny, I.; Rozanov, E.; Rüedi, I.; Schmutz,
W.; Wenzler, T.
2002ESASP.508..209H Altcode: 2002soho...11..209H
We present our analysis of data taken by SUSIM onboard UARS. We
reconstruct the variability of the UV irradiance and compare it to
available data. Up to now we model the solar irradiance according to
the 3-component model by Unruh et al. (1999) based on LTE synthetic
spectra modeled with Kurucz' ATLAS9 code. Our new approach will be that
with COSI (COde for Solar Irradiance) we model solar continuum and
line formation in spherical symmetry and in non-local thermodynamic
equilibrium (non-LTE). We present our first synthetic solar spectra
(calculated in LTE) and validate them against spectra computed with
Kurucz' ATLAS9 code.

---------------------------------------------------------
Title: Quasi-molecular lines in Lyman wings of cool DA white dwarfs:
Application to FUSE observations of G231-40
Authors: Hébrard, G.; Allard, N. F.; Hubeny, I.; Lacour, S.; Ferlet,
R.; Vidal-Madjar, A.
2002sf2a.conf..527H Altcode:
Comprehensive, calculations of the Lyman-alpha and Lyman-beta line wings
which include perturbations by both neutral hydrogen and protons and
all possible quasi-molecular states of H2 and H2+ are used to predict
synthetic spectra for cool DA white dwarfs. They are compared with
FUSE observation of G231-40. The appearance of the line wings between
Lyman-alpha and Lyman-beta is shown to be sensitive to the relative
abundance of hydrogen ions and neutral atoms, and thereby to provide
a temperature diagnostic for stellar atmospheres and laboratory plamas.

---------------------------------------------------------
Title: Lyman gamma satellites due to H-H+ collisions: Application
to FUSE observations of hot white dwarfs
Authors: Bourdreux, S.; Allard, N.; Kielkopf, J.; Desert, J. -M.;
Hebrard, G.; Hubeny, I.; Peek, J.; Ferlet, R.; Vidal-Madjar, A.
2002sf2a.conf..509B Altcode:
Lyman gamma profiles perturbed by protons using molecular potential
of Peek allow us to identificate a large feature near 995 A in FUSE
observations of hot white dwarfs. It is visible also in the HUT
spectrum of Wolf1346 and in some ORFEUS spectra. This feature is due
to Quasi-molecular Lyman gamma satellites due to H-H+ collisions near
991 and 996A.

---------------------------------------------------------
Title: A High S/N View of the Photosphere of the Hot White Dwarf
G191-B2B from STIS
Authors: Holberg, J. B.; Barstow, M. A.; Hubeny, I.; Sahu, M. S.;
Bruhweiler, F. C.; Landsman, W. B.
2002AAS...200.7205H Altcode: 2002BAAS...34..765H
We have assembled a unique, high signal-to-noise (S/N) spectrum of
the hot metal-rich DA white dwarf, G191-B2B, from a coaddition of 22
E140H and 40 E230H STIS calibration spectra. This superb data set fully
covers the wavelength range from 1155 A to 3169 A with S/N ranging from
over 100 to 30; affording an unprecedentedly detailed view of the UV
stellar absorption lines in a white dwarf star. We use this spectrum
to search for a number of previously undiscovered heavy elements in
the G191-B2B photosphere and to estimate elemental abundances and
place upper limits on the abundances of a number of key species. An
accurate photospheric inventory of heavy elements in hot degenerate
stars is basic to our understanding of the chemical diffusion in these
stars. These observations highlight the wealth of information present
in echelle resolution spectra of white dwarfs such as G191-B2B and
the need for a succeeding generation of UV instrumentation capable
of providing data of this type for other degenerate stars. This work
funded by STScI Grant AR 9202.

---------------------------------------------------------
Title: Multidimensional Non-LTE Radiative Transfer. I. A Universal
Two-dimensional Short-Characteristics Scheme for Cartesian, Spherical,
and Cylindrical Coordinate Systems
Authors: van Noort, Michiel; Hubeny, Ivan; Lanz, Thierry
2002ApJ...568.1066V Altcode:
We have developed an efficient and robust two-dimensional non-LTE
radiation transfer solver appropriate for line transfer in the
equivalent two-level atom formalism. The numerical method applies
the accelerated lambda iteration technique together with the
short-characteristics scheme. The code presented in this paper
incorporates all three standard geometries (Cartesian, cylindrical,
and spherical) in a transparent way while allowing for arbitrary
(three-dimensional) velocity fields. The geometry-specific parts of
the radiative transfer solver are modularized so that the change of
geometry is accomplished by simply setting the appropriate switch. We
have also developed a parallel version of the code, in which we
use a parallelization in spatial subdomains, and showed that such a
scheme is sufficiently robust. We have performed a number of tests
of the performance of the solver in all three geometries. Finally,
we discuss the internal accuracy of the transfer solutions depending
on the number of spatial, angular, and frequency grid points.

---------------------------------------------------------
Title: FUSE observations of PG1342+444: new insights into the nature
of the hottest DA white dwarfs
Authors: Barstow, M. A.; Good, S. A.; Holberg, J. B.; Burleigh, M. R.;
Bannister, N. P.; Hubeny, I.; Napiwotzki, R.
2002MNRAS.330..425B Altcode:
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations
of the very hot (Teff~60000K) DA white dwarf PG1342+444,
describing our data reduction and analysis techniques. The spectrum
reveals a number of photospheric absorption lines from high ionization
species along with numerous interstellar features. The photospheric
detections include the 1031.9- and 1037.0-Å OVI lines which are seen
for the first time in a hot DA atmosphere and are usually associated
with the much hotter PG1159 stars and so-called OVI central stars
of planetary nebulae. Estimates of the stellar effective temperature
made independently using both the Balmer and Lyman series lines are
in disagreement (Teff~67000 and ~54000K respectively),
when taking into account just the statistical uncertainties in the
analyses. However, the presence of weak absorption from the CIII
multiplet near 1176Å, which is predicted to be much stronger if the
star were as cool as the Lyman measurement suggests, leads us to favour
the higher temperature. PG1342+444 appears to have enhanced C, Fe and Ni
abundances in its atmosphere compared with all the other G191-B2B-like
DA white dwarfs, which might affect the temperature structure of the
atmosphere if not homogeneously distributed, as assumed in this study.

---------------------------------------------------------
Title: HST STIS spectroscopy of VW Hydri during early quiescence
following a superoutburst
Authors: Sion, E. M.; Cheng, F. -H.; Szkody, P.; Gänsicke, B. T.;
Sparks, W. M.; Hubeny, I.
2002ASPC..261...69S Altcode: 2002pcvr.conf...69S
We have obtained two Hubble Space Telescope STIS spectra of the
SU UMa-type dwarf nova VW Hydri at two points in early quiescence
following a superoutburst. Multi-spectral component model fitting to
the data reveals a 23 000 K heated white dwarf with chemical abundances
indicative of nucleosynthetic processing (supporting a link between
dwarf novae and classical novae), and a rotation of 400 km/s. Our
best-fit models result when a rapidly spinning accretion belt with
solar composition is present in addition to the cooler, more slowly
rotating photosphere.

---------------------------------------------------------
Title: The Origin of Hot Subluminous Horizontal-Branch Stars in ω
Centauri and NGC 2808
Authors: Sweigart, A. V.; Brown, T. M.; Lanz, T.; Landsman, W. B.;
Hubeny, I.
2002ASPC..265..261S Altcode: 2002ocuw.conf..261S; 2002astro.ph..3063S
Hot subluminous stars lying up to 0.7 mag below the extreme horizontal
branch (EHB) are found in the UV color-magnitude diagrams of omega Cen
and NGC 2808. Such stars are unexplained by canonical HB theory. In
order to explore the origin of these subluminous stars, we evolved a
set of low-mass stars from the main sequence through the helium-core
flash to the HB for a wide range in the mass loss along the red-giant
branch (RGB). Stars with the largest mass loss evolve off the RGB to
high effective temperatures before igniting helium in their cores. Our
results indicate that the subluminous EHB stars, as well as the gap
within the EHB of NGC 2808, can be explained if these stars undergo
a late helium-core flash on the white-dwarf cooling curve. Under
these conditions the flash convection will penetrate into the stellar
envelope, thereby mixing most, if not all, of the envelope hydrogen
into the hot helium- burning interior. This phenomenon is analogous
to the "born-again" scenario for producing hydrogen-deficient stars
during a very late helium-shell flash. "Flash mixing" greatly enhances
the envelope helium and carbon abundances and, as a result, leads to
an abrupt increase in the HB effective temperature. We argue that
the EHB gap in NGC 2808 is caused by this theoretically predicted
dichotomy in the HB morphology. Using new helium- and carbon-rich
stellar atmospheres, we show that the flash-mixed stars have the same
reduced UV flux as the subluminous EHB stars. Moreover, we demonstrate
that models without flash mixing lie, at most, ~0.1 mag below the EHB
and hence fail to explain the observations. Flash mixing may also
provide a new evolutionary channel for producing the high gravity,
He-rich sdO and sdB stars.

---------------------------------------------------------
Title: The D/H Abundance Ratio in Local Interstellar Gas
Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Holberg, J.;
Hubeny, I.; Barstow, M.; Linsky, J.; Gull, T.; Lindler, D.; Lanz,
T.; Feggans, K.
2001AAS...199.1101S Altcode: 2001BAAS...33.1323S
Variations of the D/H ratio on scales of 100 pc are important for two
reasons: (1) they complicate the use of deuterium as a cosmological
probe, and (2) they imply non-uniform deuterium production/destruction
and an inefficient mixing of interstellar gas. The Local Interstellar
Medium (LISM) is an ideal location to test whether the D/H abundance
ratio varies or not, because the heating sources and radiation
fields are well-studied and we have a detailed knowledge of the
three-dimensional (3D) structure of the diffuse clouds in the LISM. This
detailed knowledge of the number, structure and velocities of the
absorbing clouds greatly helps in reducing errors in the derived
D/H ratios. We are working on a project to obtain high-precision
D/H abundance ratios in the interstellar gas within 100 pc. For this
purpose, we primarily use HST-STIS data towards nearby hot, white dwarfs
(WDs). We have also begun an HST archival research project to study
about 20 WD sightlines. In this poster, we present new STIS observations
for GD 153 and HZ 43 and report the results of our HST Archival project.

---------------------------------------------------------
Title: The Origin of Hot Subluminous Horizontal-Branch Stars in ω
Cen and NGC 2808
Authors: Sweigart, A. V.; Brown, T. M.; Moehler, S.; Lanz, T.;
Landsman, W. B.; Hubeny, I.; Dreizler, S.; Napiwotzki, R.
2001AAS...19913704S Altcode: 2001BAAS...33.1512S
Ultraviolet (UV) observations of the globular clusters ω Cen and NGC
2808 have revealed an unexpected population of hot subluminous stars
lying up to 0.7 mag below the extreme horizontal branch (EHB) in the
UV, which are not explained by canonical stellar models. In order to
explore the evolutionary status of these stars, we have evolved a set
of low-mass stars from the main sequence through the helium flash to
the horizontal branch (HB) for a wide range in the mass loss along the
red-giant branch (RGB). Stars with the largest mass loss evolve off
the RGB to high effective temperatures before igniting helium in their
cores. Our results indicate that the subluminous stars can be explained
if these stars undergo a late helium flash while descending the white
dwarf cooling curve. Under these conditions the convection zone produced
by the helium flash will penetrate into the stellar envelope, thereby
mixing the envelope hydrogen into the hot helium-burning interior, where
it is rapidly consumed. Such “flash-mixed” stars will have helium-
and carbon-rich envelopes and will lie at higher effective temperatures
than the hottest canonical (i.e., unmixed) EHB stars. Using new stellar
atmospheres, we show that these changes in the envelope abundances
will suppress the UV flux in the spectra of the flash-mixed stars by
the amount needed to explain the hot subluminous stars in ω Cen and
NGC 2808. To test this evolutionary scenario, we have obtained medium
resolution spectra of a sample of the hottest HB stars in ω Cen. We
find that these stars are indeed helium-rich compared to classical
EHB stars and also considerably hotter than the hottest EHB models
without flash mixing.

---------------------------------------------------------
Title: Non-LTE, Relativistic Accretion Disk Fits to 3C 273 and the
Origin of the Lyman Limit Spectral Break
Authors: Blaes, Omer; Hubeny, Ivan; Agol, Eric; Krolik, Julian H.
2001ApJ...563..560B Altcode: 2001astro.ph..8451B
We fit general relativistic, geometrically thin accretion disk models
with non-LTE atmospheres to near-simultaneous multiwavelength data of
3C 273, extending from the optical to the far-ultraviolet. Our model
fits show no flux discontinuity associated with a hydrogen Lyman edge,
but they do exhibit a spectral break which qualitatively resembles that
seen in the data. This break arises from relativistic smearing of Lyman
emission edges which are produced locally at tens of gravitational radii
in the disk. We discuss the possible effects of metal line blanketing on
the model spectra, as well as the substantial Comptonization required
to explain the observed soft X-ray excess. Our best-fit accretion
disk model underpredicts the near-ultraviolet emission in this source
and also has an optical spectrum which is too red. We discuss some of
the remaining physical uncertainties and suggest in particular that
an extension of our models to the slim disk regime and/or including
nonzero magnetic torques across the innermost stable circular orbit
may help resolve these discrepancies.

---------------------------------------------------------
Title: NLTE Models of Vertical structure of Accretion Disks around
Stellar Mass Black Holes
Authors: Hubeny, I.; Blaes, O.; Krolik, J. H.; Agol, E.; Lanz, T.
2001AAS...19915908H Altcode: 2002BAAS...34Q.569H
Recent upgrades of our computer program TLUSDISK are briefly
described. These include a self-consistent treatment of Compton
scattering, and the effects of X-ray continuum opacities of the most
important metal species (C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni). In the
case the central degenerate object is a neutron star or a black hole,
we allow for a full general relativistic treatment. We show the effects
of Comptonization and metal opacities on the structure of disk under
various conditions. We also present a simple analytic prescription
for the vertical temperature structure of the disk in the presence
of Comptonization, and show under what conditions a hot outer layer
(a corona) is formed.

---------------------------------------------------------
Title: Solar UV Radiation and the Origin of Life on Earth
Authors: Heap, S. R.; Gaidos, E.; Hubeny, I.; Lanz, T. M.
2001AAS...199.0314H Altcode: 2001BAAS...33.1305H
We have embarked on a program aimed at understanding the atmosphere of
the early Earth, because of its importance as a greenhouse, radiation
shield, and energy source for life. Here, we give a progress report on
the first phase of this program: to establish the UV radiation from
the early Sun. We are presently obtaining ultraviolet spectra (STIS,
FUSE, EUVE) of carefully selected nearby, young solar-type stars,
which act as surrogates for the early Sun. We are currently making
detailed non-LTE analyses of the spectra and constructing models of
their photospheres + chromospheres. Once validated, these models will
allow us to extrapolate our theoretical spectra to unobserved spectral
regions, and to proceed to the next step: to develop photochemical
models of the pre-biotic and Archean atmosphere of the Earth.

---------------------------------------------------------
Title: NLTE Model Atmospheres of Supersoft X-Ray Sources
Authors: Lanz, T.; Hubeny, I.
2001AAS...199.1712L Altcode: 2001BAAS...33.1336L
Supersoft X-Ray Sources are most probably very hot white dwarfs
undergoing steady nuclear burning in their envelopes. They are high
luminosity (L &gt; 0.1 L Edd), soft (kT≈ 50 eV) low
mass binary with a white dwarf primary accreting at a sub-Eddington
rate. High-resolution spectra in the soft X-rays are now becoming
available with Chandra and XMM-Newton. Photospheric line spectroscopy
has the potential to provide accurate diagnostics of the white dwarf
properties. We have therefore embarked on a project aimed at providing
new NLTE model atmospheres of supersoft sources. We are building a
series of model atmospheres of hot white dwarfs (20&lt;= kT&lt;= 100
eV; 8&lt;= log g&lt;= 10) with various surface composition. The models
incorporate the most abundant species, H, He, C, N, O, Ne, Mg, Si, S,
and Fe, which are allowed to depart from Saha-Boltzmann statitics. Our
models include the influence of multi-level model atoms, metal line
blanketing, and Auger ionization on the atmospheric structure. A
preliminary comparison of models calculated assuming coherent (Thomson)
and non-coherent (Compton) scattering is presented. This work was
supported through a NASA ATP grant (NRA 00-01-ATP-153).

---------------------------------------------------------
Title: Hubble Space Telescope STIS Spectroscopy of VW Hydri during
Early Quiescence following a Superoutburst
Authors: Sion, Edward M.; Cheng, Fu-Hua; Szkody, Paula; Gänsicke,
Boris; Sparks, Warren M.; Hubeny, Ivan
2001ApJ...561L.127S Altcode:
Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS)
observations of VW Hydri 2 and 7 days after the end of a superoutburst
reveal a heated white dwarf with deep broad Lyα, narrow metallic
absorption features and evidence of a hotter Keplerian-broadened
component. We confirm the existence of enhanced abundances of
odd-numbered nuclear species P, Mn, and Al as well as an N/C
ratio indicative of CNO H-burning thermonuclear processing. Our
best single-temperature white dwarf reduced χ2
fit to the first spectrum reveals (1) a DAZQ white dwarf with
Teff=22,500+/-500 K, logg=8.0, and photospheric abundances
C=0.3 solar, N=3.0 solar, O=3.0 solar, Si=0.3 solar, Al=2 solar,
Fe=0.5 solar, Mg=3.0 solar, Mn=50 solar, Ni=0.3 solar, P=15 solar, and
Ti=0.1 solar. The best-fit white dwarf + accretion belt composite model
yields a large improvement in the reduced χ2 value. The
accretion belt temperature is 32,000 K and covers a fractional area
of 3%, contributing 11% of the flux. The second spectrum 5 days later
reveals slightly increased metal abundances except that P is elevated
to 20 times solar while Fe has declined to 0.05 times solar. The
white dwarf has cooled by ~1000 K, the belt temperature is 32,000 K,
and the fractional area and flux contribution of the belt are 5%
and 20%, respectively. These STIS observations confirm that a past
(prehistoric?) thermonuclear runaway has occurred on the white dwarf in
VW Hyi. It is expected that the thermonuclear runaway would be strong
enough to produce a nova outburst. Therefore, these two classes of
close binaries, namely, dwarf novae and classical novae, are linked
and can overlap.

---------------------------------------------------------
Title: A comparison of DA white dwarf temperatures and gravities
from Lyman and Balmer line studies
Authors: Barstow, M. A.; Holberg, J. B.; Hubeny, I.; Good, S. A.;
Levan, A. J.; Meru, F.
2001MNRAS.328..211B Altcode: 2001astro.ph..7529B
We present measurements of the effective temperatures and surface
gravities for a sample of hot DA white dwarfs, using the Lyman line data
available from the HUT, ORFEUS and FUSE FUV space missions. Comparing
the results with those from the standard Balmer line technique, we
find that there is a general good overall agreement between the two
methods. However, significant differences are found for a number of
stars, but not always of a consistent nature in that sometimes the
Balmer temperature exceeds that derived from the Lyman lines and
in other instances it is lower. We conclude that, with the latest
model atmosphere calculations, these discrepancies probably do not
arise from an inadequate theoretical treatment of the Lyman lines but
rather from systematic effects in the observation and data reduction
processes, which dominate the statistical errors in these spectra. If
these systematic data reduction effects can be adequately controlled,
the Lyman line temperature and gravity measurements are consistent
with those obtained from the Balmer lines when allowance is made for
reasonable observational uncertainties.

---------------------------------------------------------
Title: Flash Mixing on the White Dwarf Cooling Curve: Understanding
Hot Horizontal Branch Anomalies in NGC 2808
Authors: Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Landsman,
Wayne B.; Hubeny, Ivan
2001ApJ...562..368B Altcode: 2001astro.ph..8040B
We present an ultraviolet color-magnitude diagram (CMD) spanning
the hot horizontal branch (HB), blue straggler, and white dwarf
populations of the globular cluster NGC 2808. These data were obtained
with the far-UV and near-UV cameras on the Space Telescope Imaging
Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show
a high-temperature gap within the hot HB population, no such gap is
evident in our UV CMD. Instead, we find a population of hot subluminous
HB stars, an anomaly only previously reported for the globular cluster
ω Cen. Our theoretical modeling indicates that the location of these
subluminous stars in the UV CMD, as well as the high-temperature gap
along the HB in optical CMDs, can be explained if these stars underwent
a late helium-core flash while descending the white dwarf cooling
curve. We show that the convection zone produced by such a late helium
flash will penetrate into the hydrogen envelope, thereby mixing hydrogen
into the hot helium-burning interior, where it is rapidly consumed. This
phenomenon is analogous to the “born again” scenario for producing
hydrogen-deficient stars following a late helium-shell flash. The
flash mixing of the envelope greatly enhances the envelope helium and
carbon abundances, and leads, in turn, to a discontinuous increase in
the HB effective temperatures at the transition between canonical and
flash-mixed stars. We argue that the hot HB gap is associated with this
theoretically predicted dichotomy in the HB properties. Moreover, the
changes in the emergent spectral energy distribution caused by these
abundance changes are primarily responsible for explaining the hot
subluminous HB stars. Although further evidence is needed to confirm
that a late helium-core flash can account for the subluminous HB stars
and the hot HB gap, we demonstrate that an understanding of these stars
requires the use of appropriate theoretical models for their evolution,
atmospheres, and spectra. Based on observations with the NASA/ESA Hubble
Space Telescope obtained at the Space Telescope Science Institute,
which is operated by AURA, Inc., under NASA contract NAS5-26555.

---------------------------------------------------------
Title: Non-LTE Models and Theoretical Spectra of Accretion Disks
in Active Galactic Nuclei. IV. Effects of Compton Scattering and
Metal Opacities
Authors: Hubeny, Ivan; Blaes, Omer; Krolik, Julian H.; Agol, Eric
2001ApJ...559..680H Altcode: 2001astro.ph..5507H
We extend our models of the vertical structure and emergent radiation
field of accretion disks around supermassive black holes described
in previous papers of this series. Our models now include both a
self-consistent treatment of Compton scattering and the effects of
continuum opacities of the most important metal species (C, N, O,
Ne, Mg, Si, S, Ar, Ca, Fe, Ni). With these new effects incorporated,
we compute the predicted spectrum from black holes accreting at nearly
the Eddington luminosity (L/LEdd~0.3) and central masses of
106, 107, and 108 Msolar. We
also consider two values of the Shakura-Sunyaev α parameter, 0.1 and
0.01, but in contrast to our previous papers, we consider a kinematic
viscosity that is independent of depth. Although it has little effect
when M&gt;108 Msolar, Comptonization grows in
importance as the central mass decreases and the central temperature
rises. It generally produces an increase in temperature with height
in the uppermost layers of hot atmospheres. Compared to models
with coherent electron scattering, Comptonized models have enhanced
extreme ultraviolet/soft X-ray emission, but they also have a more
sharply declining spectrum at very high frequencies. Comptonization
also smears the hydrogen and the He II Lyman edges. The effects
of metals on the overall spectral energy distribution are smaller
than the effects of Comptonization for these parameters. Compared to
pure hydrogen-helium models, models with metal-continuum opacities
have reduced flux in the high-frequency tail, except at the highest
frequencies, where the flux is very low. Metal photoionization edges
are not present in the overall disk-integrated model spectra. The
viscosity parameter α has a more dramatic effect on the emergent
spectrum than do metal-continuum opacities. As α increases (and
therefore the disk column density decreases), the flux at both the high-
and low-frequency extremes of the spectrum increases, while the flux
near the peak decreases. Multitemperature blackbodies are a very poor
approximation to accretion disk spectra in the soft X-ray region, and
such crude modeling may greatly overestimate the accretion luminosity
required to explain observed soft X-ray excesses in active galactic
nuclei. In addition to our new grid of models, we also present a
simple analytic prescription for the vertical temperature structure
of the disk in the presence of Comptonization, and show under what
conditions a hot outer layer (a corona) is formed.

---------------------------------------------------------
Title: Far-ultraviolet spectroscopy of the hot DA white dwarf WD
2218+706 (DeHt5) with STIS
Authors: Barstow, M. A.; Bannister, N. P.; Holberg, J. B.; Hubeny,
I.; Bruhweiler, F. C.; Napiwotzki, R.
2001MNRAS.325.1149B Altcode: 2000astro.ph..1172B
We report a study of the photospheric composition of the hot DAQ3 white
dwarf WD 2218+706, which is also the central star of the old planetary
nebula DeHt5. Helium is detected in the far-UV spectrumQ4. In addition,
the star clearly contains significant quantities of elements heavier
than He at abundances generally a factor of 2 to 10 higher than those
found in the archetypal heavy element-rich DA G191-B2B. This is the
first detection of trace He using the Heii λ1640 line in an isolated
DA white dwarf, but the low surface gravity is more indicative of a
binary evolution route from the red giant branch rather than a path
along the asymptotic giant branch (AGB) as a single star. However,
the absence of any evidence for a companion star and the uncertainty
in the measured mass for WD 2218+706 still allow the possibility of
an origin along an AGB evolutionary track. We reanalyse the existing
optical spectra of WD 2218+706 using our latest pure H and heavy
element-rich model atmospheres, obtaining a good match between the
observed and synthetic spectra with either set of models. We find
little evidence of any inconsistency in the temperature required to
fit individual Balmer lines, as reported elsewhere for this star. Any
discrepancies we see are confined to the Hα line and the core of Hβ
but they do not compromise our analysis.

---------------------------------------------------------
Title: HST/STIS spectroscopy of the exposed white dwarf in the
short-period dwarf nova EK TrA
Authors: Gänsicke, B. T.; Szkody, P.; Sion, E. M.; Hoard, D. W.;
Howell, S.; Cheng, F. H.; Hubeny, I.
2001A&A...374..656G Altcode: 2001astro.ph..6023G
We present high resolution Hubble Space Telescope ultraviolet
spectroscopy of the dwarf nova EK TrA obtained in deep
quiescence. The Space Telescope Imaging Spectrograph data reveal
the broad Lyalpha absorption profile typical of a moderately cool
white dwarf, overlayed by numerous broad emission lines of He, C,
N, and Si and by a number of narrow absorption lines, mainly of
\C I and \Si II. Assuming a white dwarf mass in the range 0.3-1.4
Msun we derive Teff=17 500-23 400 K for the
primary in EK TrA; Teff =18 800 K for a canonical mass of
0.6 Msun. From the narrow photospheric absorption lines,
we measure the white dwarf rotational velocity, vsin i=200+/-100
km s-1. Even though the strong contamination of the
photospheric white dwarf absorption spectrum by the emission lines
prevents a detailed quantitative analysis of the chemical abundances
of the atmosphere, the available data suggest slightly sub-solar
abundances. The high time resolution of the STIS data allows us to
associate the observed ultraviolet flickering with the emission lines,
possibly originating in a hot optically thin corona above the cold
accretion disk. Based on observations made with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: Understanding Horizontal Branch Anomalies in NGC 2808
Authors: Brown, T. M.; Sweigart, A. V.; Lanz, T.; Landsman, W. B.;
Hubeny, I.
2001AAS...198.4302B Altcode: 2001BAAS...33..844B
We present an ultraviolet color-magnitude diagram (CMD) spanning the hot
horizontal branch (HB), blue straggler, and white dwarf populations
of the globular cluster NGC 2808. These data, obtained with the
Space Telescope Imaging Spectrograph (STIS), demonstrate that NGC
2808 harbors a significant population of hot subluminous HB stars,
an anomaly only previously reported for the globular cluster Omega
Cen. Our theoretical modeling indicates that the location of these
subluminous stars in the CMD, as well as the high temperature gap along
the HB of NGC 2808, can be explained if these stars underwent a late
helium-core flash while descending the white dwarf cooling curve. We
show that the convective zone produced by such a late helium flash will
penetrate into the hydrogen envelope, thereby mixing hydrogen into
the hot helium-burning interior, where it is rapidly consumed. This
phenomenon is analogous to the "born again" scenario for producing
hydrogen-deficient stars following a late helium-shell flash. The flash
mixing of the envelope greatly enhances the envelope helium and carbon
abundances that, in turn, leads to a discontinuous increase in the HB
effective temperatures. We argue that the hot HB gap is associated with
this theoretically predicted dichotomy in the HB properties. Moreover,
the changes in the emergent spectral energy distribution caused by
these abundance changes are primarily responsible for explaining
the hot subluminous HB stars. Although further evidence is needed to
confirm that a late helium-core flash can account for the subluminous HB
stars and the hot HB gap, we demonstrate that an understanding of these
stars requires use of sophisticated models for the stellar evolution,
atmospheres, and synthetic spectra.

---------------------------------------------------------
Title: Heavy Elements in DA White Dwarfs
Authors: Barstow, M. A.; Burleigh, M. R.; Bannister, N. P.; Holberg,
J. B.; Hubeny, I.; Bruhweiler, F. C.; Napiwotzki, R.
2001ASPC..226..128B Altcode: 2001ewwd.work..128B
No abstract at ADS

---------------------------------------------------------
Title: Preface (Interacting astronomers: a symposium on Mirek Plavec's
favorite stars. Proceedings)
Authors: Hubeny, Ivan; Malkan, Matt
2001PAICz..89D...5H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: FUSE Observations of the Hottest DA White Dwarfs
Authors: Barstow, M. A.; Burleigh, M. R.; Bannister, N. P.; Holberg,
J. B.; Hubeny, I.
2001ASPC..226...94B Altcode: 2000astro.ph.10447B; 2001ewwd.work...94B
We report early FUSE observations of the very hot DA white dwarfs
PG1342+444 and REJ0558-371. Detection of photospheric absorption lines
allows us to estimate the abundances of C, O, Si, P, S and Fe, the first
measurements reported for DA stars at such high temperatures. Values
of Teff and log g determined for PG1342+444 from the Lyman line series
disagree with the results of the standard Balmer line analysis, an
issue that requires further investigation.

---------------------------------------------------------
Title: NLTE Model Atmospheres for OB Stars
Authors: Lanz, Thierry; Hubeny, Ivan
2001ASPC..247..351L Altcode: 2001scpp.conf..351L
No abstract at ADS

---------------------------------------------------------
Title: From Escape Probabilities to Exact Radiative Transfer
Authors: Hubeny, Ivan
2001ASPC..247..197H Altcode: 2001scpp.conf..197H
No abstract at ADS

---------------------------------------------------------
Title: From stars to quasars. Modeling radiation-dominated objects
in astrophysics
Authors: Hubeny, Ivan
2001PAICz..89....1H Altcode:
There is a whole class of astronomical objects for which radiation is
not only a probe of the physical state, but is in fact an important
energy balance agent. In these objects, radiative transfer plays
a crucial role. Typical examples of such a situation are stellar
atmospheres, interstellar matter and various structures within,
and accretion disks, both around stellar mass degenerate objects
(cataclysmic variables), and around supermassive black holes
(quasars). We concentrate on two objects - stellar atmospheres and
quasar accretion disks, and show that since they have many similarities
they can be modeled using an analogous approach. We will give a
brief overview of recent progress in their modeling. On the stellar
atmospheres side, very sophisticated non-LTE models including millions
of lines are now being constructed. It is shown that they provide an
excellent match to high-quality ground- and space-based spectra. In
quasar modeling, we have constructed a grid of non-LTE disk models for
a wide range of parameters, and begun a study of analysing observed
data using the new models.

---------------------------------------------------------
Title: Interacting astronomers: a symposium on Mirek Plavec's favorite
stars. Proceedings
Authors: Harmanec, P.; Hadrava, P.; Hubeny, I.
2001PAICz..89.....H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Properties of the z = 2.73 galaxy, MS1512-cB58
Authors: Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry M.
2001ApSSS.277..263H Altcode:
MS1512-cB58 is an z=2.73 galaxy whose apparent brightness is amplified
by30-50X due to gravitationally lensing. The restframe far-UV spectrum
of cB58 that was obtained by Keck-I/LRIS is `one of the best ultraviolet
spectra of a starburst galaxy obtained at any redshift' (Pettini et
al., 2000). We have analyzed and modeled the spectrum of this galaxy in
order to learn the properties of high-redshift galaxies. We find that
our model spectrum is a near match to the observed spectrum of cB58 if
the galaxy has a SMC-like metallicity, and has a Salpeter IMF extending
up to ~ 100 M. The spectrum of cB58 also shows many absorption lines
formed in a giant, expanding gas shell surrounding the star-forming
complex. We show preliminary findings of the properties of the giant
HII region based on our CLOUDSPEC model.

---------------------------------------------------------
Title: STIS Observations of the Photospheric Stratification of Heavy
Elements in Hot DA White Dwarfs
Authors: Holberg, J. B.; Barstow, M. A.; Bruhweiler, F. C.; Hubeny, I.
2000AAS...197.8304H Altcode: 2000BAAS...32.1543H
We present results from echelle spectra obtained with the Space
Telescope Imaging Spectrograph (STIS) of the hot DA white dwarfs, GD 659
(WD0050-332) and GD 246 (WD2309+105), which provide new information on
the occurrence of heavy elements in the photospheres of these stars. In
the case of GD 659, we show that the C IV, N V and Si IV resonance
lines, previously observed with IUE, and thought to be interstellar or
circumstellar in origin, are in fact photospheric in nature. Further,
the strength of the highly excited N V lines in a star as cool as GD
659 (Teff = 35,300 K) can be explained only if the nitrogen
is stratified in the stellar photosphere. Thus, GD 659 is apparently
similar to REJ 1032+532 and perhaps REJ 1614-085, which also show super
nitrogen abundances, and have photospheres with stratified N abundances.

---------------------------------------------------------
Title: GALSPEC: A Tool for Computing the Spectra of Star-Forming
Galaxies of Low Metallicity
Authors: Heap, S. R.; Hubeny, I.; Lanz, T. M.; Lindler, D. J.
2000AAS...197.7813H Altcode: 2000BAAS...32R1531H
The far-ultraviolet spectral region is rich in diagnostics of
star formation in galaxies; it gives information not only about the
star-forming complex, but also about the dust and circumstellar gas. We
have therefore developed a tool, called GALSPEC, for computing far-UV
spectra of star-forming galaxies. The user controls the process through
a graphical user interface (GUI) by specifying the input parameters for
the stars, dust, and gas. GALSPEC computes the integrated spectrum of a
star-forming complex, given the rate of star formation, metallicity (0.2
Zsun or 0.5 Zsun), and IMF upper mass limit. To
compute the integrated spectrum, GALSPEC makes use of a library
of stellar spectra composed of observed spectra from HST and model
spectra calculated with the TLUSTY/SYNSPEC program (Hubeny &amp; Lanz
1995). GALSPEC then applies wavelength-dependent extinction according
to Calzetti's starburst extinction law. The mechanical luminosity from
massive stars and supernovae sweeps up interstellar material thereby
forming an expanding shell with a central cavity. GALSPEC computes the
absorption spectrum of the shell using the CLOUDSPEC program (Hubeny
2000), a combination of Ferland's CLOUDY program and SYNSPEC. This work
was supported by the STIS GTO program at Goddard and by STScI grants
(GO 7437, AR 7985).

---------------------------------------------------------
Title: A New Grid of NLTE Model Atmospheres of Hot Stars
Authors: Hubeny, I.; Lanz, T.
2000AAS...197.7812H Altcode: 2000BAAS...32Q1531H
We briefly describe improvements in our non-LTE model atmosphere code,
TLUSTY. The upgrades include opacity sampling, improved Fe collisional
strengths, resonance-averaged photoionization cross-sections, and a
number of numerical enhancements. Using TLUSTY, version 200, we have
calculated a grid of NLTE, fully-blanketed, model atmospheres covering
the range of O to early B-type stars ( 20000 &lt;= T eff
&lt;= 55000 K, 3.0 &lt;= log g &lt;= 4.75) with a solar or 1/5 solar
metallicity. The models incorporate over 70,000 NLTE atomic levels of
about 30 ions of H, He, C, N, O, Ne, Mg, Si, S, Fe and Ni, which are
grouped into about 700 superlevels. We compare our predicted EUV fluxes
to other existing grids of model atmospheres (Atlas9, CoStar models), as
well as to actual UV and optical observations of representative stars.

---------------------------------------------------------
Title: The D/H Abundance Ratio in Local Interstellar Gas
Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Linsky, J.;
Hubeny, I.; Barstow, M.; Holberg, J.; Gull, T.; Bowers, C.; Lindler,
D.; Lanz, T.; Feggans, K.
2000AAS...197.0703S Altcode: 2000AAS...197..703S; 2000BAAS...32.1401S
Does the D/H ratio vary within ~ 100 pc of the Sun? If it does, what
is the range of these variations? Are the D/H variations correlated to
any physical property of the absorbing cloud? To answer these questions
we have begun a project to obtain high-precision D/H abundance ratios
in the Local Interstellar Medium within ~ 100 pc (Sahu et al. 1999,
ApJ 523, L159). For this purpose, we primarily use HST-Space Telescope
Imaging Spectrograph (STIS) data towards nearby hot, white dwarfs
(WDs). The advantages of using STIS for this study are: (1) The high
velocity resolution of STIS in the UV ( ~ 2 km s-1 in the
E140H and E230H modes) allows the velocity structure in the sightline
to be resolved and reliable component-to-component variations can be
studied (2) WDs provide a strong, smooth UV continuum against which
other diagnostic interstellar absorption lines can be measured. The
wider wavelength coverage ( ~ 200Å per setting) of STIS in the
high-resolution E140H mode allows important interstellar lines such as N
i, Si ii, O i and S ii to be measured in addition to D i and H i, at no
extra cost in HST time (3) The superior echelle scatter and background
corrections possible with the 2-dimensional STIS-MAMA detectors allow
more accurate D/H determinations from WD spectra. In this poster, we
present a status report of our project along with some recent results.

---------------------------------------------------------
Title: NGC 346-12, a Rapidly Rotating O9.5V Star in the SMC: Test
Case of Weak Winds
Authors: Lanz, T.; Bouret, J. -C.; Heap, S. R.; Hubeny, I.; Hillier,
D. J.; Lennon, D. J.; Smith, L. J.; Evans, C. J.; Owocki, S. P.
2000AAS...197.7811L Altcode: 2000BAAS...32R1531L
We have analyzed the UV and optical spectrum of the O9.5V Star, NGC
346-12, using sophisticated, NLTE line-blanketed model atmospheres
calculated with our code TLUSTY. The following stellar parameters
were derived: T eff=30,000 K, log g=3.5. An abundance
analysis yields a metallicity, [Fe/H]=-1.0. The N/C abundance ratio
is 25 times the solar ratio, indicating that material processed
through the CNO-cycle has been brought up to the surface. Assuming
a distance modulus, (m-M)=18.9, for the SMC, we have derived the
luminosity, radius, and mass of the star. We found, similarly to higher
luminosity galactic stars, a discrepancy between the mass derived
from the spectroscopic analysis and from stellar evolution theory. We
conclude that it is very likely that NGC 346-12 is a fast rotator,
whose evolution has been affected by rotation. Furthermore, the wind
of NGC 346-12 appears abnormal: while the C IV resonance lines do not
reveal any indication of a wind, a weak P-Cygni profile is observed
in N 5 1240. Various possible explanations for the low inferred ion
density in the stellar wind, including an enhanced degree of wind
ionization associated with ion frictional heating, or ion runaway
due to frictional decoupling from the hydrogen-helium bulk plasma,
are examined. This work was supported through a NASA/NRC RA award and
STScI grants (GO 7437, AR 7985).

---------------------------------------------------------
Title: NLTE Model Atmosphere Analysis of Main-Sequence O Stars in
NGC 346
Authors: Bouret, J. -C.; Heap, S. R.; Hubeny, I.; Lanz, T.; Hillier,
D. J.; Lennon, D. J.; Smith, L. J.; Evans, C. J.
2000AAS...197.7810B Altcode: 2000BAAS...32.1531B
To serve as spectral templates of young, low-metallicity populations,
high-resolution, high-quality, ultraviolet (STIS) and optical spectra
of several main-sequence O stars in NGC 346 have been obtained. Stellar
parameters and abundance of light elements are derived from the optical
spectrum using NLTE, line-blanketed, photospheric models calculated
with TLUSTY. In addition, the UV spectrum is analyzed with the NLTE,
line-blanketed wind model code, CMFGEN, to derive wind parameters
(mass-loss rate, clumping properties, terminal velocity). Iron
abundances, derived from fitting Fe 4 and Fe 5 lines with the two NLTE
codes, compare favorably. We achieve good matches of the observed
spectra, thus supporting our modeling work of spectra of starburst
galaxies at high redshift. We finally discuss the dependence of the
mass-loss rate in terms of metallicity and luminosity. This work
was supported through a NASA/NRC RA award and STScI grants (GO 7437,
AR 7985).

---------------------------------------------------------
Title: Stellar Atmospheres: Early Type Stars
Authors: Hubeny, I.
2000eaa..bookE1837H Altcode:
The topic `Stellar atmospheres of early type stars' has two individual
ingredients, a (stellar) atmosphere, and an early type star. We shall
specify these two terms in turn....

---------------------------------------------------------
Title: Temporal Variations of the White Dwarf and Disk in OY Carinae
Following the 1992 Superoutburst
Authors: Cheng, F. H.; Horne, Keith; Marsh, T. R.; Hubeny, Ivan;
Sion, E. M.
2000ApJ...542.1064C Altcode:
Hubble Space Telescope observations of the eclipsing dwarf nova
OY Carinae after its 1992 April superoutburst are used to isolate
ultraviolet spectra (1150-2500 Å at 9.2 Å FWHM resolution) of the
white dwarf, the accretion disk, and the bright spot. The white dwarf
spectra have a Stark-broadened photospheric Lyα absorption feature
but are veiled by a forest of absorption features that we attribute to
absorption by intervening disk material (a curtain). All the spectral
fits required supersonic turbulence in the curtain material with
Mach numbers of 6-8. All curtain temperatures were between 10,000 and
11,000 K. There was a curtain temperature increase ~3 months after the
superoutburst. We find that the white dwarf temperature changed from
19,700 K just 27 days after the end of the superoutburst to 18,000 K
roughly 3 months after the superoutburst; the exponential (e-folding)
decay time of the white dwarf temperature was 66 days. We present
evidence that the heating of the white dwarf was more extensive during
the superoutburst than the normal outburst. The thermal response of the
OY Car white dwarf to outburst heating is compared with WZ Sagittae,
VW Hydri (the most similar dwarf nova to OY Car), and the cooling
timescales of other dwarf novae after superoutburst. The measured
cooling timescales of the five systems with superoutbursts appear to
be shorter the longer the orbital period (accretion rate). Possible
implications are discussed. There is evidence of a disk flux variation,
independent of the effect of white dwarf cooling, which suggests a
possible contradiction of the disk instability model. To establish
this, however, data are required throughout a quiescent cycle. Based
on observations with the NASA/ESA Hubble Space Telescope obtained
at the Space Telescope Science Institute, which is operated by the
Association of University Research in Astronomy, Inc., under NASA
contract NAS 5-26555.

---------------------------------------------------------
Title: Heavy element abundances in DA white dwarfs
Authors: Barstow, M. A.; Burleigh, M. R.; Bannister, N. P.; Holberg,
J. B.; Hubeny, I.; Bruhweiler, F. C.; Napiwotzki, R.
2000astro.ph.10448B Altcode:
We present a series of systematic abundance measurements for a group
of hot DA white dwarfs in the temperature range 20,000-75,000K, based
on far-UV spectroscopy with STIS on HST, IUE and FUSE. Using our latest
heavy element blanketed non-LTE stellar atmosphere calculations we have
addressed the heavy element abundance patterns for the hottest stars
for the first time, showing that they are similar to objects like
G191-B2B. The abundances observed in the cooler (&lt;50,000K) white
dwarfs are something of a mystery. Some of the patterns (e.g. REJ1032)
can be explained by self-consistent levitation-diffusion calculations
but there is then a serious difficulty in understanding the appearance
of the apparently pure H atmospheres. We also report the detection of
photospheric HeII in the atmosphere of WD2218+706.

---------------------------------------------------------
Title: Space Telescope Imaging Spectrograph Coronagraphic Observations
of β Pictoris
Authors: Heap, Sara R.; Lindler, Don J.; Lanz, Thierry M.; Cornett,
Robert H.; Hubeny, Ivan; Maran, S. P.; Woodgate, Bruce
2000ApJ...539..435H Altcode: 1999astro.ph.11363H
We present new coronagraphic images of β Pictoris obtained with the
Space Telescope Imaging Spectrograph (STIS) in 1997 September. The
high-resolution images (0.1") clearly detect the circumstellar disk as
close to the star as 0.75", corresponding to a projected radius of 15
AU. The images define the warp in the disk with greater precision and
at closer radii to β Pic than do previous observations. They show
that the warp can be modeled by the projection of two components:
the main disk and a fainter component, which is inclined to the main
component by 4°-5° and extends only as far as ~4" from the star. We
interpret the main component as arising primarily in the outer disk
and the tilted component as defining the inner region of the disk. The
observed properties of the warped inner disk are inconsistent with a
driving force from stellar radiation. However, warping induced by the
gravitational potential of one or more planets is consistent with the
data. Using models of planet-warped disks constructed by Larwood &amp;
Papaloizou, we derive possible masses of the perturbing object. Based
on observations with the NASA/ESA Hubble Space Telescope, obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Inc., under
NASA contract NAS5-26555.

---------------------------------------------------------
Title: The D/H Abundance Ratio in Local Interstellar Gas
Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Linsky, J. L.;
Hubeny, I.; Barstow, M.; Holberg, J.; Gull, T.; Bowers, C.; Lindler,
D.; Lanz, T.; Feggans, K.
2000AAS...196.2620S Altcode: 2000BAAS...32..713S
Does the D/H ratio vary within ~ 100 pc of the Sun? If it does, what
is the range of these variations? Are the D/H variations correlated to
any physical property of the absorbing cloud? To answer these questions
we have begun a project to obtain high-precision D/H abundance ratios
in the Local Interstellar Medium within ~ 100 pc (Sahu et al. 1999,
ApJ 523, L159). For this purpose, we primarily use HST-Space Telescope
Imaging Spectrograph (STIS) data towards nearby hot, white dwarfs
(WDs). The advantages of using STIS for this study are: (1) The high
velocity resolution of STIS in the UV ( ~ 2 km s-1 in the
E140H and E230H modes) allows the velocity structure in the sightline
to be resolved and reliable component-to-component variations can be
studied (2) WDs provide a strong, smooth UV continuum against which
other diagnostic interstellar absorption lines can be measured. The
wider wavelength coverage ( ~ 200 Angstroms per setting) of STIS in the
high-resolution E140H mode allows important interstellar lines such as N
i, Si ii, O i and S ii to be measured in addition to D i and H i, at no
extra cost in HST time (3) The superior echelle scatter and background
corrections possible with the 2-dimensional STIS-MAMA detectors allow
more accurate D/H determinations from WD spectra. In this poster, we
present a status report of our project along with some recent results.

---------------------------------------------------------
Title: Analysis of the Spectrum of the z=2.73 Galaxy, MS1512-cB58:
A Unified Model of the Stellar and Interstellar Contributions
Authors: Hubeny, I.; Heap, S. R.; Lanz, T. M.
2000AAS...196.2914H Altcode: 2000BAAS...32R.718H
We perform a detailed analysis of the restframe far-UV spectrum of
cB58 that was obtained by Pettini et al. (ApJ 528,96, 2000) using
the KECK-I/LRIS. In the previous paper (AAS 195, 9.19), we modeled
the spectrum of the galaxy and found that the observed spectrum is
well described by a starburst model having a SMC-like metallicity
and a constant star formation rate. Here, we extend the analysis to
model the total spectrum that includes stellar as well as interstellar
contributions. The latter is modeled using a newly developed program
CLOUDSPEC that combines the photoionization program CLOUDY (Ferland,
http://www.pa.uky.edu/ gary/cloudy) with our spectrum synthesis program
SYNSPEC (Hubeny &amp; Lanz, http://tlusty.gsfc.nasa.gov). With the
ionization structure of the interstellar medium given by CLOUDY,
CLOUDSPEC solves the radiative transfer along the line-of-sight to
the starburst and yields the profiles of the interstellar absorption
lines. We show that such a “unified” model of the stellar +
interstellar spectrum provides an excellent match to the observed
spectrum of cB58.

---------------------------------------------------------
Title: The discovery of photospheric nickel in the hot DO white
dwarf REJ 0503-289
Authors: Barstow, M. A.; Dreizler, S.; Holberg, J. B.; Finley, D. S.;
Werner, K.; Hubeny, I.; Sion, E. M.
2000MNRAS.314..109B Altcode:
We present the first evidence for the direct detection of nickel
in the photosphere of the hot DO white dwarf REJ 0503-289. While
this element has been seen previously in the atmospheres of hot
H-rich white dwarfs, this is one of the first similar discoveries
in a He-rich object. Intriguingly, iron, which is observed to
be more abundant than Ni in the hot DA stars, is not detected,
the upper limit to its abundance (Fe/He=10-6) implying
an Fe/Ni ratio a factor of 10 lower than seen in the H-rich objects
(Ni/He=10-5 for REJ 0503-289). The abundances of nickel and
various other elements heavier than He were determined from Goddard High
Resolution Spectrograph spectra. We used two completely independent
sets of non-local thermodynamic equilibrium model atmospheres, which
both provide the same results. This not only reduces the possibility of
systematic errors in our analysis, but is also an important consistency
check for both model atmosphere codes. We have also developed a more
objective method of determining Teff and logg, from the He
lines in the optical spectrum, in the form of a formal fitting of the
line profiles to a grid of model spectra, an analogue of the standard
procedure utilizing the Balmer lines in DA white dwarfs. This gives the
assigned uncertainties in Teff and logg a firm statistical
basis and allows us to demonstrate that inclusion of elements heavier
than H, He and C in the spectral calculations, exclusively considered
in most published optical analyses, yields a systematic downward shift
in the measured value of Teff.

---------------------------------------------------------
Title: STIS Observations of HE II Gunn-Peterson Absorption toward
Q0302-003
Authors: Heap, Sara R.; Williger, Gerard M.; Smette, Alain; Hubeny,
Ivan; Sahu, Meena S.; Jenkins, Edward B.; Tripp, Todd M.; Winkler,
Jonathan N.
2000ApJ...534...69H Altcode:
The ultraviolet spectrum (1145-1720 Å) of the distant quasar
Q0302-003 (z=3.286) was observed at 1.8 Å resolution with the Space
Telescope Imaging Spectrograph aboard the Hubble Space Telescope. A
total integration time of 23,280 s was obtained. The spectrum clearly
delineates the Gunn-Peterson He II absorption trough, produced by He II
Lyα along the line of sight over the redshift range z=2.78-3.28. Its
interpretation was facilitated by modeling based on Keck HIRES spectra
of the H I Lyα forest (provided by A. Songaila and by M. Rauch and
W. Sargent). We find that near the quasar, He II Lyα absorption
is produced by discrete clouds, with no significant diffuse gas;
this is attributed to a He II “proximity effect” in which the
quasar fully ionizes He in the diffuse intergalactic medium, but
not the He in denser clouds. By two different methods we calculate
that the average He II Lyα opacity at z~3.15 is τ&gt;=4.8. In the
Dobrzycki-Bechtold void in the H I Lyα forest near z=3.18, the average
He II opacity τ=4.47+0.48-0.33. Such large
opacities require the presence of a diffuse gas component as well as
a soft UV background spectrum, whose softness parameter, defined as
the ratio of the photoionization rate in H I over the one in He II
S≡ΓJHI/ ΓJHeII~=800,
indicating a significant stellar contribution. At z=3.05, there is a
distinct region of high He II Lyα transmission that most likely arises
in a region where helium is doubly ionized by a discrete local source,
quite possibly an AGN. At redshifts z&lt;2.87, the He II Lyα opacity
detected by STIS, τ=1.88, is significantly lower than at z&gt;3. Such
a reduction in opacity is consistent with Songaila's report that the
hardness of the UV background spectrum increases rapidly from z=3 to
z=2.9. Based on observations with the NASA/ESA Hubble Space Telescope,
obtained at the Space Telescope Science Institute, which is operated
by the Association of Universities for Research in Astronomy, Inc.,
under NASA contract NAS5-26555.

---------------------------------------------------------
Title: On the Age Estimation of LBDS 53W091
Authors: Yi, Sukyoung; Brown, Thomas M.; Heap, Sara; Hubeny, Ivan;
Landsman, Wayne; Lanz, Thierry; Sweigart, Allen
2000ApJ...533..670Y Altcode: 1999astro.ph.11067Y
The recent spectral analysis of LBDS 53W091 by Spinrad and his
collaborators has suggested that this red galaxy at z=1.552 is at
least 3.5 Gyr old. This imposes an important constraint on cosmology,
suggesting that this galaxy formed at z&gt;~6.5, assuming recent
estimates of cosmological parameters. While their analysis was heavily
focused on the use of some UV spectral breaks as age indicators,
we have performed χ2 tests to the continuum of this
galaxy using its UV spectrum and photometric data (R, J, H, and K:
2000-9000 Å in rest frame). We have used the updated Yi models
that are based on the Yale tracks. We find it extremely difficult to
reproduce such large age estimates, under the assumption of the most
probable input parameters. Using the same configuration as in Spinrad
et al. (conventional solar abundance models), our analysis suggests
an age of approximately 1.4-1.8 Gyr. We have improved our models over
conventional ones by taking into account convective core overshoot in
the stellar model calculations and realistic metallicity distributions
in the galaxy population synthesis. Overshoot affects the visible
continuum normalized to the UV and raises the photometry-based age
estimates by 25%. The use of metallicity mixtures affects the whole
spectrum and raises all continuum-based age estimates by up to a factor
of 2. If the mean metallicity of the stars in this galaxy is assumed
to be twice solar, the models including these two effects match the UV
spectrum and photometric data of LBDS 53W091 near the age of 1.5-2.0
Gyr. Our results cannot be easily reconciled with that of Spinrad et
al. The discrepancy between Spinrad et al.'s age estimate (based on
the Jimenez models) and ours originates from the large difference in
the model integrated spectrum: the Jimenez models are much bluer than
the Yi models and the Bruzual &amp; Charlot models. We propose to apply
some viable tests to them for verification and search for the origin
of the difference through a more thorough investigation. Considering
the significance of the age estimates of distant galaxies as probes
of cosmology, it would be an urgent task.

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Title: Non-LTE Models and Theoretical Spectra of Accretion Disks in
Active Galactic Nuclei. III. Integrated Spectra for Hydrogen-Helium
Disks
Authors: Hubeny, Ivan; Agol, Eric; Blaes, Omer; Krolik, Julian H.
2000ApJ...533..710H Altcode: 1999astro.ph.11317H
We have constructed a grid of non-LTE disk models for a wide range of
values of black hole mass and mass accretion rate, for several values
of the viscosity parameter α, and for two extreme values of the black
hole spin: the maximum-rotation Kerr black hole, and the Schwarzschild
(nonrotating) black hole. Our procedure calculates self-consistently
the vertical structure of all disk annuli together with the radiation
field, without any approximations imposed on the optical thickness
of the disk, and without any ad hoc approximations to the behavior
of the radiation intensity. The total spectrum of a disk is computed
by summing the spectra of the individual annuli, taking into account
the general relativistic transfer function. The grid covers nine
values of the black hole mass between M=1/8×109 and
32×109 Msolar with a twofold increase of mass
for each subsequent value; and eleven values of the mass accretion
rate, each a power of 2 times 1 Msolar yr-1. The
highest value of the accretion rate corresponds to the total luminosity
L/LEdd~0.3. We show the vertical structure of individual
annuli within the set of accretion disk models, along with their local
emergent flux, and discuss the internal physical self-consistency of the
models. We then present the full disk-integrated spectra and discuss
a number of observationally interesting properties of the models,
such as optical/ultraviolet colors, the behavior of the hydrogen Lyman
limit region, polarization, and the number of ionizing photons. Our
calculations are far from definitive in terms of the input physics,
but generally we find that our models exhibit rather red optical/UV
colors. Flux discontinuities in the region of the hydrogen Lyman limit
are only present in cool, low-luminosity models, while hotter models
exhibit blueshifted changes in spectral slope.

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Title: The He II Gunn-Peterson Effect
Authors: Heap, Sara R.; Hubeny, Ivan; Smette, Alain; Williger, Gerard
2000fist.conf..323H Altcode:
The UV background at a given redshift is representative of the starlight
and QSO-light generated in earlier redshifts. It holds key information
on the populations of stars and QSO's at high redshift. We have used
observations of the He II Gunn-Peterson (G-P) effect along the line
of sight to Q 0302-003 at z = 3.286 to constrain the spectral energy
distribution of the UV background spectrum over the redshift range, z =
2.78 - 3.28. Assuming that stars have softer spectra than do QSO's, we
used the softness of the UV background to discriminate between these
two possible classes of ionizing sources. We find that stars appear
to be significant contributors to the UV background at z &gt; 3, but
below z = 3, there is a hardening of the UV background suggestive of
an increased contribution by QSO's.

---------------------------------------------------------
Title: Model Atmospheres and Radiatively Driven Winds of Metal-Poor
Stars
Authors: Hubeny, Ivan; Lanz, Thierry; Heap, Sally
2000fist.conf..133H Altcode:
We present a grid of non-LTE metal-line-blanketed hydrostatic
model atmospheres for various effective temperatures, surface
gravities (luminosities), and metallicities. The metallicities range
from an essentially metal-free, primordial composition to a solar
composition. For each model, we calculate the exact radiation force and
compare to gravity acceleration. We thus find for each metallicity,
the maximum surface gravity as a function of effective temperature
required for the radiation force to exceed gravity and thus to initiate
a radiatively-driven wind.

---------------------------------------------------------
Title: Commission 36: Theory of Stellar Atmospheres: (Theorie des
Atmospheres Stellaires)
Authors: Pallavicini, R.; Dravins, D.; Barbuy, B.; Cram, L.; Hubeny,
I.; Owocki, S.; Saio, H.; Sasselov, D.; Spite, M.; Stepien, K.;
Wehrse, R.
2000IAUTA..24..219P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Properties of the z=2.73 Galaxy, MS1512-cB58
Authors: Heap, S. R.; Bouret, J. -C.; Hubeny, I.; Lanz, T. M.
1999AAS...195.0919H Altcode: 1999BAAS...31.1387H
MS1512-cB58 is an z=2.73 galaxy whose apparent brightness is amplified
by 30X due to gravitationally lensing. The restframe far-UV spectrum of
cB58 that was obtained by Keck-I/LRIS is “one of the best ultraviolet
spectra of a starburst galaxy obtained at any redshift” (Pettini et
al. 1999, astro-ph/9908007). We have therefore analyzed and modelled the
spectrum of this galaxy in order to obtain information about the stellar
content of high-redshift galaxies. We find that our model spectrum is
a near match to the observed spectrum of cB58 if we assume a constant
rate of star formation, a SMC-like metallicity, and a Salpeter IMF
extending up to 100 Msun. In our paper, we will present
the details of the spectral synthesis (based on both observed and
model spectra of OB stars), and we will investigate the reliability
of current evolutionary tracks (based on single, non-rotating stars)
for interpreting the spectra of star-forming galaxies.

---------------------------------------------------------
Title: The Eclipsing Cataclysmic Variable V347 Puppis Revisited
Authors: Diaz, Marcos P.; Hubeny, Ivan
1999ApJ...523..786D Altcode:
An observational study of the nova-like cataclysmic binary V347 Pup (LB
1800) is presented. An analysis of optical and UV spectroscopy is made
with the aim of defining the physical properties of the binary system
and of the accretion disk. The study of the line profile behavior and
the determination of the primary radial velocity are pursued using a
variety of methods. We also present the detection of secondary spectral
signatures that were used to derive the object's radial velocity
curve. A tentative companion spectral classification and spectroscopic
parallax are also given. A Doppler tomography study of Balmer and
He II lines lead us to an estimate of the average surface brightness
distribution of these lines in the accretion disk. Exploratory modeling
of the accretion disk in V347 Pup and comparison with UV observations
is carried on using the system parameters constrained by the radial
velocity study.

---------------------------------------------------------
Title: The D/H Ratio in Interstellar Gas toward G191-B2B
Authors: Sahu, M. S.; Landsman, W.; Bruhweiler, F. C.; Gull, T. R.;
Bowers, C. A.; Lindler, D.; Feggans, K.; Barstow, M. A.; Hubeny, I.;
Holberg, J. B.
1999ApJ...523L.159S Altcode: 1999astro.ph..8123S
Recent analysis of Goddard High-Resolution Spectrograph (GHRS) echelle
spectra suggests ~30% variations in the D/H abundance ratio along the
line of sight to the nearby (69 pc) hot white dwarf (WD) G191-B2B
(Vidal-Madjar et al.). Variations in the D/H ratio on such short
length scales imply nonuniform production/destruction of deuterium
and an inefficient mixing of gas in the local interstellar medium
(LISM). We reinvestigate the question of the spatial variation
of the local D/H abundance using both archival GHRS spectra and
new echelle spectra of G191-B2B obtained with the Space Telescope
Imaging Spectrograph (STIS) aboard the Hubble Space Telescope. The
STIS spectra were obtained in the high-resolution (E140H) mode
and cover the wavelength region ranging from 1140 to 1700 Å. Our
analysis uses stratified line-blanketed non-LTE model atmosphere
calculations to determine the shape of the intrinsic WD Lyα profile
and to estimate the WD photospheric contamination of the interstellar
lines. Although three velocity components were reported previously
toward G191-B2B, we deduce only two velocity components. The first
component is at vhel~8.6 km s-1, and the second
is at vhel~19.3 km s-1, which we identify
with the local interstellar cloud (LIC). From the STIS data, we
derive D/H = 1.60+0.39-0.27×10-5
for the LIC component and D/H&gt;1.26×10-5 for the 8.6 km
s-1 component (uncertainties denote 2σ or 95% confidence
limits). The derived D/H values in both components are consistent with
(D/H)LIC = (1.5+/-0.1)×10-5, which was determined
by Linsky in 1998. The STIS data provide no evidence for local or
component-to-component variation in the D/H ratio. Our reanalysis of
the GHRS data gives essentially the same results as Vidal-Madjar et al.,
despite using two velocity components for the profile fitting (vs. three
by Vidal-Madjar et al.) and a more physically realistic WD Lyα
profile for G191-B2B. The GHRS data indicate a component-to-component
variation as well as a variation of the D/H ratio in the LISM, neither
of which are supported by the newer STIS data. The D I absorption
in the GHRS spectrum is shallower than in the STIS spectrum. The
most probable cause for this difference in the two data sets is the
characterization of the background due to scattered light in the GHRS
and STIS spectrographs. The D/H ratios derived are sensitive to the
background-subtraction procedures employed. The two-dimensional MAMA
detectors of STIS measure both the spatial and wavelength dependences
of scattered light, allowing more accurate scattered-light corrections
than was possible with GHRS. Based on observations done with the
NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science
Institute, which is operated by the Association of Universities for
Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555.

---------------------------------------------------------
Title: Evidence for the stratification of Fe in the photosphere
of G191-B2B
Authors: Barstow, M. A.; Hubeny, I.; Holberg, J. B.
1999MNRAS.307..884B Altcode: 1999astro.ph..6191B
The presence of heavy elements in the atmospheres of the hottest H-rich
DA white dwarfs has been the subject of considerable interest. While
theoretical calculations can demonstrate that radiative forces,
counteracting the effects of gravitational settling, can explain
the detections of individual species, the predicted abundances
do not accord well with observation. However, accurate abundance
measurements can only be based on a thorough understanding of the
physical structure of the white dwarf photospheres, which has proved
elusive. Recently, the availability of new non-local thermodynamic
equilibrium model atmospheres with improved atomic data has allowed
self-consistent analysis of the extreme ultraviolet (EUV), far UV
and optical spectra of the prototypical object G191-B2B. Even so, the
predicted and observed stellar fluxes remain in serious disagreement at
the shortest wavelengths (below ~190Å), while the inferred abundances
remain largely unaltered. We show here that the complete spectrum of
G191-B2B can be explained by a model atmosphere where Fe is stratified,
with increasing abundance at greater depth. This abundance profile may
explain the difficulties in matching observed photospheric abundances,
usually obtained by analyses using homogeneous model atmospheres,
to the detailed radiative levitation predictions, particularly as
the latter are only strictly valid for regions deeper than where the
EUV/far UV lines and continua are formed. Furthermore, the relative
depletion of Fe in the outer layers of the atmosphere may be evidence
for radiatively driven mass-loss in G191-B2B.

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Title: RXTE, ROSAT, EUVE, IUE, and Optical Observations through the
45 Day Supercycle of V1159 Orionis
Authors: Szkody, Paula; Linnell, A.; Honeycutt, Kent; Robertson, Jeff;
Silber, Andrew; Hoard, D. W.; Pastwick, L.; Desai, V.; Hubeny, Ivan;
Cannizzo, John; Liller, William; Zissell, Ronald; Walker, Gary
1999ApJ...521..362S Altcode:
A complete 45 day supercycle of the cataclysmic variable V1159
Ori comprising a superoutburst and eight normal outbursts was
observed. Coverage included ground-based optical observations as well as
observations with RXTE for 38 days, ROSAT for 34 days, IUE for 27 days,
and Extreme Ultraviolet Explorer (EUVE) for 10 days. The resulting light
curves reveal that the optical and UV light variations are inversely
correlated with the RXTE and ROSAT fluxes, with the largest change in
intensity occurring in the ROSAT bandpass. There is no evidence for a
strong EUV/soft X-ray component during outburst. An outflowing wind
is evident from the C IV line profile during each brief outburst as
well as the superoutburst. The transitions from outburst states of the
disk to quiescent states take place on timescales of hours. Accretion
disk models can fit the UV line and continuum energy distributions
near outburst only if the disk radial temperature profile is modified
from the standard case to produce a hotter distribution in the outer
annuli. The high mass transfer rate, the hot disk distribution, and
the similarity of outbursts and superoutbursts argue for outside-in
outbursts in this system.

---------------------------------------------------------
Title: Opacities along the line of sight to and in the atmosphere
of the white dwarf in the close detached DAO+dM binary <ASTROBJ>RE
J0720-318</ASTROBJ>
Authors: Dobbie, P. D.; Barstow, M. A.; Burleigh, M. R.; Hubeny, I.
1999A&A...346..163D Altcode: 1999astro.ph..5205D
We present the results from a multi-wavelength study of the mixed H+He
composition DAO white dwarf <ASTROBJ>RE J0720-318</ASTROBJ>. A detailed
analysis of UV and EUV spectroscopic data with state-of-the-art non-LTE
photospheric models demonstrates that the observed opacity to EUV
radiation probably results from a more complex structure than a simple
H+He, chemically layered atmosphere. Instead, EUV photometry and phase
resolved EUV spectroscopy indicate a likely spatial non-uniformity in
the surface distribution of helium, which is consistent with a model
in which material is accreted from the wind of the dM secondary. The
rotational modulation of the spatially inhomogeneous EUV opacity
allows us to estimate the rotation period of the white dwarf (0.463 +/-
0.004 days). We have also reviewed two plausible origins proposed by
Burleigh et al. (\cite{burleigh97}) and Dupuis et al. (\cite{dupuis97a})
to account for the unusual N(HI)/N(HeI) ~ 1 along this line of sight. We
conclude that it is probably due to the presence of a cloud of ionized
gas along this line of sight, rather than a circumbinary disk. The
cloud, residing between 123-170 pc distant in the direction of the
CMa ISM tunnel, may be ~&gt; 40 pc in length.

---------------------------------------------------------
Title: Far-Ultraviolet Space Telescope Imaging Spectrograph Spectra
of the White Dwarf REJ 1032+532. II. Stellar Spectrum
Authors: Holberg, J. B.; Barstow, M. A.; Bruhweiler, F. C.; Hubeny,
I.; Green, E. M.
1999ApJ...517..850H Altcode:
We discuss the Hubble Space Telescope Space Telescope Imaging
Spectrograph UV echelle spectrum of the hot DA white dwarf REJ
1032+532. The interstellar data from this spectrum are presented by
Holberg and coworkers. In this paper we discuss a number of strong
photospheric features due to C, N, and Si that are present in the
REJ 1032+532 spectrum. While the inferred heavy element content of
REJ 1032+532 roughly matches the predictions of radiative levitation
for carbon and silicon, the observed nitrogen abundance greatly
exceeds predictions by a factor of 50. The observed shapes of the N V
lines provide the first evidence, at UV wavelengths, of heavy element
stratification in a hot DA white dwarf. Homogeneous models are unable to
reproduce the shape of the REJ 1032+532 N V lines, nor can they account
for the relatively low degree of EUV opacity in the star. We present
a simple stratified nitrogen model that resolves these problems. The
high degree of stratification in REJ 1032+532 is the signature of
ongoing mass loss in this star. The radial velocity of REJ 1032+532
obtained with Space Telescope Imaging Spectrograph differs by 44 km
s-1 from that obtained from the Balmer H I lines with the
Multiple Mirror Telescope. This suggests that REJ 1032+532 is likely
a member of a binary system containing either a late M star or another
white dwarf. Based on observations with the NASA/ESA Hubble Space
Telescope obtained at the Space Telescope Science Institute, which is
operated by the Association of Universities for Research in Astronomy,
Inc., under NASA contract NAS 5-26555. Observations reported here
have been obtained in part with the Multiple Mirror Telescope, a joint
facility of the University of Arizona and Smithsonian Institution.

---------------------------------------------------------
Title: Hot stellar population synthesis from the UV spectrum: the
globular cluster M79 (NGC 1904)
Authors: Vink, Jorick S.; Heap, Sara R.; Sweigart, Allen V.; Lanz,
Thierry; Hubeny, Ivan
1999A&A...345..109V Altcode: 1999astro.ph..3258V
We have analyzed the far UV-spectrum of the globular cluster
M79. We show that the nearly Gaussian mass distribution of zero-age
horizontal-branch stars, as derived by Dixon et al. (1996), is able
to reproduce the far-UV Hopkins Utraviolet Telescope (HUT) spectrum,
if there is a luminous UV-bright star of about T_eff\ = 9,500 K
within the HUT entrance slit, or, more likely, if the horizontal
branch morphology becomes considerably redder in the core of M79, as
observed in some other centrally condensed globular clusters. Agreement
between the synthetic and observed far-UV spectra for M79 would also
be improved if the surface abundances of the heavy elements in the hot
horizontal-branch stars were enhanced by radiative diffusion. Contrary
to Dixon et al. (1996) we do not need extremely low gravities to
reproduce the width of Ly alpha .

---------------------------------------------------------
Title: The D/H Ratio in Interstellar Gas towards G191-B2B from STIS
Echelle Observations
Authors: Sahu, M. S.; Landsman, W. B.; Bruhweiler, F. C.; Gull, T. R.;
Bowers, C. A.; Lindler, D.; Feggans, K.; Barstow, M. A.; Hubeny, I.;
Holberg, J. B.
1999AAS...194.7116S Altcode: 1999BAAS...31..945S
We present STIS echelle observations of interstellar D i and H i Lyα
and N i (1199.5, 1200.2 and 1200.7 Angstroms), C ii 1334.5 Angstroms,
C(*) ii 1335.7 Angstroms, O i 1302 Angstroms, Si ii (1190, 1193,
1260, 1304 and 1526 Angstroms), Si iii 1206.5 Angstroms, Al ii 1670.8
Angstroms, S ii 1259.5 Angstroms and Fe ii 1608.5 Angstroms in the line
of sight to the nearby (69 pc) hot, white dwarf (WD) G191-B2B. Compared
to the GHRS study of G191-B2B by Vidal-Madjar et al. 1998 (VM98),
the STIS E140H spectra have a higher velocity resolution (3 km s(-1)
), better S/N (between 20 to 50) and broader wavelength coverage
(1150 to 1700 Angstroms). We use the Barstow &amp; Hubeny stratified
non-LTE model atmosphere calculations which include the effects of
line-blanketing from more than 9x10(6) atomic transitions (mainly
Ni and Fe), both to determine the NLTE shape of the stellar Lyalpha
profile and to estimate the contamination of the interstellar lines
by WD photospheric lines. The interstellar N i 1200.7 Angstroms, Si
ii 1193 &amp; 1304 Angstroms and Fe ii lines show no contamination by
WD photospheric lines and are given more weight in our analysis. VM98
reported three components while we detect only two velocity components
in all the interstellar species observed: one at ~ 8.5 km s(-1) and one
at ~ 19.3 km s(-1) which we identify as the LIC component. Using the
NLTE stellar Lyα profile and a total column density of N(H i) ~ 2 x
10(18) cm(-2) for both components (consistent with EUVE observations),
we derive confidence contours. We find the D/H ratio with 2sigma
confidence limits to lie within 1.77+/-0.2x10(-5) . This value is
consistent with the value of (D/H)LIC = 1.6+/-0.1x10(-5)
determined towards Capella (Linsky et al. 1995). The STIS data provide
no evidence for local or cloud-to-cloud variation in the D/H ratio as
suggested by VM98. Re-analysis of the GHRS data and comparison to the
STIS data is in progress.

---------------------------------------------------------
Title: Detailed non-LTE Analysis of the High-S/N STIS Spectrum of
the Hot Subdwarf, BD+28{(deg}4211)
Authors: Hubeny, I.; Lanz, T.; Haas, S.; Heap, S. R.; Lindler, D. J.;
Kaiser, M. E.
1999AAS...194.6702H Altcode: 1999BAAS...31..930H
During the course of commissioning the Space Telescope Imaging
Spectrograph (STIS), observations were obtained to demonstrate the
feasibility of achieving very high S/N spectra in the ultraviolet
with the MAMA detectors. The hot subdwarf, BD+28{(deg}4211) , was
selected as the test case. The two series of FP-SPLIT observations
(E140M and E230M) have been shown to provide spectra with S/N=370 per
resolution element in both the far-UV and near-UV. The observations
were reduced by an iterative procedure that uses models of all relevant
scatter sources including echelle scatter, MAMA detector halo and OTA
scatter. We have performed a detailed analysis of the spectrum, using
sophisticated non-LTE model atmospheres including several million
metal lines in non-LTE. First, we derive basic parameters of this
star (effective temperature, surface gravity, chemical composition)
and compare to earlier results. We present a detailed comparison of
predicted and observed spectra in order to provide a benchmark for
future detailed spectroscopic studies of hot compact objects. Finally,
we present a preliminary analysis of the effects of elemental abundance
stratification on the selected line profiles with the aim of studying
the detectability of non-homogeneous abundance patterns.

---------------------------------------------------------
Title: Synthetic spectra of young starbursts: exploring the
metallicity dependence
Authors: de Koter, A.; Heap, S. R.; Hillier, D. J.; Hubeny, I.
1999IAUS..193..485D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dating intermediate-age populations with main-sequence A and
F-type stars
Authors: Lanz, T.; Heap, S.; Brown, T. M.; Hubeny, I.; Yi, S.
1999ASPC..192..106L Altcode: 1999sdsg.conf..106L
No abstract at ADS

---------------------------------------------------------
Title: On the Age Estimation of High Redshift Galaxies
Authors: Yi, S.; Brown, T. M.; Heap, S.; Hubeny, I.; Landsman, W.;
Lanz, T.; Sweigart, A.
1999ASPC..192..126Y Altcode: 1999sdsg.conf..126Y
No abstract at ADS

---------------------------------------------------------
Title: On the Age Estimation of LBDS 53W091
Authors: Yi, S.; Brown, T.; Heap, S.; Hubeny, I.; Landsman, W.; Lanz,
T.; Sweigart, A.
1999ASPC..193..185Y Altcode: 1999hrug.conf..185Y
No abstract at ADS

---------------------------------------------------------
Title: Mid-UV Spectroscopic Dating of LBDS 53W091
Authors: Heap, S. R.; Lanz, T. M.; Brown, T.; Hubeny, I.
1999ASPC..193..167H Altcode: 1999hrug.conf..167H
No abstract at ADS

---------------------------------------------------------
Title: Non-LTE model atmospheres of hot DA white dwarfs
Authors: Hubeny, I.; Barstow, M. A.; Lanz, T.; Holberg, J. B.
1999ASPC..169..445H Altcode: 1999ewwd.conf..445H
No abstract at ADS

---------------------------------------------------------
Title: Spectrophotometric Dating of Stars and Galaxies
Authors: Hubeny, Ivan; Heap, Sally; Cornett, Robert
1999ASPC..192.....H Altcode: 1999sdsg.conf.....H
No abstract at ADS

---------------------------------------------------------
Title: New Light Synthesis and Spectrum Synthesis Constraints on a
Model for β Lyrae
Authors: Linnell, A. P.; Hubeny, I.; Harmanec, P.
1998ApJ...509..379L Altcode:
A suite of programs that calculates both synthetic light curves and
synthetic spectra for a binary system with an optically thick accretion
disk has been applied for the first time to β Lyrae A. Our results
demonstrate that the standard accretion disk model by Hubeny &amp;
Plavec shows significant residuals from observations, both photometric
and spectroscopic, and that no changes in adjustable parameters are
able to remove the residuals. The basic problem is that the Hubeny
&amp; Plavec model requires a small visible segment of the hot star
at the center of the accretion disk and this requirement conflicts
with the photometric evidence. As an alternative, we investigate
standard accretion disk models in which the central star is hidden from
view. We find that no model of this type can satisfy either the observed
photometry or the IUE spectra. To resolve this impasse, we suggest the
presence of a light-scattering region above the accretion disk faces,
which scatters light from the central star into the line of sight and
provides the high-Teff radiation component required by both
the light curves and the observed spectra. This source of radiation is
very likely related to the jets proposed by Harmanec et al. and Hoffman
et al. We calculate the location of stream impact on the accretion disk
rim. Observations show no detectable photometric signature of a rim
bright spot. We suggest that the liberated kinetic energy is spread
over a region sufficiently large and at a sufficient optical depth to
suppress appearance of a bright spot. A slight asymmetry of primary
minimum may indicate swelling of the accretion disk rim downstream
from the stream impact point, with an attendant slight increase in
obscuration of the background light sources.

---------------------------------------------------------
Title: Age and Metallicity of F-type Stars
Authors: Lanz, T.; Heap, S. R.; Brown, T. M.; Hubeny, I.; Yi, S.
1998AAS...193.0302L Altcode: 1998BAAS...30.1246L
The rest-frame ultraviolet spectrum of intermediate-age stellar
populations (1 to few Gyr) is dominated by the UV flux of F-type
stars at the main sequence turn-off. We have obtained high-resolution
spectra of four well-studied F stars with STIS aboard HST to calibrate
ultraviolet spectral features in terms of age and metallicity. The UV
spectrum is mostly degenerate in terms of effective temperature and
metallicity, but we found that two spectral indices, [2850/2770] and
[2310/3040], allow a discrimination between these two parameters. The
first index measures mainly the strength of Mg I 2852, while the second
index is the mid-UV color. We find that the Mg I index is sensitive
to the effective temperature, but insensitive to metallicity; the
UV color is sensitive to both metallicity and T_eff. We discuss the
importance of departures from LTE and chromospheric activity on these
two spectral indices. We apply our results to estimate the time since
the last major star formation episode in the early-type galaxy LBDS
53W091 at redshift z=1.55.

---------------------------------------------------------
Title: STIS Observations of He II Gunn-Peterson Absorption Toward
Q0302-003
Authors: Heap, S. R.; Williger, G. M.; Smette, A.; Hubeny, I.; Sahu,
M.; Jenkins, E. B.; Tripp, T. M.; Winkler, J. N.
1998astro.ph.12429H Altcode:
The ultraviolet spectrum (1145--1720A) of the distant quasar Q
0302--003 (z=3.286) was observed at 1.8A resolution with the Space
Telescope Imaging Spectrograph aboard the Hubble Space Telescope. A
total integration time of 23,280 s was obtained. The spectrum clearly
delineates the Gunn-Peterson HeII absorption trough, produced by HeII
Lya, along the line of sight over the redshift range z=2.78-3.28. Its
interpretation was facilitated by modeling based on Keck HIRES spectra
of the HI Lya forest (provided by A. Songaila and by M. Rauch and
W. Sargent). We find that near the quasar HeII Lya absorption is
produced by discrete clouds, with no significant diffuse gas; this
is attributed to a HeII "proximity effect" in which the quasar fully
ionizes He in the diffuse intergalactic medium, but not the He in denser
clouds. By two different methods we calculate that the average HeII
Lya opacity at z~3.15 is tau &gt;= 4.8. In the Dobrzycki-Bechtold
void in the HI Lya forest near z=3.18, the average HeII opacity
tau=4.47^{+0.48}_{-0.33}. Such large opacities require the presence of
a diffuse gas component as well as a soft UV background spectrum, whose
softness parameter, defined as the ratio of the photo-ionization rate
in HI over the one in HeII, S=Gamma^J_HI/Gamma^J_HeII~=800, indicating a
significant stellar contribution. At z=3.05, there is a distinct region
of high HeII Lya transmission which most likely arises in a region where
helium is doubly ionized by a discrete local source, quite possibly an
AGN. At redshifts z&lt;2.87, the HeII Lya opacity detected by STIS,
tau=1.88, is significantly lower than at z&gt;3. Such a reduction in
opacity is consistent with Songaila's (1998) report that the hardness
of the UV background spectrum increases rapidly from z=3 to z=2.9.

---------------------------------------------------------
Title: Detailed Mid- and Far-Ultraviolet Model Spectra for Accretion
Disks in Cataclysmic Binaries
Authors: Wade, Richard A.; Hubeny, Ivan
1998ApJ...509..350W Altcode: 1998astro.ph..6399W
We present a large grid of computed far- and mid-ultraviolet spectra
(850-2000 Å) of the integrated light from steady-state accretion
disks in luminous cataclysmic variables. The spectra are tabulated at
0.25 Å intervals with an adopted FWHM resolution of 1.0 Å, so they
are suitable for use with observed spectra from a variety of modern
space-borne observatories. Twenty-six different combinations of white
dwarf mass Mwd and mass accretion rate ṁ are considered,
and spectra are presented for six different disk inclinations i. The
disk models are computed self-consistently in the plane-parallel
approximation, assuming LTE and vertical hydrostatic equilibrium, by
solving simultaneously the radiative transfer, hydrostatic equilibrium,
and energy balance equations. Irradiation from external sources is
neglected. Local spectra of disk annuli are computed taking into account
line transitions from elements 1-28 (H through Ni). Limb darkening as
well as Doppler broadening and blending of lines are taken into account
in computing the integrated disk spectra. The radiative properties
of the models are discussed, including the dependence of ultraviolet
fluxes and colors on Mwd, ṁ, and i. The appearance of
the disk spectra is illustrated, with regard to changes in the same
three parameters. Finally, possible future improvements to the present
models and spectra are discussed.

---------------------------------------------------------
Title: An Empirical Isochrone of Very Massive Stars in R136a
Authors: de Koter, Alex; Heap, Sara R.; Hubeny, Ivan
1998ApJ...509..879D Altcode:
We report on a detailed spectroscopic study of 12 very massive and
luminous stars (M &gt;~ 35M⊙) in the core of the compact
cluster R136a, near the center of the 30 Doradus complex. The three
brightest stars of the cluster, R136a1, R136a2, and R136a3, have been
investigated earlier by de Koter, Heap, &amp; Hubeny. Low-resolution
spectra (&lt;200 km s-1) of the program stars were obtained
with the GHRS and FOS spectrographs on the Hubble Space Telescope. These
instruments covered the spectral range from 1200 to 1750 Å and from
3200 to 6700 Å, respectively. Fundamental stellar parameters were
obtained by fitting the observations by model spectra calculated with
the unified ISA-WIND code of de Koter et al. supplemented by synthetic
data calculated using the program TLUSTY. We find that the stars are
almost exclusively of spectral type O3. They occupy only a relatively
narrow range in effective temperatures between 40 and 46 kK. The
reason for these similar Teff's is that the isochrone of
these very massive stars, which we determined to be at ~2 Myr, runs
almost vertically in the H-R diagram. We present a quantitative method
of determining the effective temperature of O3-type stars based on the
strength of the O V λ1371 line. Present-day evolutionary calculations
by Meynet et al. imply that the program stars have initial masses in the
range of Mi ~ 37-76 M⊙. The observed mass-loss
rates are up to 3 (2) times higher than is assumed in these evolution
tracks when adopting a metallicity Z = 0.004 (0.008) for the LMC. The
high observed mass-loss rates imply that already at an age of ~2 Myr
the most luminous of our program stars will have lost a significant
fraction of their respective initial masses. For the least luminous
stars investigated in this paper, the observed mass loss agrees with
the prediction by the theory of radiation-driven winds (Kudritzki et
al.). However, for increasing luminosity the observed mass loss becomes
larger, reaching up to 3-4 times what is expected from the theory. Such
an increasing discrepancy fits in with the results of de Koter et al.,
where an observed overpredicted mass-loss ratio of up to 8 was reported
for the brightest members of the R136a cluster, for which Mi
~ 100 M⊙ was found. The failure of the theory is also
present when one compares observed over predicted wind momentum as
a function of wind performance number. This strongly indicates that
the shortcoming of the present state of the theory is connected to the
neglect of effects of multiple photon momentum transfer. Based on
observations with the NASA/ESA Hubble Space Telescope obtained at the
Space Telescope Science Institute, which is operated by AURA, Inc.,
under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: Multidimensional, non-LTE Radiation Transport in Astrophysical
Media
Authors: Hubeny, I.; van Noort, M.; Lanz, T.
1998AAS...193.6918H Altcode: 1998BAAS...30.1352H
We present first results of our long-term program whose ultimate goal
is to develop multidimensional, self-consistent, non-LTE radiation
hydrodynamic and magnetohydrodynamic spectroscopic diagnostics of
astronomical objects. As the first step, we have developed an efficient
and robust 2-D and 3-D radiation transfer solver, based on the short
characteristics method. We assume the source function either to be
fully specified (i.e., the so-called formal solution of the transfer
equation), or to be given in terms of the equivalent-two-level
atom formalism. In the latter case, we have adopted the idea of
the Accelerated Lambda Iteration technique to treat iteratively the
non-LTE coupling of radiation and matter. We compare the efficiency and
numerical accuracy of several variants of the short-characteristics
scheme. Finally, we present some illustrative examples of radiative
transfer for 2-D and 3-D media with the specified temperature, density,
and velocity structure.

---------------------------------------------------------
Title: STIS Spectroscopy of Q0302-003: The HeII Gunn Peterson Effect
Authors: Heap, S. R.; Hubeny, I.; Jenkins, E. B.; Sahu, M. S.; Smette,
A.; Tripp, T. M.; Williger, G. M.; HST/STIS IDT
1998AAS...193.0404H Altcode: 1998BAAS...30.1248H
We present a new far-UV spectrum of the z=3.29 QSO, Q0302-003, obtained
by STIS. This spectrum traces redshifted HeII Lyman-alpha lambda 304
absorption over a redshift range, z=2.8 - 3.28, at a resolution of
Delta lambda =1 Angstroms. In agreement with previous observations by
the FOC (Jakobsen et al. 1994) and the GHRS (Hogan et al. 1997), the
STIS spectrum clearly shows the “Gunn-Peterson absorption trough”
formed by redshifted HeII Lyman-alpha . The STIS spectrum, however,
also shows that the HeII opacity is not constant with redshift:
there is a hole in the opacity at z=3.05 as well as numerous minor
fluctuations in opacity at z&lt;3.05. We will describe the major
features of the absorption trough and will compare the observed
distribution of opacities with theoretical predictions.

---------------------------------------------------------
Title: Non-LTE Models and Theoretical Spectra of Accretion Disks in
Active Galactic Nuclei. II. Vertical Structure of the Disk
Authors: Hubeny, Ivan; Hubeny, Veronika
1998ApJ...505..558H Altcode: 1998astro.ph..4288H
We have calculated several representative models of vertical structure
of an accretion disk around a supermassive Kerr black hole. The
interaction of radiation and matter is treated self-consistently,
taking into account departures from LTE for calculating both the
disk structure and the radiation field. The structural equations are
described in detail, and various approximations are discussed. We have
demonstrated that departures from LTE are very important for determining
the disk structure, even at the midplane, as well as the emergent
radiation, particularly for hot and electron-scattering-dominated
disks. We have shown that at least for the disk parameters studied
in this paper, non-LTE effects tend to reduce the value of the Lyman
jump with respect to the LTE predictions, regardless whether LTE
predicts an emission or absorption jump. We have studied the effects
of various values of viscosity on the model structure and predicted
spectral energy distribution. The viscosity is parameterized through a
parameter α0 that describes the vertically averaged viscous
stress, two power-law exponents, ζ0 and ζ1,
and the division point md between these two forms. The disk
structure and emergent radiation is sensitive mainly to the values of
α0, while the other parameters influence the disk structure
to a much lesser extent. However, although the detailed shape of the
predicted spectrum is sensitive to adopted value of α0,
the overall appearance of the spectrum is quite similar.

---------------------------------------------------------
Title: An alternative explanation of the EUV spectrum of the white
dwarf G191-B2B invoking a stratified H+He envelope including heavier
elements
Authors: Barstow, M. A.; Hubeny, I.
1998MNRAS.299..379B Altcode:
Only recently has it been possible to find an effective temperature and
composition for the hot DA white dwarf G191-B2B which can consistently
match the optical, far-UV and EUV data simultaneously, by using new
non-LTE model calculations that include the effects of line blanketing
from more than 9 million (mainly Fe and Ni) transitions. However, to
maintain agreement below the Heii 228-Angstroms Lyman limit requires
the inclusion of additional Heii opacity in the form of photospheric,
circumstellar or interstellar material. If photospheric, the homogeneous
H+He structure adopted predicted a Heii 1640-Angstroms line strength
well above the IUE detection limit, while an unusally high He ionization
fraction was required in this particular line of sight if the material
was in the local ISM. We show that, by considering the effects of a
stratified H upon He photospheric structure within the heavy-element
blanketed models, a lower interstellar Heii column density is obtained,
leading to a more reasonable He ionization fraction for the G191-B2B
line of sight. In addition, the absence of a Heii 1640-Angstroms
feature in the far-UV data is consistent with the equivalent width
predicted by the stratified model. However, while this alternative
interpretation offers a better description of certain aspects of the
G191-B2B optical-to-EUV spectra, a new problem is introduced, namely
that the Heii Lyman lines are predicted to be stronger than can be
comfortably accommodated by the observational data.

---------------------------------------------------------
Title: The effect of photospheric heavy elements on the hot DA white
dwarf temperature scale
Authors: Barstow, M. A.; Hubeny, I.; Holberg, J. B.
1998MNRAS.299..520B Altcode:
Using the latest non-local thermodynamic equilibrium (non-LTE) synthetic
spectra and stellar model calculations, we have evaluated the potential
effect of the presence of heavy elements in the photospheres of hot
H-rich DA white dwarfs. In particular, we have examined their influence
on the effective temperature and surface gravity perceived from analysis
of the Balmer line profiles. It is apparent that both the inclusion
of non-LTE effects in the models and significant quantities of heavy
elements act independently to lower the value of T_eff determined from a
particular spectrum. Hence, the true effective temperatures of the heavy
element-rich DA white dwarfs, currently estimated to be above 55000 K,
are apparently lower than previously reported from pure-H LTE analyses,
by some 4000-7000 K. We do not see any similar influence on measurements
of log g. This work concentrates on a group of relatively bright
well-studied objects, for which heavy element abundances are known. As
a consequence of this, establishment of correct temperatures for all
other hot white dwarfs will require a programme of far-UV spectroscopy
in order to obtain the essential compositional information. Since only
stars with effective temperatures lying notionally in the range from
~55000 to 70000 K (52000-62000 K when the non-LTE effects and heavy
elements are taken into account) have been considered here, important
questions remain regarding the magnitude of any similar effects in even
hotter white dwarfs and pre-white dwarfs. The resulting implications
for the plausibility of the evolutionary link between the main hot DA
population and their proposed precursors, the H-rich central stars of
planetary nebulae, need to be investigated.

---------------------------------------------------------
Title: HST spatially resolved spectra of the accretion disc and gas
stream of the nova-like variable UX Ursae Majoris
Authors: Baptista, Raymundo; Horne, Keith; Wade, Richard A.; Hubeny,
Ivan; Long, Knox S.; Rutten, Rene G. M.
1998MNRAS.298.1079B Altcode: 1998astro.ph..4002B
Time-resolved eclipse spectroscopy of the nova-like variable UX UMa
obtained with the HST/FOS on 1994 August and November is analysed
with eclipse mapping techniques to produce spatially resolved spectra
of its accretion disc and gas stream as a function of distance from
the disc centre. The inner accretion disc is characterized by a blue
continuum filled with absorption bands and lines, which cross over
to emission with increasing disc radius, similar to that reported
by Rutten et al. at optical wavelengths. The comparison of spatially
resolved spectra at different azimuths reveals a significant asymmetry
in the disc emission at ultraviolet (UV) wavelengths, with the disc
side closest to the secondary star showing pronounced absorption by an
`iron curtain' and a Balmer jump in absorption. These results suggest
the existence of an absorbing ring of cold gas whose density and/or
vertical scale increase with disc radius. The spectrum of the infalling
gas stream is noticeably different from the disc spectrum at the same
radius suggesting that gas overflows through the impact point at the
disc rim and continues along the stream trajectory, producing distinct
emission down to 0.1R_L1. The spectrum of the uneclipsed light shows
prominent emission lines of Lyalpha, Nv lambda1241, Siiv lambda1400,
Civ lambda1550, Heii lambda1640, and Mgii lambda2800, and a UV continuum
rising towards longer wavelengths. The Balmer jump appears clearly
in emission indicating that the uneclipsed light has an important
contribution from optically thin gas. The lines and optically thin
continuum emission are most probably emitted in a vertically extended
disc chromosphere + wind. The radial temperature profiles of the
continuum maps are well described by a steady-state disc model in the
inner and intermediate disc regions (R&lt;=0.3R_L1). There is evidence
of an increase in the mass accretion rate from August to November (from
M^. = 10^-8.3 +/- 0.1 to 10^-8.1 +/- 0.1M/yr^-1), in accordance with the
observed increase in brightness. Since the UXUMa disc seems to be in a
high mass accretion, high-viscosity regime in both epochs, this result
suggests that the mass transfer rate of UX UMa varies substantially
(~= 50 per cent) on time-scales of a few months. It is suggested that
the reason for the discrepancies between the prediction of the standard
disc model and observations is not an inadequate treatment of radiative
transfer in the disc atmosphere, but rather the presence of additional
important sources of light in the system besides the accretion disc
(e.g. optically thin continuum emission from the disc wind and possible
absorption by circumstellar cool gas).

---------------------------------------------------------
Title: A Grid of Non-LTE Models and Theoretical Spectra of AGN
Accretion Disks
Authors: Hubeny, I.; Blaes, O.; Agol, E.
1998AAS...192.3606H Altcode: 1998BAAS...30..862H
We present a grid of non-LTE models of the vertical structure and
emergent spectra of accretion disks around supermassive Kerr black
holes. For computing the disk structure, the interaction of radiation
and matter is treated self-consistently, taking into account departures
from LTE for calculating both the disk structure and the radiation
field. The basic parameters of the system are the black hole mass,
M, the mass accretion rate, dot {M}, the angular momentum (rotation)
of the black hole (a/M), and the chemical composition of the disk. The
viscosity is parameterized through a parameter alpha which describes the
vertically-averaged viscous stress, two power-law exponents zeta_0 and
zeta_1 , and the division point m_d between these two forms. The disk
structure and emergent radiation is sensitive mainly to the values of
alpha , while the other parameters influence the disk structure to a
much lesser extent. We have constructed a grid of non-LTE disk models
for M = 1, 2, 4, 8, 10, 16, and 32 (in units of 10(9) M_sun), and the
mass accretion rates 1, 2, 4, 8, 10, 16 M_sun/yr, for all combinations
of these parameters which yield the total luminosity below the Eddington
limit. In the present study, we have considered only H-He models with
a solar helium abundance. We have assumed the maximum stable rotation
of the black hole, a/M=0.998, and alpha was set to 0.01. Each disk
was divided into a set of 20 - 25 concentric annuli covering the
region between radii 1.3 to 50 gravitational radii, R_g = G M/c(2)
. These annuli provide all emergent UV and harder disk radiation, and
most of its optical radiation. The disk structure (including general
relativistic effects) was computed with the code TLUSDISK; the local
spectra of the annuli by program SYNSPEC; and the integrated spectrum
of the disk taking into account all general relativistic effects was
done by program KERRTRANS (Agol 1997).

---------------------------------------------------------
Title: Hubble Space Telescope Eclipse Observations of the Nova-like
Cataclysmic Variable UX Ursae Majoris
Authors: Knigge, Christian; Long, Knox S.; Wade, Richard A.; Baptista,
Raymundo; Horne, Keith; Hubeny, Ivan; Rutten, René G. M.
1998ApJ...499..414K Altcode: 1998astro.ph..1206K
We present and analyze Hubble Space Telescope observations of the
eclipsing nova-like cataclysmic variable UX UMa obtained with the
Faint Object Spectrograph. Two eclipses each were observed with the
G160L grating (covering the ultraviolet waveband) in 1994 August and
with the PRISM (covering the near-ultraviolet to near-infrared) in
November of the same year. The system was ~50% brighter in November
than in August, which, if due to a change in the accretion rate,
indicates a fairly substantial increase in Ṁacc by
&gt;~50%. The eclipse light curves are qualitatively consistent with
the gradual occultation of an accretion disk with a radially decreasing
temperature distribution. The light curves also exhibit asymmetries
about mideclipse that are likely due to a bright spot at the disk
edge. Bright-spot spectra have been constructed by differencing the
mean spectra observed at pre- and posteclipse orbital phases. These
difference spectra contain ultraviolet absorption lines and show
the Balmer jump in emission. This suggests that part of the bright
spot may be optically thin in the continuum and vertically extended
enough to veil the inner disk and/or the outflow from UX UMa in some
spectral lines. Model disk spectra constructed as ensembles of stellar
atmospheres provide poor descriptions of the observed posteclipse
spectra, despite the fact that UX UMa's light should be dominated
by the disk at this time. Suitably scaled single temperature model
stellar atmospheres with Teff ~= 12,500-14,500 K actually
provide a better match to both the ultraviolet and optical posteclipse
spectra. Evidently, great care must be taken in attempts to derive
accretion rates from comparisons of disk models to observations. One
way to reconcile disk models with the observed posteclipse spectra
is to postulate the presence of a significant amount of optically
thin material in the system. Such an optically thin component
might be associated with the transition region (“chromosphere”)
between the disk photosphere and the fast wind from the system, whose
presence has been suggested by Knigge &amp; Drew. In any event, the
wind/chromosphere is likely to be the region in which many, if not
most, of the UV lines are formed. This is clear from the plethora of
emission lines that appear in the mideclipse spectra, some of which
appear as absorption features in spectra taken at out-of-eclipse
orbital phases. Based on observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS 5-2655.

---------------------------------------------------------
Title: Light Synthesis and Spectrum Synthesis Constraints on a Model
for beta Lyrae
Authors: Linnell, A. P.; Hubeny, I.; Harmanec, P.
1998AAS...192.2602L Altcode: 1998BAAS...30..850L
We have used the program suite BINSYN (Linnell &amp; Hubeny 1996)
to simulate recent optical light curves (Harmanec et al. 1996), OAO2
light curves, and IUE spectra of beta Lyrae. beta Lyrae A is a mass
transfer system in which the mass donor is the component eclipsed at
primary minimum (optical wavelengths). That component has a mass of
about 2.9 M_⊙ (Harmanec &amp; Scholz 1993). The companion has a mass
of about 13.1 M_⊙ and is embedded in a thick accretion disk. The
only successful simulation of observed beta Lyrae light curves,
particularly OAO2 UV light curves, has been by Wilson &amp; Lapasset
(1981,WL). WL modeled the mass gainer as a greatly flattened oblate
spheroid. A standard model accretion disk representation was developed
by Hubeny &amp; Plavec (1981,HP). The continuum requires the presence
of a high T_eff component which HP attribute to a small visible segment
of the hot central star. HP point out that the WL model, which is not a
standard accretion disk model, cannot represent the required high T_eff
component, and so cannot be an acceptable model. The HP model leads to
deep UV stellar eclipses of the visible segment of the hot central star;
but the OAO2 data exclude this model. The OAO2 data require an accretion
disk thick enough completely to hide the central star. Extensive tests
with BINSYN show that no standard model accretion disk can represent the
optical wavelength light curves, the OAO2 data, and the IUE spectra. The
crux of the problem is an inability to provide the high T_eff source,
required by both the UV light curves and the IUE spectra. To resolve
this impasse, we propose the existence of a corona which scatters light
from the otherwise hidden hot central star into the line of sight to the
observer. We plan to develop a BINSYN addition to test this proposal.

---------------------------------------------------------
Title: Continuum spectra of quasar accretion disk models
Authors: Agol, Eric; Hubeny, Ivan; Blaes, Omer
1998AIPC..431..175A Altcode: 1998apas.conf..175A; 1997astro.ph.12162A
We have calculated the spectrum and polarization of a standard thin
accretion disk with parameters appropriate for a bright quasar. This
model improves upon previous work by including ultraviolet metal
line opacities, assumed for now to be in LTE. Though not yet fully
self-consistent, our calculations demonstrate that metal lines can
change the spectral slope, reduce the polarization, and reduce the Lyman
edge feature in accretion disk spectra. Some observational differences
between quasar spectra and accretion disk models might be reconciled
with the inclusion of metal lines.

---------------------------------------------------------
Title: Model atmospheres and synthetic spectra of quiescent accretion
discs
Authors: Still, Martin D.; Horne, Keith; Hubeny, Ivan
1998AIPC..431..133S Altcode: 1998apas.conf..133S
We aim to employ a fully self-consistent atmosphere code to determine
surface density, Σ, effective temperature, Teff, and microturbulence,
Vturb, from emission line spectra of quiescent accretion discs. We
have built a grid of solar-abundance, LTE, isothermal disc atmosphere
models and investigate the sensitivity of the optical spectrum to Teff,
Σ and Vturb. The results of fitting the grid to a synthetic quiescent
disc spectrum and the spectrum of the dwarf nova SS Cyg are presented.

---------------------------------------------------------
Title: Non-LTE models and theoretical spectra of accretion disks in
Active Galactic Nuclei
Authors: Hubeny, Ivan; Hubeny, Veronika
1998AIPC..431..171H Altcode: 1998apas.conf..171H
We present self-consistent models of the vertical structure and emergent
spectrum of AGN accretion disks. The central object is assumed to be
a supermassive Kerr black hole. We demonstrate that non-LTE effects
and the effects of a self-consistent vertical structure of a disk
play a very important role in determining the emergent radiation,
and therefore should be taken into account. In particular, non-LTE
models exhibit a largely diminished H I Lyman discontinuity, and strong
emission in the He II Lyman discontinuity. Consequently, the number of
ionizing photons in the He II Lyman continuum predicted by non-LTE disk
models is by 1-2 orders of magnitude higher than that following from the
black-body approximation. We have also shown that metal line opacities
are very important in predicting emergent spectra from AGN disks.

---------------------------------------------------------
Title: A Photometric and Spectrophotometric Study of MR Cygni
Authors: Linnell, A. P.; Etzel, P. B.; Hubeny, I.; Olson, E. C.
1998ApJ...494..773L Altcode:
A self-consistent, physically accurate program suite has been used
in an accurate simulation of new spectroscopy and photometry of MR
Cygni. Analysis of both the spectroscopic and photometric data used
spectrum synthesis techniques and a synthetic photometry augmentation of
a light synthesis program package. The theoretical light curves closely
fit the observational data. The same self-consistent parameters from
the light synthesis solution produced synthetic spectra precisely
fitting the observed spectra at all orbital phases. The
IRAF-reduced spectroscopy has produced an accurate double-lined
radial velocity curve. The derived mass ratio differs greatly from
photometric mass ratios in the literature. New UBV photometry closely
replicates existing data and indicates photometric stability of the
binary system. A synthetic spectrum fitted to IUE data established the
primary component Teff. The light curve solution determined
a single set of system parameters used to calculate U, B, and V light
curves. We conclude that MR Cygni is a member of the relatively rare
class of hot Algol systems defined by Popper. It is likely that mass
transfer still is in progress, but there is no evidence, either from
orbital period variation or from a bright spot on the mass gainer,
for its existence. The lack of Hα emission in any of our spectra,
including one at phase 0.063, suggests a small current rate of mass
transfer. The fact that our computationally self-consistent procedure
has successfully represented both the photometry and the spectroscopy
for a binary system whose components are appreciably distorted
demonstrates the overall power of the procedure.

---------------------------------------------------------
Title: Assessing the sensitivity of atmosphere FITS to accretion
disc spectra
Authors: Still, M. D.; Horne, K.; Hubeny, I.
1998ASPC..137..552S Altcode: 1998wsow.conf..552S
No abstract at ADS

---------------------------------------------------------
Title: HST Observations of the Fe II curtain in CVs
Authors: Catalán, M. S.; Horne, K.; Cheng, F. H.; Marsh, T. R.;
Hubeny, I.
1998ASPC..137..426C Altcode: 1998wsow.conf..426C
No abstract at ADS

---------------------------------------------------------
Title: Non-LTE Line-Blanketed Model Atmospheres of O Stars
Authors: Hubeny, I.; Heap, S. R.; Lanz, T.
1998ASPC..131..108H Altcode: 1998phls.conf..108H
No abstract at ADS

---------------------------------------------------------
Title: Non-LTE line-blanketed model atmospheres of hot stars
Authors: Hubeny, I.
1998ASPC..138..139H Altcode: 1998stas.conf..139H
No abstract at ADS

---------------------------------------------------------
Title: Ultraviolet Spectral Dating of Stars and Galaxies
Authors: Heap, S. R.; Brown, T. M.; Hubeny, I.; Landsman, W.; Yi,
S.; Fanelli, M.; Gardner, J. P.; Lanz, T.; Maran, S. P.; Sweigart,
A.; Kaiser, M. E.; Linsky, J.; Timothy, J. G.; Lindler, D.; Beck,
T.; Bohlin, R. C.; Clampin, M.; Grady, J.; Loiacono, J.; Krebs, C.
1998ApJ...492L.131H Altcode:
An echelle spectrogram (R = 30,000) of the 2300-3100 Å region in
the ultraviolet spectrum of the F8 V star 9 Comae is presented. The
observation is used to calibrate features in the mid-ultraviolet spectra
of similar stars according to age and metal content. In particular, the
spectral break at 2640 Å is interpreted using the spectral synthesis
code SYNSPEC. We use this feature to estimate the time since the last
major star formation episode in the early-type galaxy LBDS 53W091 at
redshift z=1.55, whose rest-frame mid-ultraviolet spectrum, observed
with the Keck Telescope, is dominated by the flux from similar stars
that are at or near the main-sequence turnoff in that system (Spinrad
et al.). Our result, 1 Gyr if the flux-dominating stellar population
has a metallicity twice solar, or 2 Gyr for a more plausible solar
metallicity, is significantly lower than the previous estimate and
thereby relaxes constraints on cosmological parameters that were implied
by the earlier work. Based on observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5-2655.

---------------------------------------------------------
Title: HST synthetic spectral analysis of U GEM in early and late
quiescence: A heated white dwarf and accretion belt?
Authors: Cheng, F. H.; Sion, E. M.; Horne, Keith; Hubeny, I.; Huang,
M.; Vrtilek, S. D.
1997AJ....114.1165C Altcode:
We have re-examined two archival HST FOS G130H spectra of the prototype
dwarf nova U Geminorum obtained during its quiescence 13 days and 70
days after a wide outburst. Using synthetic spectral fitting with two
flux-emitting components, a slowly rotating white dwarf photosphere,
and a rapidly spinning accretion belt (Vrot sin i=3200 km
s-1) significantly improves the spectral fit but does not
provide a unique solution. We found clear evidence for the cooling
of the white dwarf, confirming earlier results, and evidence for the
cooling of the accretion belt or gas in Keplerian motion as well. If
an accretion belt is really present, then for the white dwarf and belt,
respectively, 13 days post-outburst, we find Twd=37 000±400
K and Tbelt=45 000 ±2500 K while at 70 days post outburst,
we find Twd=33 500 ±700 K and Tbelt=37 500 ±4000
K. These results are compared with our HST GHRS G160M observations
obtained 13 days and 61 days after a narrow outburst of U Gem.

---------------------------------------------------------
Title: Non-LTE Line-blanketed Model Atmospheres of Hot Stars. III. Hot
Subdwarfs: The sdO Star BD +75°325
Authors: Lanz, Thierry; Hubeny, Ivan; Heap, Sara R.
1997ApJ...485..843L Altcode:
We have made a detailed comparison of results of spectroscopic analysis
using three differents types of model atmospheres: classical non-LTE
H-He models; approximate non-LTE line-blanketed models (with only
a subset of Fe and Ni lines: those originating from transitions
between levels with measured energies); and non-LTE fully blanketed
models. The three models were applied to the sdO star BD +75°325,
adopted as a test case. We demonstrate that the effects of
line blanketing are very important: the best fit of the observed H
and He lines is achieved for Teff = 58,000 K when using
H-He models, Teff = 55,000 K for approximate non-LTE
line-blanketed models, and Teff = 52,000 K for fully
blanketed non-LTE models. Using the high-resolution Goddard
High Resolution Spectrograph spectrum of BD +75°325 and our final
fully blanketed model, we have derived reliable abundances of He, C,
N, O, Si, Fe, and Ni. We find that BD +75°325 is an He-enriched star
(He/H = 1 by number), whose surface exhibits CNO-cycle products, i.e.,
N-rich (AN = 1.5 × 10-3 by number, or 4.2 ×
10-3 by mass fraction), and C and O deficient (by factor of
about 100 with respect to the solar value). We also find a significant
surface depletion of silicon and an enhancement of iron and nickel. We
argue that these anomalous abundances reflect some mixing with processed
material from the core, with subsequent modification at the surface by
diffusion processes. Finally, BD +75°325 possesses a weak wind. Using
a simplified description of the wind, we have derived a preliminary
value of the mass loss rate: Ṁ=1.5×10-11 M⊙
yr-1. Based on observations with the ESA/NASA Hubble
Space Telescope obtained at the Space Telescope Science Institute,
which is operated by AURA, Inc., under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: Hubble Space Telescope Ultraviolet Spectroscopy of Two Hot
White Dwarfs
Authors: Holberg, J. B.; Barstow, M. A.; Lanz, T.; Hubeny, I.
1997ApJ...484..871H Altcode:
We have obtained Hubble Space Telescope high-dispersion spectroscopy of
two hot white dwarfs using the Goddard High-Resolution Spectrograph. The
stars were selected on the basis of EUV flux as examples of H-rich DA
stars containing high and low levels of short-wavelength opacity. The
high-opacity star, REJ 1614-085, possesses strong features due to C
IV, Si IV, and N V. Carbon and silicon in this star are underabundant
with respect to the predictions of radiative levitation; nitrogen,
however, is superabundant. This is in sharp contrast to the apparently
similar DA star GD 394, where silicon is in extreme overabundance. The
low-opacity star PG 1057+719, as expected, shows no clear evidence of
these or any other heavy ions. We find evidence of weak blueshifted
components in the REJ 1614-085 C IV and Si IV lines, which may
indicate the star is experiencing ongoing mass loss. Based
on observations with the NASA/ESA Hubble Space Telescope obtained
at the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Incorporated
under NASA contract NAS 5-26555. Observations reported here have
been obtained in part with the Multiple Mirror Telescope, a joint
facility of the University of Arizona and Smithsonian Institution.

---------------------------------------------------------
Title: Non-LTE Models and Theoretical Spectra of Accretion Disks in
Active Galactic Nuclei
Authors: Hubeny, Ivan; Hubeny, Veronika
1997ApJ...484L..37H Altcode: 1997astro.ph..5085H
We present self-consistent models of the vertical structure and emergent
spectrum of active galactic nucleus (AGN) accretion disks. The central
object is assumed to be a supermassive Kerr black hole. We demonstrate
that non-LTE (NLTE) effects and the effects of a self-consistent
vertical structure of a disk play a very important role in determining
the emergent radiation and therefore should be taken into account. In
particular, NLTE models exhibit a largely diminished H I Lyman
discontinuity when compared to LTE models, and the He II discontinuity
appears strongly in emission for NLTE models. Consequently, the number
of ionizing photons in the He II Lyman continuum predicted by NLTE disk
models is 1-2 orders of magnitude higher than that following from the
blackbody approximation. This prediction has important implications
for ionization models of AGN broad-line regions and for models of
the intergalactic radiation field and the ionization of helium in the
intergalactic medium.

---------------------------------------------------------
Title: Hubble Space Telescope GHRS Spectroscopy of U Geminorum during
Two Outbursts
Authors: Sion, Edward M.; Cheng, Fuhua; Szkody, Paula; Huang, Min;
Provencal, Judi; Sparks, Warren; Abbott, Brian; Hubeny, Ivan; Mattei,
Janet; Shipman, Harry
1997ApJ...483..907S Altcode:
We obtained Hubble Space Telescope Goddard High-Resolution Spectrograph
medium-resolution (G160M grating) phase-resolved spectroscopic
observations of the prototype dwarf nova U Geminorum during different
stages of two different outbursts. The spectral wavelength ranges
were centered on three different line regions: N V (1238 Å, 1242 Å),
Si III (1300 Å), and He II (1640 Å). The spectrum corresponding to
the early decline phase of outburst 1 is essentially featureless
except for weak N V absorption and narrow interstellar lines,
while the spectrum at the peak of outburst 2 reveals broad emission
peaks separated by narrow central absorption. The double-peaked
emission-line profile structure with low-velocity central absorption
seen in the second outburst suggests a disk origin, but the emission
velocity widths appear narrower than the widths of the optical disk
emission features. We interpret the high-excitation emission lines,
with central absorption below the continuum, to be due to photoionized
material (coronal?) above the disk plane with the thickened outer disk
absorbing the boundary layer or inner disk radiation. The possibility
of a wind origin for the profiles is also discussed, as well as the
possibility of an ejected optically thin shell. The N V absorption
velocity versus orbital phase traces the motion of the white dwarf, but
the He II absorption velocity appears to deviate from the white dwarf
motion. We present the results of synthetic accretion disk spectral
fitting to the data of both outbursts and derive accretion rates for the
two outbursts of 6 × 10-10 M⊙ yr-1
and 2 × 10-9 M⊙ yr-1. Implications
are discussed. Based on observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS 5-26555.

---------------------------------------------------------
Title: The Properties and Evolution of Very Massive Stars
Authors: Heap, S. R.; de Koter, A.; Hubeny, I.; Malumuth, E.
1997AAS...190.2505H Altcode: 1997BAAS...29..807H
We present HST/FOS and GHRS spectra of 15 stars in the dense
star cluster, R136a, in the Large Magellanic Cloud. These stars
define an empirical isochrone for very massive stars (M &gt; 40
Msun). We find that the cluster age is about 2.0 Myr, a time
when the isochrone is very nearly vertical on the HR diagram. We have
therefore used these stars: to refine the luminosity classification
of early O-type stars to calibrate spectra of early O stars in terms
of their fundamental parameters; to derive a new relation between
the rate of mass-loss and the fundamental stellar parameters. The new
mass-loss law is significantly different from previous prescriptions
used by stellar evolutionary models. We therefore expect that it will
have a significant impact on our knowledge of massive-star evolution.

---------------------------------------------------------
Title: Testing the AGN Accretion Disk Paradigm by New Self-Consistent,
Non-LTE Models
Authors: Hubeny, I.; Hubeny, V.
1997AAS...190.3910H Altcode: 1997BAAS...29..830H
We present self-consistent models of the vertical structure and
emergent spectrum of an accretion disk around a supermassive Kerr
black hole. For the radial structure, we take into account general
relativistic corrections after Riffert &amp; Herold (1996). For
computing the vertical structure, the interaction of radiation and
matter is treated self-consistently, taking into account departures from
LTE for calculating both the disk structure and the radiation field. We
demonstrate that non-LTE effects and the effects of self-consistent
vertical structure of a disk play a very important role in determining
the emergent radiation. In particular, non-LTE models exhibit a
largely diminished H I Lyman discontinuity when compared to LTE
models. Moreover, the He II discontinuity appears strongly in emission
for non-LTE models. Consequently, the number of ionizing photons in
the He II Lyman continuum increases significantly for non-LTE disk
models. This prediction may be important for constraining models of
the intergalactic radiation field and the ionization of helium in the
intergalactic medium. We also compare the vertical structure models
of a disk ring with the non-LTE stellar atmosphere models computed
for the same effective temperature and as close surface gravity as
possible. We found that athough the predicted flux roughly mimicks
the results of self-consistent calculations, using stellar atmosphere
models for approximating the AGN disk emergent radiation is risky
because the predicted flux in the vicinity of the H I and He II Lyman
discontinuities is very sensitive to the assumed value of the surface
gravity.

---------------------------------------------------------
Title: Dynamic Processes in Be Star Atmospheres. V. Helium Line
Emissions from the Outer Atmosphere of λ Eridani
Authors: Smith, Myron A.; Cohen, D. H.; Hubeny, I.; Plett, K.; Basri,
G.; Johns-Krull, C. M.; MacFarlane, J. J.; Hirata, R.
1997ApJ...481..467S Altcode:
The He I lines of the mild B2e star λ Eri often exhibit rapid,
small-amplitude emissions that can occur at random places in
their photospheric lines, even when the star is in a “nonemission
state.” New simultaneous observations of the triplet λ5876 and singlet
λ6678 lines show that the emission ratio for these lines is near unity,
contrary to the predictions of either non-LTE model atmospheres or
nebular recombination theory. Several He I emission events point to
the formation of short-lived structures near the star's surface. On 1995
September 12 the line λ6678 exhibited a strong (0.13Icont)
emission lasting some 20 minutes. The rapid decay of this feature
implies a density of &gt;=1011.5 cm-3 for an
emitting plasma structure near the star. This value is consistent with
density estimates for slabs which may be responsible for ephemeral
“dimples” in this star's He I lines on other occasions. We argue that
photospheric helium emissions during Hα-quiescent phases are caused
by foreground material and ask what mechanism might produce these
features against the stellar background. To answer this question
we have simulated He I line emission from model slabs having various
properties and suspended over the star. We find that illumination by
a source of extreme-ultraviolet (EUV) or X-ray flux depletes the He I
column density so that it is difficult to form observable He I lines. A
more interesting set of conditions occur for slabs with high densities
(~1012 cm-3) and moderately large optical
thicknesses in optical He I lines. Under these modified assumptions
modest amounts of emission can be reproduced in singlet and triplet
lines, and in the observed ratio. The key to producing this emission is
for the slab to feel its own Lyman continuum radiation. This condition
causes λ584 and other resonance lines to partially depopulate the
ground state and to overpopulate the first few excited levels, ensuring
that the departure coefficients of relevant atomic levels approach
common values. The second necessary ingredient is a high density,
which tends to equalize the departure coefficients of excited levels
through recombinations and through redistribution of electrons among the
l-sublevels. The combination is a kind of “Lyman-pumped recombination”
because it relies on the Lyman continuum being marginally optically
thick. Our results are consistent with studies of He I emission from
planetary nebulae, symbiotic variables, and active galactic nuclei
(AGNs), and may have a bearing on other “detached atmospheres”
problems as well. This study appears to be the first application of
such a recombination mechanism to a quasi-photospheric setting.

---------------------------------------------------------
Title: Evidence of a Thermonuclear Runaway and Proton Capture Material
on a White Dwarf in a Dwarf Nova
Authors: Sion, E. M.; Cheng, F. H.; Sparks, W. M.; Szkody, P.; Huang,
M.; Hubeny, I.
1997AAS...190.2704S Altcode: 1997BAAS...29Q.812S
We present Hubble Space Telescope GHRS G160M spectra of the white dwarf
in VW Hydri, exposed during quiescence, one month after the end of a
normal dwarf nova outburst. Our spectra reveal strong photospheric Si
II 1260 Angstroms, 1265 Angstroms absorption features, and a previously
unidentified broad feature centered around 1250 Angstroms. This feature
is due to a blend of phosphorus lines. From line-shift measurements
we determine a gravitational redshift of 58 +/- 33 km s(-1) yielding a
white dwarf mass, M = 0.86 (+0.18,-0.32) M_sun, white dwarf radius R =
6.5 (+3.1,-1.5) x 10(8) cm, and gravity log g = 8.43 (+0.31,-0.54). Our
best fitting synthetic spectra yield white dwarf effective temperature
T = 22,000 K, a rotational velocity v sin i = 400 km s(-1) . The
chemical abundances in number relative to solar are: C = 0.5, N =
5.0, O = 2.0, Fe - 0.5, Si = 0.1, P = 900, and all other metals Z
= 0.3. The abundance of phosphorus being 900 times solar, coupled
with the elevated aluminum abundance reported by Sion and coworkers,
suggest nucleosynthetic production of these odd-numbered nuclei from
proton capture on the even-numbered nuclei during a CNO thermonuclear
runaway (TNR) on the white dwarf. It is clear that the white dwarf has
undergone a runaway sometime in the past, the first such evidence of
a TNR in a dwarf nova. A TNR on a slowly accreting 0.86 M_sun white
dwarf should produce a classical nova explosion. If our interpretation
is correct, then we have found the first direct spectroscopic link
between a dwarf nova and a classical nova by using the white dwarf
surface chemical abundance. This is also the first direct evidence
of proton capture-processed material in the atmosphere of a white
dwarf. We acknowledge with gratitude the support of this work by NASA
through grant GO6084.01-95A from the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5-26555.

---------------------------------------------------------
Title: Evidence of a Thermonuclear Runaway and Proton-Capture Material
on a White Dwarf in a Dwarf Nova
Authors: Sion, Edward M.; Cheng, F. H.; Sparks, Warren M.; Szkody,
Paula; Huang, Min; Hubeny, Ivan
1997ApJ...480L..17S Altcode:
We present Hubble Space Telescope Goddard High-Resolution Spectrograph
G160M spectra of the white dwarf in VW Hydri, exposed during quiescence,
1 month after the end of a normal dwarf nova outburst. Our spectra,
covering the wavelength interval 1236-1272 Å, were obtained at
orbital phase 0.06-1.60 they reveal strong photospheric Si II
λλ1260, 1265 absorption features and a previously unidentified
broad feature centered around 1250 Å. This feature is due to a
blend of phosphorus lines. From line-shift measurements we determine
a gravitational redshift of 58 +/- 33 km s-1, yielding a
white dwarf mass Mwd = 0.86+0.18-0.32
Msolar (this is only the second gravitational redshift
determined for a cataclysmic variable white dwarf), white dwarf radius
Rwd = 6.5+3.1-1.5 × 108
cm, and gravity log g = 8.43+0.31-0.54. Our
best-fitting synthetic spectra yield white dwarf effective temperature
Twd = 22,000 K and a rotational velocity Vrot =
400 km s-1. The chemical abundances, in number relative to
solar, are, for C, 0.5; N, 5.0; O, 2.0; Fe, 0.5; Si, 0.1; P, 900; and
all other metals, 0.3. The abundance of phosphorus being 900 solar,
coupled with the elevated aluminum abundance reported by Sion and
coworkers, suggest nucleosynthetic production of these odd-numbered
nuclei from proton capture on the even-numbered nuclei during a CNO
thermonuclear runaway (TNR) on the white dwarf. It is clear that the
white dwarf has undergone a runaway sometime in the past, the first
such evidence of a TNR in a dwarf nova. A TNR on a slowly accreting
0.86 Msolar white dwarf should produce a classical nova
explosion. If our interpretation is correct, then we have found the
first direct spectroscopic link between a dwarf nova and a classical
nova by using the white dwarf surface chemical abundance. This is
also the first direct evidence of proton capture-processed material
in the atmosphere of a white dwarf. Nova explosions on more numerous,
lower mass C-O white dwarfs may therefore account for some fraction
of the short-lived radionuclide 26Al in the Galaxy. This
nuclide is observed from its Galactic gamma-ray line emission and is
postulated to have an important role in the heating of small bodies in
the solar system. Based on observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5-26555.

---------------------------------------------------------
Title: On the Evolutionary Phase and Mass Loss of the Wolf-Rayet--like
Stars in R136a
Authors: de Koter, Alex; Heap, Sara R.; Hubeny, Ivan
1997ApJ...477..792D Altcode:
We report on a systematic study of the most massive stars, in which
we analyzed the spectra of four very luminous stars in the Large
Magellanic Cloud. The stars lie in the 30 Doradus complex, three of
which are located in the core of the compact cluster, R136a (R136a1,
R136a3, and R136a5), and the fourth (Melnick 42), located about 8"
north of R136a. Low-resolution spectra (&lt;200 km s-1) of these four
stars were obtained with the GHRS and FOS spectrographs on the Hubble
Space Telescope. The GHRS spectra cover the spectral range from 1200
to 1750 A, and the FOS spectra from 3200 to 6700 A. We derived the
fundamental parameters of these stars by fitting the observations by
model spectra calculated with the "ISA-WIND" code of de Koter et al. We
find that all four stars are very hot (~45 kK), luminous, and rich in
hydrogen. Their positions on the HR-diagram imply that they are stars
with masses in the range 60--90 M⊙ that are 2 million years old
at most, and hence, they are O-type main-sequence stars still in the
core H-burning phase of evolution. Nevertheless, the spectra of two of
the stars (R136a1, R136a3) mimic those of Wolf-Rayet stars in showing
very strong He II emission lines. According to our calculations, this
emission is a natural consequence of a very high mass-loss rate. We
conjecture that the most massive stars in R136a---those with initial
masses of ~100 M⊙ or more---are born as WR-like stars and that
the high mass loss may perhaps be connected to the actual stellar
formation process. Because the observed mass-loss rates are up to 3
times higher than assumed by evolutionary models, the main-sequence
and post--main-sequence tracks of these stars will be qualitatively
different from current models. The mass-loss rate is 3.5--8 times
that predicted by the analytical solutions for radiation-driven winds
of Kudritzki et al. (1989). However, using sophisticated Monte Carlo
calculations of radiative driving in unified model atmospheres, we show
that---while we cannot say for sure what initiates the wind---radiation
pressure is probably sufficient to accelerate the wind to its observed
terminal velocity, if one accounts for the effects of multiple photon
scattering in the dense winds of the investigated stars.

---------------------------------------------------------
Title: O Stars in Transition. II. Fundamental Properties and
Evolutionary Status of Ofpe/WN9 Stars from HST Ultraviolet
Observations
Authors: Pasquali, Anna; Langer, Norbert; Schmutz, Werner; Leitherer,
Claus; Nota, Antonella; Hubeny, Ivan; Moffat, Anthony F. J.
1997ApJ...478..340P Altcode:
We present new HST/FOS ultraviolet spectroscopic observations of
seven LMC Ofpe/WN9 stars. We find that Ofpe/WN9 stars have slow winds
with terminal velocities of about 400 km s-1 and high
mass-loss rates of the order of 2-5 × 10-5 M⊙
yr-1. Ofpe/WN9 stellar temperatures and radii are in the
range 30,000-39,000 K, and 19-39 R⊙, respectively. Stellar
luminosities are between log (L/L⊙) = 5.6 and 6.3. We study the Ofpe/WN9 stars winds and examine their evolutionary
status. We find that Ofpe/WN9 stars are intermediate between O and W-R
stars in terms of the wind momentum flux. We also find that the stellar
properties and wind momentum of the Ofpe/WN9 sample place them in the
evolutionary sequence: O --&gt; Of --&gt; H-rich WNL --&gt; Ofpe/WN9,
for initial stellar masses less than ~100 M⊙. In
view of persisting discrepancies of standard massive star models with
observations, we compute massive main-sequence models according to three
different evolutionary scenarios. We find that both higher mass-loss
rate and enhanced mixing between core and envelope are required in order
to yield models compatible with the derived stellar and wind properties
of Ofpe/WN9 stars. The emerging picture may be consistent with earlier
evidence of Ofpe/WN9 stars being quiescent luminous blue variables
(LBVs). This idea is further strengthened by the highly reduced surface
H mass fractions of the Ofpe/WN9 stars. We derive Xs = 0.5
to 0.3, which still excludes Ofpe/WN9 stars from being core He-burning
objects, but is almost identical to the Xs values recently
measured in LBVs. Based on observations with the NASA/ESA Hubble
Space Telescope, obtained at the Space Telescope Science Institute,
which is operated by AURA for NASA under contract NAS5-26555.

---------------------------------------------------------
Title: Interstellar and photospheric opacity from EUV spectroscopy
of DA white dwarfs
Authors: Barstow, M. A.; Dobbie, P. D.; Holberg, J. B.; Hubeny, I.;
Lanz, T.
1997MNRAS.286...58B Altcode:
We present a detailed analysis of the extreme-ultraviolet (EUV)
spectra of 13 hydrogen-rich DA white dwarfs, observed by the Extreme
Ultraviolet Explorer (EUVE) satellite, paying attention to the
possible sources of absorbing material along the lines of sight both
in the local interstellar medium (ISM) and in the photospheres of the
stars themselves. The range of interstellar column densities seen are
consistent with our previous understanding of the local distribution of
material. Absorption from interstellar Heii is found in the direction of
five stars, allowing us to measure directly the He ionization fraction
and estimate, indirectly, that of H. The weighted mean ionization
fractions along these lines of sight are 0.27+/-0.04 and 0.35+/-0.1
respectively. Where Heii is directly detected, the observed ionization
fractions are not correlated with direction or with the volume/column
density of material along the line of sight. Furthermore, the limits
on the amount of Heii established in all other directions completely
encompass the range of observed values. Indeed, all the data can be
consistent with more or less constant He and H ionization fractions
throughout the local ISM. It is clear that there is little photospheric
opacity, from either He or heavier elements, in the majority of the
stars we have studied. This poses further difficulties in explaining
the observed division of white dwarfs into H- and He-rich groups,
the temperature gap in the He-rich sequence and the detailed spectral
evolution of the H-rich DA white dwarfs as they cool. A striking
observational result is that our spectroscopic evidence indicates that
radiative levitation effects are only important at temperatures above
50000K, rather than the 40000K suggested by broad-band photometry. There
is clearly an urgent need for further theoretical work on the mechanisms
that determine the photospheric composition of white dwarf stars.

---------------------------------------------------------
Title: NLTE Line Blanketed Model Atmospheres of A-Type Stars
Authors: Hubeny, I.; Lanz, T.
1997IAUJD..16E.102H Altcode:
We present new non-LTE metal line-blanketed model atmospheres
of A-type stars. We show that thanks to the recent progress in
developing fast and robust numerical schemes, such as the hybrid
complete linearization/accelerated lambda iteration method, one may
now construct model atmospheres of unprecedented degree of complexity
and realism. Other basic ingredient of the recent progress are the
new atomic data calculations, like the Opacity and Iron Projects,
which provide a substantial quantity of high-accuracy data. In this
review we discuss the influence of the non-LTE approach, the influence
of metal line blanketing, and the impact of new atomic data on model
atmospheres and predicted spectra of A-type stars.

---------------------------------------------------------
Title: HST synthetic spectral analysis of U Gem in early and late
quiescence: a heated white dwarf and accretion belt
Authors: Cheng, F. H.; Sion, E. M.; Horne, K.; Hubeny, I.; Huang,
M.; Vrtilek, S. D.
1997ASSL..214..359C Altcode: 1997whdw.work..359C
No abstract at ADS

---------------------------------------------------------
Title: Heavy elements in white dwarf envelopes
Authors: Barstow, M. A.; Holberg, J. B.; Hubeny, I.; Lanz, T.
1997ASSL..214..237B Altcode: 1997whdw.work..237B
No abstract at ADS

---------------------------------------------------------
Title: Stellar Atmosphers Theory: an Introduction
Authors: Hubeny, I.
1997LNP...497....1H Altcode: 1997sato.conf....1H
Fundamentals Concepts What Is a Stellar Atmosphere, and Why Do We
Study It? Basic Structural Equations LTE Versus Non-LTE Radiative
Transfer Equation Intensity of Radiation and Related Quantities
Absorption and Emission Coefficient Phenomenological Derivation of
the Transfer Equation Optical Depth and the Source Function Elementry
Solutions Moments of the Transfer Equation Lambda Operator Diffusion
Approximation Radiative Transfer with Constraints; Escape Probality
Two-Level Atom Escape Probability Numerical Methods Formal Solution
of the Transfer Equation Linear Coupling Problems Accelerated Lambda
Iteration Non-linear Coupling Problems Model Atmospheres Definition and
Terminology Basic Equations of Classical Stellar Atmospheres LTE-Grey
Model: A Tool to Understand the Temperature Structure LTE and NLTE Model
Atmospheres Line Blanketing Using Model Atmospheres to Analyse Observed
Spectra A Scheme of Spectroscopic Diagnostics Spectrum Synthesis
Spectrum Fitting Determination of Fundamental Stellar Parameters

---------------------------------------------------------
Title: A Self-consistent Optical, Ultraviolet, and Extreme-Ultraviolet
Model for the Spectrum of the Hot White Dwarf G191-B2B
Authors: Lanz, T.; Barstow, M. A.; Hubeny, I.; Holberg, J. B.
1996ApJ...473.1089L Altcode:
The star G191-B2B is one of a number of hot DA white dwarfs whose
atmospheres have been found to contain significant quantities of
heavy elements, including C, N, 0, Si, Fe, and Ni. Several earlier
studies have measured their abundances using IUE echelle data in
conjunction with synthetic spectra derived from theoretical model
atmosphere calculations of varying degrees of sophistication. However,
predictions of the EUV spectrum based on these observations failed
completely to match either its shape or absolute flux level. We
present here the results of new non-LTE model calculations including
the effects of line blanketing from more than 9 million (mainly Fe and
Ni) transitions. For the first time, we are able to find an effective
temperature and composition that can consistently match the optical, FUV
and EUV data. However, to maintain this agreement below the He II λ228
Lyman limit, it is necessary to incorporate additional He II opacity
in the form of photo spheric, circumstellar or interstellar material.

---------------------------------------------------------
Title: The Cooling White Dwarf in VW Hydri after Normal Outburst
and Superoutburst: HST Evidence of a Sustained Accretion Belt
Authors: Sion, Edward M.; Cheng, Fu-Hua; Huang, Min; Hubeny, Ivan;
Szkody, Paula
1996ApJ...471L..41S Altcode:
Hubble Faint Object Spectrograph (FOS) observations of VW Hyi one day
after the end of a normal dwarf nova outburst reveal a heated white
dwarf with deep, broad Ly alpha , narrow metallic absorption features,
and evidence of a hotter Keplerian-broadened component manifested in
quiescence as a broad continuum hump. Our best reduced chi 2 fit to
the data reveals (1) a DAZQ white dwarf with Teff = 22,500 +/- 500 K,
log g = 8, and photospheric abundances C = 0.5 solar, N = 5.0 solar,
O = 2.0 solar, Si = 0.2 solar, Fe = 0.5 solar, with all other metals
being 0.3 solar, and (2) a rapidly spinning accretion belt with Vrot =
3350 km s-1, Tbelt = 26,000 +/- 1000 K, log g = 6.0, and a fractional
belt area of approximately 11%. Our earlier FOS spectrum obtained 10
days after superoutburst reveals a cooler DAZQ white dwarf (20,500 +/-
1000 K), relatively lower metal abundances, and a smaller fractional
area (3%) for the accretion belt. Thus, 1 day after a normal outburst,
the white dwarf is ~2000 K hotter, the accretion belt fractional area
is a factor of 3 greater, the accretion belt temperature appears to be
cooler than at 10 days post-superoutburst, and the accreted atmosphere
has relatively higher metal abundances. Finally, the accretion belt
maintained during quiescence may provide a natural explanation for
the 14 s soft X-ray oscillations, requires a deeper source of heating
(compression and shear mixing), and implies a lower limit to the
viscous spin-down timescale of 10 days.

---------------------------------------------------------
Title: Ultraviolet Spectra and Limb Darkening of Accretion Disks
Authors: Diaz, M. P.; Wade, R. A.; Hubeny, I.
1996RMxAC...4Q..95D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Spectrum Synthesis and Light Synthesis Program for Binary
Stars with Optically Thick Accretion Disks
Authors: Linnell, A. P.; Hubeny, I.
1996ApJ...471..958L Altcode:
We have completed a new program that calculates both synthetic spectra
and synthetic light curves for binary systems that contain a standard
model optically thick accretion disk. Our initial model is physically
and geometrically simple. It is capable of substantial elaboration. By
basing both synthetic light curves and synthetic spectra on a single
self-consistent model, we anticipate useful tests of systems for which
both types of data are available. Alternatively, explicit simulations
now become possible for systems for which only light curves or only
spectra are available.

---------------------------------------------------------
Title: HST/GHRS Observations of the beta Pictoris
System. II. Exploring the Potential for Tracking Comet-like Objects
Orbiting the Star
Authors: Hubeny, Ivan; Heap, Sara R.
1996ApJ...470.1144H Altcode:
We present a systematic study of the transient absorption features
observed in the spectra of β Pictoris. We calculate detailed
synthetic spectra of a star, which is partially obscured by a permanent
circumstellar disk and by an absorbing body of a given size, position,
radial velocity with respect to the star, and total column density. We
show that because of the nonuniform surface brightness of the star,
the strength of an absorption feature is an intricate function of both
position and size. Consequently, one cannot determine the projected area
of the body or its position from a single measurement only. However, by
studying several closely spaced lines simultaneously, like those in the
region of the Fe II UV1 multiplet at λ ≍2600 Å, one can determine
both the position and size of the body. We present a preliminary
analysis of Goddard High Resolution Spectrograph observations of β
Pic in this region and show that while existing spectra do not have
sufficient time coverage to allow us to track a single body during its
transit across the stellar disk, our approach provides a robust means
of tracking cometlike objects transiting the star and should be used to
interpret future monitoring of β Pic and other stars showing similar
variable absorption features. Conversely, the effect of body position
is so great that it must be taken into account; absorption-line analyses
based on techniques developed for interstellar lines should not be used.

---------------------------------------------------------
Title: Observations of the Dwarf Nova VW Hydri in Quiescence with
the Hopkins Ultraviolet Telescope
Authors: Long, Knox S.; Blair, William P.; Hubeny, Ivan; Raymond,
John C.
1996ApJ...466..964L Altcode:
The dwarf nova VW Hydri was in quiescence when it was observed in 1995
March with the Hopkins Ultraviolet Telescope on the Astro-2 space
shuttle mission. The far-UV (820-1840 Å) spectra are punctuated by
broad Lyα and Lyβ absorption profiles and narrow absorption lines
which can be identified with transitions expected in the atmosphere of
a moderate-temperature white dwarf. There is no detectable emission
shortward of 980 Å. The only emission line seen is C IV λλ1548,
1551. If the emission from VW Hyi is due to a uniform-temperature
white dwarf; then our spectra suggest that the temperature of the
white dwarf was ∼17,000 K at the time of our observations and that
abundances in the atmosphere were subsolar. Improved fits to the data
are obtained using models in which the far-UV emission arises in part
from a white dwarf with near-solar abundances and in part from the
accretion disk. However, given the uncertainties in model spectra of
metal-enriched atmospheres in this temperature range and our limited
knowledge of quiescent accretion disks, higher S/N spectra are needed
to unambiguously assess the disk contribution to the far-UV spectrum
of VW Hyi in quiescence.

---------------------------------------------------------
Title: Toward Resolving the “Mass Discrepancy” in O-Type Stars
Authors: Lanz, Thierry; de Koter, Alex; Hubeny, Ivan; Heap, Sara R.
1996ApJ...465..359L Altcode:
We show that metal line blanketing has an important effect on the
atmospheres of hot stars, and we argue that the omission of metal line
blanketing in previous non-LTE model atmospheres of 0-type stars has led
to underestimates of surface gravity and stellar mass. In addition to
wind emission, metal line blanketing contributes to the solution of the
long-standing discrepancy between spectroscopic and evolutionary masses
for 0-type stars. To support our argument, we calculated a series
of non-LTE stellar atmospheres for 0 and Of-type stars. We compared the
predicted profiles of hydrogen and helium lines produced by (1) a static
plane- parallel H-He model, (2) a metal line-blanketed static model,
and (3) an extended expanding model atmosphere. We find that simple
H-He models produce stronger lines than do our metal line-blanketed
models. Consequently, they lead to underestimated gravities. Wind
emission is more effective in filling in the wings of H or He lines in
the case of large mass-loss rates typical of extreme Of stars. These
findings apply equally to young, massive 0-type stars and to highly
evolved stars, i.e., central stars of planetary nebulae. The lower
gravities derived from H-He models yield underestimated masses for
young 0-type stars, and overestimated masses for highly evolved stars.

---------------------------------------------------------
Title: The Accretion Disk (Belt?) During the Quiescence of VW Hydri
Authors: Huang, M.; Sion, E. M.; Hubeny, I.; Cheng, F. H.; Szkody, P.
1996AJ....111.2386H Altcode:
We considered the contribution of an accretion disk to the UV spectrum
of the dwarf nova (DN) VW Hydri during its quiescence, obtained with
the Faint Object Spectrograph (FOS) on the Hubble Space Telescope
(HST). Our analysis indicates that the UV emitting area of the disk is
a ring of width 0.06 RWD, inclination angle i=60°, rotating
with Keplerian velocity on the white dwarf surface of 3350 km/s. The
other parameters of the ring are log g=6.0, Teff=28,000 K, C
and Si enhanced to 20 and 15 times their solar abundances, respectively,
and the abundance of all other heavy elements having solar values. The
disk area outside this UV emitting region has a Teff
much lower than 10,000 K. Our modelling does not rule out that the
ring could actually be a belt on the white dwarf. For the central
white dwarf, our analysis yields a Teff=22,000±1000 K,
log g=8.0, with the abundance of all heavy elements 0.3 times their
solar values. The disk contributes about 12% of the total far UV flux
in the FOS far ultraviolet region.

---------------------------------------------------------
Title: Photoionization resonances of SiII in stellar spectra.
Authors: Lanz, T.; Artru, M. -C.; Le Dourneuf, M.; Hubeny, I.
1996A&A...309..218L Altcode:
We have studied the influence of the Si^+^ photoionization on stellar
atmospheres. Recent calculations using the R-matrix code provide
detailed cross-sections with autoionization resonances for the first
50 energy levels of Si^+^. These photoionization cross-sections
are included in the calculation of the far ultraviolet synthetic
spectrum of A and late B-type stars. The opacity from the six lowest
excited states of Si^+^ provides observable features in the emergent
spectrum of a solar-composition A0 star. The photoionization from
higher states of Si^+^ should be included when modeling hotter or
silicon-rich stars. These new cross-sections increase dramatically
the opacity in the far UV spectrum of Ap Si stars and allow to
reproduce the most characteristic UV features of these stars, as
illustrated by a comparison with IUE data for the hot Ap Si star HD
34452. Broad features in its spectrum are definitely assigned to Si
ii autoionization resonances, and the flux deficiency below 130nm is
well matched with a predicted continuum edge near this wavelength. The
successful stellar spectrum synthesis brings a strong support to
the R-matrix photoionization calculations, which cannot otherwise be
compared to laboratory data.

---------------------------------------------------------
Title: Solving the mystery of the heavy-element opacity in the DA
white dwarf GD 394.
Authors: Barstow, M. A.; Holberg, J. B.; Hubeny, I.; Lanz, T.;
Bruhweiler, F. C.; Tweedy, R. W.
1996MNRAS.279.1120B Altcode:
The white dwarf GD 394 is one of a very small number of bright hot DAs
which have detectable quantities of highly ionized heavy elements in
their spectra. Whether this material resides in the photosphere of GD
394 or is in the form of a circumstellar shell has been the subject of
much debate, with the large discrepancy between the published radial
velocity of the star and the measured velocities of the Si III/IV
lines having apparently settled the argument in favour of the latter
idea. However, GD 394 is also the coolest DA white dwarf in the ROSAT
sky survey showing convincing evidence for the presence of absorbing
material which strongly suppresses the EUV continuum. This result has
reopened the argument about the nature of the silicon lines. From a
combined analysis of the IUE, HST and EUVE spectra, coupled with new
measurements of the photospheric radial velocity from Hα data, the
authors have established that the Si III and Si IV lines are indeed
photospheric. Furthermore, other heavy elements must be present to
account for the EUV opacity, but at levels below the thresholds for
detecting absorption lines in either far-UV or EUV bands.

---------------------------------------------------------
Title: EUVE Spectroscopy of beta Canis Majoris (B1 II--III) from
500 Angstrom to 700 Angstrom
Authors: Cassinelli, J. P.; Cohen, D. H.; Macfarlane, J. J.; Drew,
J. E.; Lynas-Gray, A. E.; Hubeny, I.; Vallerga, J. V.; Welsh, B. Y.;
Hoare, M. G.
1996ApJ...460..949C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Ultraviolet Limb Darkening and Spectra for Accretion Disks
in Cataclysmic Variables
Authors: Diaz, Marcos P.; Wade, Richard A.; Hubeny, Ivan
1996ApJ...459..236D Altcode:
Limb darkening is an important effect, more so for flat objects such
as accretion disks (ADs) than for stars, since no averaging over
emergent angles occurs for disks. As with stars, limb darkening
in ADs is especially important in the ultraviolet, and at a given
wavelength it is more important for low- temperature atmospheres. We
study the size of the limb-darkening effect in the ultraviolet (925 A
to 1750 A) spectra of ADs in cataclysmic variables, using a grid of
models and spectra of disk atmospheres. Radial emissivity (surface
brightness) distributions in the ultraviolet are presented, along
with a brief comparison of limb darkening in stars and disks. The
changes in the absorption-line profiles in the model disk spectra as
the disk inclination is varied, because of both limb darkening and
kinematic broadening, are also discussed. Limb-darkening correction
factors for the integrated disk brightness at 1448 A are derived for
a representative set of disk parameters and orbital inclinations.

---------------------------------------------------------
Title: EE Pegasi Revisited: A Spectrum Synthesis and New Light
Synthesis Study
Authors: Linnell, A. P.; Hubeny, I.; Lacy, C. H. S.
1996ApJ...459..721L Altcode:
A self-consistent physical model, described in an earlier paper
by Linnell &amp; Hubeny, has permitted fits of synthetic spectra
to observed spectra of EE Pegasi. The synthetic spectra determine
abundances for iron, calcium, and silicon. The same model has been the
basis of an optimized light synthesis solution of an accurate light
curve by Ebbighausen. The solution, requiring use of a model atmospheres
option to represent the component star radiative properties, agrees
with the standard solution by Lacy &amp; Popper. The derived component
parameters place them on an isochrone, determine a compositional Z,
and are in accordance with evolution tracks by Schaller et al. The
model is not restricted to systems with small distortion.

---------------------------------------------------------
Title: Hubble Space Telescope/FOS Spectroscopy of VW Hydri in
Superoutburst
Authors: Huang, M.; Sion, E. M.; Hubeny, I.; Cheng, F. H.; Szkody, P.
1996ApJ...458..355H Altcode:
We present an analysis of two HST/FOS UV spectra of the SU UMa type
dwarf nova, VW Hyi, obtained on 1993 October 24, ∼5 days after the
optical rise of a superoutburst The absorption features in the first
spectrum appear to consist of two components: a broad-winged component
(with velocity width of 3000 km s-1) and a sharp core narrow
component. This is the first time the narrow core is clearly resolved
in super-outburst spectra of a dwarf nova system. The sharp core appears
absent in the second spectrum obtained ∼10 minutes later. The broader
component is mainly from the accretion disk. By comparing the spectra
with a grid of LTE model accretion disk atmospheres constructed with
TLUSTY, SYNSPEC, and DISKSYN, we present two possible disk fits to the
observed spectra: a steady state disk with solar abundance and Mdot =
3 × 109 Msun yr-1 which can account
for all the broad absorption features except for N V λ1240, and a model
with a discontinuous Teff(r) distribution in which there is a
contribution to the N V λ1240 absorption feature. We provide arguments
supporting the possibility that the sharp cores are due to gas streams
in the system. We also point out the far less likely possibility that
the sharp cores form in a hot, high-gravity atmosphere. The synthetic
fitting results may imply that the hot matter is accreted from the
inner part of the disk onto the surface of the white dwarf through a
highly inhomogeneous gas flow. We relate this discussion to our FOS
detection of highly asymmetric inverse P Cygni profile structure in
the narrow stellar components at C IV λ1550.

---------------------------------------------------------
Title: Non-LTE spectroscopic analysis of the wind of the central
star of NGC 6543
Authors: de Koter, A.; Hubeny, I.; Heap, S. R.; Lanz, T.
1996ASPC...96..141D Altcode: 1996hds..conf..141D
No abstract at ADS

---------------------------------------------------------
Title: Spectroscopic Constraints on the Helium Abundance in Globular
Cluster Stars
Authors: Landsman, W. B.; Crotts, A. P. S.; Hubeny, I.; Lanz, T.;
O'Connell, R. W.; Whitney, J.; Stecher, T. P.
1996ASPC...99..199L Altcode: 1996coab.proc..199L
No abstract at ADS

---------------------------------------------------------
Title: EUV Radiation from Hot Star Photoepheres: Theory Versus
Observations
Authors: Hubeny, Ivan; Lanz, Thierry
1996aeu..conf..381H Altcode: 1996IAUCo.152..381H
No abstract at ADS

---------------------------------------------------------
Title: A quantitative study of limb darkening in accretion disks
Authors: Diaz, M. P.; Wade, R. A.; Hubeny, I.
1996ASSL..208..123D Altcode: 1996cvro.coll..123D; 1996IAUCo.158..123D
No abstract at ADS

---------------------------------------------------------
Title: Fundamental Properties of Ofpe/WN9 Stars from Ultraviolet
HST Spectra1
Authors: Pasquali, A.; Schmutz, W.; Leitherer, C.; Nota, A.; Hubeny,
I.; Langer, N.; Drissen, L.; Robert, C.
1996swhs.conf..386P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Model Spectra for Accretion Disks Truncated at the Inner Edge
Authors: Wade, Richard A.; Diaz, Marcos; Hubeny, Ivan
1996aeu..conf..355W Altcode: 1996IAUCo.152..355W
No abstract at ADS

---------------------------------------------------------
Title: Fully line-blanketed NLTE model atmospheres for hot
hydrogen-deficient stars
Authors: Hubeny, I.; Lanz, T.
1996ASPC...96..249H Altcode: 1996hds..conf..249H
No abstract at ADS

---------------------------------------------------------
Title: The possible contribution from the accretion disk during the
quiescence of VW HYI
Authors: Huang, M.; Sion, E. M.; Hubeny, I.; et al.
1996ASSL..208..247H Altcode: 1996cvro.coll..247H; 1996IAUCo.158..247H
No abstract at ADS

---------------------------------------------------------
Title: Spectrally resolved maps of optically thick accretion disks
Authors: Baptista, B.; Horne, K.; Hubeny, I.; et al.
1996ASSL..208...17B Altcode: 1996cvro.coll...17B; 1996IAUCo.158...17B
No abstract at ADS

---------------------------------------------------------
Title: The Composition and Structure of White Dwarf Atmospheres
Revealed by Extreme Ultraviolet Spectroscopy
Authors: Barstow, Martin A.; Hubeny, Ivan; Lanz, Thierry; Holberg,
Jay B.; Sion, Edward M.
1996aeu..conf..203B Altcode: 1996IAUCo.152..203B
No abstract at ADS

---------------------------------------------------------
Title: Atomic data for non-LTE model stellar atmospheres
Authors: Lanz, T.; Hubeny, I.; de Koter, A.
1996PhST...65..144L Altcode:
We briefly review the state-of-the-art in non-LTE modeling of hot
star atmospheres. During the last decade, the development of a
new class of radiation transfer methods, based on the Accelerated
Lambda Iteration method, have brought considerable progress in this
field. Severe limitations of the size of model atoms used in non-LTE
model atmospheres have been removed, so that realistic model atoms
can now be considered. In recent years, the problem of non-LTE line
blanketing, which aims to include the influence of thousands to millions
of lines on the atmospheric structure, has been attacked, and first
non-LTE line-blanketed models have been calculated. We illustrate
the importance of non-LTE line blanketing with the example of a hot
subdwarf, BD +75° 325. As a consequence of these developments,
there is a need for extensive radiative and collisional atomic databases
to build non-LTE models. Recent projects, like the Opacity Project,
provide a substantial quantity of the necessary data. We discuss the
current needs of atomic data for the purpose of building reliable
non-LTE line-blanketed model atmospheres. A critical evaluation of the
quality of such extensive sets of atomic data is essential. Finally,
we present a new, IDL-based, graphic tool intended to facilitate the
manipulation of large amounts of data needed to build the realistic
model atoms that we use.

---------------------------------------------------------
Title: A Spectrum Synthesis and new Light Synthesis Study of EE Pegasi
Authors: Linnell, A. P.; Hubeny, I.; Lacy, C. H. S.
1995AAS...187.6006L Altcode: 1995BAAS...27.1372L
EE Peg is among the binary systems with accurately known masses and
radii (Andersen 1991, Astron. Astrophys. Rev., 3, 91). With component
masses of 2.15 M_⊙, 1.33 M_⊙ and an orbital period of 2.63() d, this
low distortion system is an excellent first test case for application
of a new spectrum synthesis program for binary stars (Linnell &amp;
Hubeny 1994, ApJ, 434, 738). The existing standard light curve solution
is by Lacy &amp; Popper (1984, ApJ, 281, 268), analyzing an excellent
B light curve by Ebbighausen. That paper also includes component mass
determinations to 1%, based on spectra by Lacy. Our synthetic spectrum
fits determine an iron abundance of log Fe=3.0x solar, and a calcium
abundance of Ca=0.5x solar, consistent with the Am spectral type of the
primary component. Calculation of a synthetic spectrum depends on prior
knowledge of component dimensions and T_eff values. Consequently there
is an interdependence with the light curve solution. Our light curve
solution applied the light synthesis program by Linnell (1984, ApJS,
54, 17). We found that use of the Planck Law to represent component
radiative properties, a common procedure, produced a B-V component
difference in strong disagreement with Popper's calibration of flux
and T_eff for main sequence stars. A program modification permitted
direct use of our synthetic spectrum results to represent the component
radiative properties, with improved results. Since our program is
applicable to systems of any distortion, successful analysis of EE
Peg invites future discussion of distorted systems for which single
values of T_eff and log g are inadequate.

---------------------------------------------------------
Title: Partial Redistribution in Multilevel Atoms. I. Method and
Application to the Solar Hydrogen Line Formation
Authors: Hubeny, I.; Lites, B. W.
1995ApJ...455..376H Altcode:
We present a robust method for solution of multilevel non-LTE
line transfer problems including the effects of partial frequency
redistribution (PRD). This method allows the self-consistent solution
for redistribution of scattered line photons simultaneously in multiple
transitions of a model atom, including the effects of resonant Raman
scattering ("cross-redistribution") among lines sharing common upper
levels. The method is incorporated into the framework of the widely used
non-LTE complete redistribution code MULTI. We have applied this method
to the problem of transfer in hydrogen lines in a plane-parallel solar
model atmosphere, including cross-redistribution between the Hα and
Lβ, using general redistribution functions for the Lα and Lβ lines
which are not restricted by the impact approximation. The convergence
properties of this method are demonstrated to be comparable to that of
the equivalent complete redistribution problem. In this solar model,
PRD in the Lα line produces the dominant influence on the level
populations. It changes considerably the populations of the excited
states of hydrogen, as well as the proton number density, in the
middle and upper chromosphere, owing to modification of the Lα wing
radiation. The population of the hydrogen ground state undergoes
only modest changes, however. The influence of cross-redistribution
and PRD in Lβ has a much smaller influence on the level populations
but a considerable influence on the wing intensity of the Lβ line.

---------------------------------------------------------
Title: Support for the Primordial Helium Abundance Derived from
Observation of Globular Cluster Stars
Authors: Stecher, T. P.; Landsman, W.; Crotts, A. P. S.; Whitney,
J.; O'Connell, R. W.; Lanz, T.; Hubeny, I.; Sweigart, A.
1995AAS...187.8606S Altcode: 1995BAAS...27.1414S
The UIT on Astro-1 found nearly 2000 hot stars in the globular cluster
Omega Centauri (Whitney et al. 1994, AJ, 108, 1350), of which 28 are at
least 1 mag brighter than the horizontal branch (HB). We have obtained
CTIO 4m and/or IUE low-dispersion spectra of eleven of the brightest
of these stars. All seven stars observed at CTIO are radial velocity
members of the clusters. Three of these stars have log Teff &gt; 60,000
K and show only Balmer lines and He II lines. These stars, which appear
to be in the post-asymptotic giant branch phase, are the hottest stars
ever found in a globular cluster. The other four stars have log Teff ~
20,000 K and show a B-type spectrum with numerous He I lines but none
of He II. We have estimated the helium abundance by comparison with
hydrogen-helium NLTE model stellar atmospheres. The He II lines in the
hottest three stars in our sample appear to be best fit by models with
a He/H ratio of about 0.1. This helium abundance should reflect the
primordial helium abundance of the gas from which Omega Cen formed
nearly 13 x 10(9) yr ago, except for the modest helium enrichment
caused by the first dredge-up during the red giant branch phase. By
analyzing these stars we have been able to derive a helium abundance
for a globular cluster which is independent of the well-known R-method
and which should provide an estimate of the helium abundance produced by
the Big Bang. The four cooler stars in our sample show a quite different
pattern of helium abundances. The three faintest of these stars have a
substantially lower helium abundance which we attribute to the effects
of diffusion during the preceding HB phase. These stars appear to
have recently evolved off of the blue end of the HB. In contrast,
the most luminous of these stars is very helium rich (He/H = 0.55),
indicating that some UV-bright stars undergo interior mixing during
their preceding AGB evolution.

---------------------------------------------------------
Title: Accretion Disk Models for VOYAGER, HUT, and FUSE Far--UV
Spectra
Authors: Wade, R. A.; Hubeny, I.
1995AAS...187.7905W Altcode: 1995BAAS...27.1397W
Several past and future space missions have the capability
to observe spectra of cataclysmic variables in the far--UV
spectral region (from just short of the Lyman limit to longward
of Lyman-alpha ). These include the VOYAGER Far--UV Spectrometers
(FUVS), the Hopkins Ultraviolet Telescope (HUT; flew on ASTRO--1
and ASTRO--2), and the planned FUSE mission. We present a model
spectrum grid for axi\-symmetric, steady--state accretion disks
in cataclysmic variable systems, where the accreting object is a
white dwarf. The white dwarf mass and radius are varied on the grid,
0.35 &lt;= Mwd/Msun &lt;= 1.21 and 1.14 &gt;=
Rwd/(10(9) cm) &gt;= 0.38 respectively. The mass accretion
rate is also varied, on the range -10.5 &lt;= log dM/dt &lt;= -8
(Msun yr(-1) ). A standard Reynolds number description
of the viscous dissipation is adopted. The local plane--parallel
approximation is used, LTE is assumed, and irradiation of the
disk or communication between different radial zones of the disk is
ignored. Within these assumptions the vertical structure is calculated
in a self--consistent manner to ensure both hydrostatic equilibrium
and energy balance, using a restricted set of opacities. Ring and
full--disk spectra are constructed from these models using a full
line list, taking into account self--consistent limb darkening
and Doppler shifts due to Keplerian orbital motion of the gas. The
spectral region from 850 Angstroms to 1350 Angstroms is covered, and
spectra at several inclinations are tabulated. The spectra will be
made available electronically with a sampling interval and resolution
sufficient to allow the study of FUVS and HUT spectra, and can be
resampled for FUSE and other missions. The models serve as a benchmark
against which more complicated models, perhaps including winds, can
be compared. Representative spectra and some interesting trends are
shown. Supported by NASA grants NAG5--2125 and NAGW--3171.

---------------------------------------------------------
Title: The IUE MEGA Campaign: Wind Variability and Rotation in
Early-Type Stars
Authors: Massa, D.; Fullerton, A. W.; Nichols, J. S.; Owocki, S. P.;
Prinja, R. K.; St-Louis, N.; Willis, A. J.; Altner, B.; Bolton, C. T.;
Cassinelli, J. P.; Cohen, D.; Cooper, R. G.; Feldmeier, A.; Gayley,
K. G.; Harries, T.; Heap, S. R.; Henriksen, R. N.; Howarth, I. D.;
Hubeny, I.; Kambe, E.; Kaper, L.; Koenigsberger, G.; Marchenko, S.;
McCandliss, S. R.; Moffat, A. F. J.; Nugis, T.; Puls, J.; Robert, C.;
Schulte-Ladbeck, R. E.; Smith, L. J.; Smith, M. A.; Waldron, W. L.;
White, R. L.
1995ApJ...452L..53M Altcode:
Wind variability in OB stars may be ubiquitous, and a connection
between projected stellar rotation velocity and wind activity is well
established. However, the origin of this connection is unknown. To
probe the nature of the rotation connection, several of the attendees
at the workshop on Instability and Variability of Hot-Star Winds
drafted an IUE observing proposal. The goal of this program was to
follow three stars for several rotations to determine whether the
rotation connection is correlative or causal. The stars selected for
monitoring all have rotation periods &lt;=5 days. They were HD 50896
(WN5), HD 64760 (B0.5 Ib), and HD 66811 [ zeta Pup; O4 If(n)]. During
16 days of nearly continuous observations in 1995 January (dubbed the
"MEGA" campaign), 444 high-dispersion IUE spectra of these stars were
obtained. This Letter presents an overview of the results of the MEGA
campaign and provides an introduction to the three following Letters,
which discuss the results for each star.

---------------------------------------------------------
Title: HST/GHRS Observations of the beta Pictoris System: Basic
Parameters of the Age of the System
Authors: Lanz, Thierry; Heap, Sara R.; Hubeny, Ivan
1995ApJ...447L..41L Altcode:
We have reevaluated the basic parameters and age of the A5 IV--V star,
beta Pictoris, making use of new observations obtained by the Goddard
High-Resolution Spectrograph aboard the Hubble Space Telescope and
using modern methods of atmospheric modeling and spectral synthesis. We
derive stellar parameters appropriate to its spectral type (Teff =
8200 +/- 150 K, log g = 4.25 +/- 0.1) including a normal (solar)
metallicity. When dated by comparison with current evolutionary tracks,
we find that (1) the star is apparently "underluminous"---a situation we
attribute to (gray) extinction by the disk---and (2) the star is either
a pre--main-sequence (PMS) star nearing the zero-age main sequence
(ZAMS), or it is a main-sequence star older than 0.3 Gyr. Given the
apparent youth of the disk as inferred from its high density, we judge
the PMS interpretation as most likely.

---------------------------------------------------------
Title: A Quantitative Study of Limb Darkening in Accretion Disks
Authors: Diaz, M. P.; Wade, R. A.; Hubeny, I.
1995AAS...186.0911D Altcode: 1995BAAS...27..825D
The effect of limb darkening in the UV spectra of accretion disks
is evaluated using disk models. Limb darkening correction factors
are derived for a grid of binary system parameters and orbital
inclinations. Continuum radial emissivity distributions and a
description of the local limb darkening effect are presented for a
wide range of physical conditions. The behavior of the synthetic line
profiles and their disk diagnostic capabilities are also discussed.

---------------------------------------------------------
Title: Non-LTE Line-blanketed Model Atmospheres of Hot Stars. II. Hot,
Metal-rich White Dwarfs
Authors: Lanz, T.; Hubeny, I.
1995ApJ...439..905L Altcode:
We present several model atmospheres for a typical hot metal-rich DA
white dwarf, Teff = 60,000 K, log g = 7.5. We consider
pure hydrogen models, as well as models with various abundances
of two typical 'trace' elements-carbon and iron. We calculte a
number of Local Thermodynamic Equilibrium (LTE) and non-LTE models,
taking into account the effect of numerous lines of these elements
on the atmospheric structure. We demostrate that while the non-LTE
effects are not very significant for pure hydrogen models, except for
describing correctly the central emission in H-alpha they are essential
for predicting correctly the ionization balance of metals, such as
carbon and iron. Previously reported discrepancies in LTE abundances
determinations using C III and C IV lines are easily explained by
non-LTE effects. We show that if the iron abundance is larger than
10-5, the iron line opacity has to be considered not only for
the spectrum synthesis, but also in the model construction itself. For
such metal abundances, non-LTE metal line-blanketed models are needed
for detailed abundance studies of hot, metal-rich white dwarfs. We also
discuss the predicted Extreme Ultraviolet (EUV) spectrum and show that
it is very sensitive to metal abundances, as well as to non-LTE effects.

---------------------------------------------------------
Title: Hubble Space Telescope FOS Spectroscopy of the
Ultrashort-Period Dwarf Nova WZ Sagittae: The Underlying Degenerate
Authors: Sion, Edward M.; Cheng, F. H.; Long, Knox S.; Szkody, Paula;
Gilliland, Ron L.; Huang, Min; Hubeny, Ivan
1995ApJ...439..957S Altcode:
Two consecutive Hubble Space Telescope (HST) Faint Object Spectrograph
(FOS) spectra of the exposed white dwarf in the ultrashort-period,
high-amplitude, dwarf nova WZ Sge, reveal a rich absorption line
spectrum of neutron carbon and ionized metals, the Stark-broadened
Lyman-alpha absorption wing, the H2 quasi-molecular Lyman-alpha
'satellite' absorption line, and a double-peaked C IV emission line
which is variable with orbital phase. A synthetic spectral analysis of
the white dwarf yields Teff = 14,900 K +/- 250 K, log g =
8.0. In order to fit the strongest C I absorption lines and account for
the weakness of the silicon absorption lines, the abundance of carbon
in the photosphere must be approximately 0.5 solar, silicon abundance
is 5 x 10-3 solar, with all other metal species appearing to
be 0.1-0.001 times solar. The H2 quasi-molecular absorption is fitted
very successfully. The photospheric metals have diffusion timescales of
fractions of a year, and thus they must have been accreted long after
the 1978 December outburst. The source of the most abundance metal,
carbon, is considered. If the time-averaged accretion rate during
quiescence is low enough for diffusive equilibrium to prevail, then the
equilibrium accretion rate pf neutron carbon is 7 x 10-16
solar mass/yr. A convective dredge-up origin for the concentration of
carbon is extremely unlikely, given that the white dwarf atmosphere
is H-rich while in single degenerates showing carbon and hydrogen,
the C and H are trace elements in a helium background. Additional
implications are explored.

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Title: Non-LTE Line-blanketed Model Atmospheres of Hot
Stars. I. Hybrid Complete Linearization/Accelerated Lambda Iteration
Method
Authors: Hubeny, I.; Lanz, T.
1995ApJ...439..875H Altcode:
A new munerical method for computing non-Local Thermodynamic Equilibrium
(non-LTE) model stellar atmospheres is presented. The method, called the
hybird complete linearization/accelerated lambda iretation (CL/ALI)
method, combines advantages of both its constituents. Its rate of
convergence is virtually as high as for the standard CL method,
while the computer time per iteration is almost as low as for the
standard ALI method. The method is formulated as the standard complete
lineariation, the only difference being that the radiation intensity
at selected frequency points is not explicity linearized; instead,
it is treated by means of the ALI approach. The scheme offers a wide
spectrum of options, ranging from the full CL to the full ALI method. We
demonstrate that the method works optimally if the majority of frequency
points are treated in the ALI mode, while the radiation intensity at a
few (typically two to 30) frequency points is explicity linearized. We
show how this method can be applied to calculate metal line-blanketed
non-LTE model atmospheres, by using the idea of 'superlevels' and
'superlines' introduced originally by Anderson (1989). We calculate
several illustrative models taking into accont several tens of thosands
of lines of Fe III to Fe IV and show that the hybrid CL/ALI method
provides a robust method for calculating non-LTE line-blanketed model
atmospheres for a wide range of stellar parameters. The results for
individual stellar types will be presented in subsequent papers in
this series.

---------------------------------------------------------
Title: Non-LTE Line Blanketed Model Atmospheres of Hot, Metal-Rich
White Dwarfs
Authors: Hubeny, I.; Lanz, T.
1995LNP...443...98H Altcode: 1995whdw.conf...98H
No abstract at ADS

---------------------------------------------------------
Title: Hydrogene line profiles
Authors: Hubeny, I.
1995HiA....10..411H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: EUVE Spectroscopy of epsilon Canis Majoris (B2 II) from 70
to 730 Angstrom
Authors: Cassinelli, J. P.; Cohen, D. H.; Macfarlane, J. J.; Drew,
J. E.; Lynas-Gray, A. E.; Hoare, M. G.; Vallerga, J. V.; Welsh, B. Y.;
Vedder, P. W.; Hubeny, I.; Lanz, T.
1995ApJ...438..932C Altcode:
We present spectra of the brightest stellar source of extreme
ultraviolet (EUV) radiation longward of 400 A, the B2 II star, epsilon
CMa. These data were taken with the three spectrometers aboard the
NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first
cycle of pointed observations. We report on our initial studies of
the continuum and line spectrum of the stellar photosphere in the
320 to 730 A region, and on the wind emission lines observed in the
170-375 A region. This is the first EUV spectrum of an early-type
star, and thus makes epsilon CMa the most comprehensively observed B
star from the X-ray to infrared regimes. The radiation in both the H
Lyman continuum and He I continuum (shortward of 504 A) are found to
be significantly greater than predicted by both Local Thermodynamic
Equilibrium (LTE) and non-LTE model atmospheres. Since epsilon CMa
also exhibits a mid-infrared excess, this points to the outer layers
being warmer than the models indicate. The anomalously large Lyman
continuum flux, combined with the very low column density measured in
the direction toward this star implies that it is the dominant source
of hydrogen ionization of the local interstellar medium in the immediate
vicinity of the sun. All of the lines predicted to be strong from model
atmospheres are present and several wind absorption features are also
identified. We have detected emission lines from highly ionized iron
that are consistent with the ROSAT Position Sensitive Proportional
Counter (PSPC) observations if a multi-temperature emission model is
used, and the assumption is made that there is significant absorption
beyond that of the neutral phase of the ISM. The spectrum shows strong
O III 374 A line emission produced by the Bowen flourescence mechanism,
which has not previously been observed in the spectra of hot stars.

---------------------------------------------------------
Title: A Spectrum Synthesis Program for Binary Stars with Optically
Thick Accretion Disks
Authors: Linnell, A. P.; Hubeny, I.
1994AAS...185.8505L Altcode: 1994BAAS...26.1460L
We recently reported a spectrum synthesis program for binary stars
(Linnell &amp; Hubeny, 1994, ApJ, 434, Oct.20). That program now has
been extended to include the case of an optically thick accretion
disk associated with either of the two stellar components. Our model
of the accretion disk uses the Pringle expression (Pringle, 1981,
ARA&amp;A, 19, 137) for T_eff values on the accretion disk face,
and the results of Hubeny and Plavec (1981, ApJ, 102, 1156) for rim
T_eff values. The treatment of the stellar components is the same
as in our 1994 paper. The program divides the rim into NRIM latitude
values, typically 9, and divides the visible accretion disk face into
NRING concentric ring boundaries, typically 31. The individual rings
(for both the rim and the face) subdivide into NSEG pixels, typically
101. An individual synthetic spectrum, appropriate to the local T_eff
value, is attached to each pixel. For illustration purposes we have used
synthetic spectra prepared from Kurucz atmospheres. The extended program
constructs a synthetic spectrum for the accretion disk face, rim, the
separate stellar components, and the entire system by producing a sum
of contributions, at each wavelength (with due allowance for Doppler
shift), from each visible pixel on the accretion disk or the separate
stellar components, weighted by the projected area of the pixel. A
separate program establishes a visibility key for each pixel and cosine
of the zenith angle of the observer as seen from each pixel, for the
current value of orbital inclination and orbital longitude. These data
combine with synthetic spectra in the spectrum synthesis program to
determine line of sight light intensities at each wavelength, i.e.,
the contributions needed for the sum. Separate data from related
programs permit a plot of the system projected on the plane of the
sky. This project received partial support from NSF grant AST9020459.

---------------------------------------------------------
Title: HST spectrally-resolved accretion disk maps of UX UMa in
the ultraviolet
Authors: Baptista, R.; Long, K.; Horne, K.; Hubeny, I.; Mauche, C. W.;
Rutten, R. G. M.; Wade, R. A.
1994AAS...18511605B Altcode: 1994BAAS...26.1514B
We present spectrally-resolved eclipse maps of the UX UMa accretion
disk in the spectral region 1170--2400 Angstroms. The analysis is
based on low-dispersion time-resolved spectroscopy during eclipse,
obtained with the HST Faint Object Spectrograph. The out-of-eclipse
spectra show prominent emission lines (CIII lambda 1176, Lyalpha ,
NV lambda 1240, SiIV lambda 1400, CIV lambda 1550) as well as many
absorption features and possibly a broad absorption band centered
at about 1900 Angstroms. HeII lambda 1640 appears as a weak emission
line. At mid-eclipse the continuum flux is reduced by a factor ~ 3,
while the emission lines are much less affected by the eclipse and
some of the lines that are seen in absorption in the out-of-eclipse
spectra appear in emission. UV continuum light curves show significant
flickering activity outside of eclipse, relatively much stronger
than observed in the optical range. The eclipses are deep and steep
in the continuum with an asymmetry in their egress portion, however
no clear evidence of a compact bright spot is seen. The spectra were
divided into passbands ( ~ 40 Angstroms in the continuum and ~ 3000
Km/s in the emission lines) and light curves were extracted for each
one. Maximum-entropy eclipse mapping techniques were used to solve
for a map of the disk brightness distribution and for the flux of
an additional uneclipsed component in each band. Radial brightness
temperature profiles and spatially-resolved disk spectra derived from
these maps are presented and discussed.

---------------------------------------------------------
Title: HST/GHRS Observations of beta Pictoris
Authors: Heap, S. R.; Lanz, T.; Hubeny, I.; Lindler, D.
1994AAS...185.4812H Altcode: 1994BAAS...26.1389H
In September 1994, we obtained HST/GHRS spectra and maps of beta
Pic (HD 39060) and its surrounding disk. In this paper, we report
our preliminary findings concerning the star and circumstellar gas
disk. Both Stromgren photometry and Geneva photometry (as listed
by SIMBAD) yield the following parameters for the star: T_eff=8200
+/- 50 K log g = 4.25 +/- 0.05 [Fe/H] = -0.1 +/- 0.2 To refine the
estimated metallicity, we have compared the GHRS ultraviolet spectra
with model spectra. Our model of beta Pic incorporates both a LTE
stellar atmosphere plus a circumstellar gas disk, treated in a first
approximation as a homogeneous, absorbing slab at T=5,000 K with
a column density, N_H=2x10(19) cm(-2) . We confirm that the star
has a near-solar metallicity. We have compared the derived stellar
parameters with evolutionary models for Z=0.017. For reference, a ZAMS
star having a T_eff=8,200 K has a mass, M ~1.72 M_sun, and a gravity,
log g= 4.37. The somewhat low surface gravity of beta Pic suggests that
the star has evolved slightly off the ZAMS or is still in the pre-main
sequence phase. In either case, it should be somewhat more luminous
than a 1.72M_sun ZAMS star. However, its absolute magnitude, M_V= 2.70
(for a distance of 17 pc) is 0.21 mag fainter than a M=1.72 M_sun star
on the ZAMS. We therefore conclude that (1) the dust disk must diminish
light from the star, and (2) it is entirely possible that beta Pic is
a very young star. Both conclusions raise problems for interpreting
observations of the dust disk (c.f. Paresce, 1991, A&amp;A, 247, L25).

---------------------------------------------------------
Title: HST/FOS Spectroscopy of the Central Star of NGC 7027
Authors: Hubeny, I.; Heap, S. R.; Lanz, T.
1994AAS...185.4710H Altcode: 1994BAAS...26.1385H
With its rich UV-optical-IR spectrum, NGC 7027 has been the target
of choice for study of important physical processes including
charge-exchange, dust-quenching of nebular lines, PDR's, extended-red
emission, etc. Surprisingly little, though, is known about the central
star, because the star is embedded in such a bright nebula. With
the success of the HST servicing mission, it is now possible to
carry out direct studies of the central star. Here, we report on
HST/FOS spectroscopy of the central star. In August 1994, we obtained
FOS/G400H, G570H spectra of NGC 7027 with the central star centered
in the 1farcs 0 aperture. The spectra cover the wavelength range,
3250-6750 Angstroms. The spectra are of excellent quality. For example,
at lambda 4800 Angstroms, the S/N=76 per resolution element, and the
star contributes 94% of the observed continuum flux. Other very hot
stars, i.e. PG 1159 stars, are known to show high-ionization lines,
such as O VI lambda lambda 3811,3834, C IV lambda 4441, or C V lambda
4945, etc. To our surprise, these lines are absent in the spectrum
of the central star. In fact, the stellar spectrum is apparently
featureless. We have therefore used the absence of spectral features
to put limits on effective temperature and chemical composition of the
central star. We are constructing a small grid of line-blanketed NLTE
model atmospheres for the effective temperatures range 100,000 K &lt;
T_eff &lt; 250,000 K, 5.8 &lt; log g &lt; 7.5, and for two compositions:
(i) a H-rich (solar) composition, and (ii) a C-O enriched atmosphere,
typical of PG 1159 stars. We will report on the results of this
analysis.

---------------------------------------------------------
Title: NLTE Line-blanketed Model Atmosphere Analysis of the hot
Subdwarf BD +75° 325
Authors: Lanz, T.; Hubeny, I.; Heap, S. R.
1994AAS...185.8005L Altcode: 1994BAAS...26.1447L
With the Goddard High Resolution Spectrograph aboard HST, we have
recorded a high-resolution, high signal-to-noise ratio, far ultraviolet
spectra of the hot subdwarf, BD +75° 325. Because of the low apparent
rotational velocity (V sin i &lt; 20 km/s) of the star, the high
quality of the spectra and our newly developed NLTE line-blanketed
model atmospheres, we have an unprecedented opportunity to derive
accurate stellar parameters. With our hybrid CL/ALI method, we have
constructed a small grid of NLTE line-blanketed model atmospheres for
hot subdwarfs. These model atmospheres include consistently about
200 NLTE levels (or superlevels) of H, He, C, N, O, Si, Fe and Ni,
and all transitions between them. Individual atomic levels of iron and
nickel are merged into a relatively small number of superlevels. The
cross-sections of the transitions between superlevels are represented by
NLTE Opacity Distribution Functions. This grid of NLTE model atmospheres
is used for a thorough analysis of BD +75° 325, which then provides
in addition a “real” check of our methodology. Our results show that
BD +75° 325 is a He enriched star, whose surface exhibit CNO-cycle
products (N-rich and C-deficient). The very rich Fe v and Fe vi line
spectra can only be reproduced by a significant overabundance of iron
(about 5 to 10 times solar). With our metal line blanketed NLTE model
atmospheres, we are now able to reproduce the deep line cores in the
ultraviolet spectrum assuming a microturbulence of 5-10 km/s. Moreover,
the line blanketed models are necessary to match the H i/He ii line
cores in the visual spectrum (models including only H and He produce
too shallow lines). Finally, we have found a weak asymmetry of the
N v resonance line profiles, which can be attributed to a weak wind
with M = 9+/- 2times 10(-11) M_sun /yr.

---------------------------------------------------------
Title: Analysis of the VW Hyi's Superoutburst FOS/HST Spectrum:
a Heated White Dwarf and a Disrupted Disk
Authors: Huang, M.; Sion, E. M.; Hubeny, I.; Cheng, F. H.; Szkody, P.
1994AAS...18513104H Altcode: 1995BAAS...27..770H
No abstract at ADS

---------------------------------------------------------
Title: GHRS Spectroscopy of Individual Stars in R136a
Authors: Heap, Sara R.; Ebbets, Dennis; Malumuth, Eliot M.; Maran,
Stephen P.; de Koter, Alex; Hubeny, Ivan
1994ApJ...435L..39H Altcode:
The installation of the Corrective Optics Space Telescope Axial
Replacement (COSTAR) Instrument on the Hubble Space Telescope (HST)
makes it possible to observe stars in very crowded regions with high
spatial and spectral purity. To demonstrate this capability, we have
used the Goddard High Resolution Spectrograph (GHRS) to obtain spectra
of two stars in the dense center of the 30 Doradus ionizing cluster:
R136a5, and its nearest neighbor, R136a2, only 0.17 sec away. R136a5 is
shown to ben an O3f/WN star, while R136a2 is a WN4-w star. From both
Wide Field/Planetary Camera (WFPC) photometry and GHRS, spectroscopy
we estimate the following properties of R136a5: Teff =
42,500 K, R = 16.4 solar radius, Lbol = 8 x 105
solar luminosity, and M approx. equals 50 solar mass -- all indicating
that, despite its spectral type, R136a5, as indicated by the strength
of He II lambda 1640 emission. The observed mass-loss rate, dot-M =
1.8 x 10-5 solar mass/yr, is an order of magnitude higher
than is assumed by current stellar evolutionary models. We argue that
this high rate of mass loss will alter drastically the evolutionary
path of R136a5. If so, evolutionary models for massive stars require
substantial revision.

---------------------------------------------------------
Title: Theoretical Modeling of GHRS Observations of the Of/WN-Type
Star R136a5
Authors: de Koter, Alex; Hubeny, Ivan; Heap, Sara R.; Lanz, Thierry
1994ApJ...435L..71D Altcode:
We present a theoretical analysis of the Hubble Space Telescope (HST)
Goddard High Resolution Spectrograph (GHRS) spectrum of R136a5,
an O3fWN star in the R136a cluster in 30 Doradus. Using non-LTE
extended and expanding model atmospheres, we find a surprisingly high
mass-loss rate dot-M = 1.8 +/- 0.5 x 10-5 solar mass/yr and
an effective temperature Teff = 42.5 +/- 2.5 kK. With the
observed visual magnitude, this implies a radius R* = 17 +/-
2 solar radius and a solar luminosity L = 8.5 +/- 1 x 105
solar luminosity. We discuss possible sources of the uncertainties
in the derived stellar parameters and conclude that the effective
temperature may be underestimated; however, if so, the value of the
mass-loss rate would not be affected.

---------------------------------------------------------
Title: A Spectrum Synthesis Program for Binary Stars
Authors: Linnell, Albert P.; Hubeny, Ivan
1994ApJ...434..738L Altcode:
A new program produces synthetic spectra of binary stars at arbitrary
values of orbital longitude, including longitudes of partial or
complete eclipse. The stellar components may be distorted, either
tidally or rotationally, or both. Either or both components may be
rotating nonsynchronously. We illustrate the program performance with
two cases: EE Peg, an eclipsing binary with small distortion, and SX
Aur, an eclipsing binary that is close to contact.

---------------------------------------------------------
Title: On the strategy of future observations and modeling of the
β Lyrae system.
Authors: Hubeny, I.; Harmanec, P.; Shore, S. N.
1994A&A...289..411H Altcode:
β Lyr is an enigmatic object. Despite several decades of concentrated
observational and theoretical effort, our understanding of the system
is still rather poor. We discuss two existing structural models of the
system, the massive torus model of Wilson (1981), and the low mass
accretion disk model of Hubeny &amp; Plavec (1991). In particular,
we answer recent criticism of the low mass disk model expressed by
the proponents of the massive disk hypothesis. We show that although
both theories have a large degree of internal consistency, there are
several serious physical objections against the massive disk model. In
particular, such a model requires unrealistically low viscosity
(large Reynolds number). Moreover, such massive disk is likely to
be dynamically unstable. Finally, we propose several observational
and theoretical approaches that could lead to construction of a more
physically realistic model of the β Lyr system.

---------------------------------------------------------
Title: Dynamic Processes in Be Star Atmospheres. II. He i 2P--nD
Line Formation in lambda Eridani (Outburst)
Authors: Smith, Myron A.; Hubeny, Ivan; Lanz, Thierry; Meylan, Thomas
1994ApJ...432..392S Altcode:
The He I lambda 6678 line of early Be stars generally shows violet (V)
and red (R) emission whenever hydrogen alpha emission is present, but
its use as a diagnostic has been handicapped by a poor understanding of
the processes that drive it into emission. In an attempt to address this
problem we obtained three series of eschelle spectra of the first two
members of the singlet and triplet 2P-nD series of lambda Eri (B2e)
during 1992 November 3-5 at Kitt Peak. During these observations
lambda 6678 showed substantial emission variability in both the
wings and central profile, providing an opportunity to compare its
behavior with that of the lambda 4922, lambda 5876, and lambda 4471
lines. We found that the responses of the lines were different in
several respects. Whereas the emissions in the V wings of all four
lines scaled together, the R wing of the lambda 4922 line invariably
responded with increased absorption whenever the R wing of lambda 6678
line showed increased emission. These same trends occurred within
the central photospheric profiles. The R-wing behavior shows that
much, but not all of the emission in lambda 6678 is caused by matter
projected against the stellar disk. The excitation temperatures of
the neighboring 21 P transitions, lambda 6678 and lambda
4922 must be greater than and less than the photospheric continuum
temperature, respectively. We have investigated departures from local
thermodynamic equilibrium (LTE) for the He I spectrum in a variety of ad
hoc, perturbed model atmospheres. We have found only one way to cause
the source function of lambda 6678 to increase so strongly, namely,
by increasing the atmospheric temperature in the line formation region
to 30,000 - 40,000 K. This effect was discovered by Auer and Mihalas
for O3-O4 atmospheric models, but it has not been applied to active
B stars. Our models suggest that lambda 6678 emission in Be stars can
be used as a sensitive monitor of localized hot spots on these stars'
surfaces. The energies involved in heating the active portions of the
atmosphere are too high to be produced by gravitational infall. This
leaves magnetically induced flares among the few known processes on
the surfaces of stars capable of sustaining this energy level.

---------------------------------------------------------
Title: A Spectrum Synthesis Program for Binary Stars
Authors: Linnell, A. P.; Hubeny, I.
1994AAS...184.0606L Altcode: 1994BAAS...26..868L
A new computer program produces synthetic spectra of binary stars
at arbitrary values of orbital longitude, including longitudes of
partial or complete eclipse. The stellar components may be distorted,
either tidally or rotationally, or both. Either or both components may
be rotating non-synchronously. We illustrate the program performance
with two cases: EE Peg, an eclipsing binary with small distortion and
SX Aur, an eclipsing binary that is close to contact.

---------------------------------------------------------
Title: HST Eclipse Mapping of Dwarf Nova OY Carinae in Quiescence:
an “Fe II Curtain” with Mach 6 Velocity Dispersion Veils the
White Dwarf
Authors: Horne, Keith; Marsh, T. R.; Cheng, F. H.; Hubeny, Ivan;
Lanz, Theirry
1994ApJ...426..294H Altcode:
Hubble Space Telescope (HST) observations of the eclipsing dwarf nova OY
Car in its quiescent state are used to isolate the ultraviolet spectrum
(1150-2500 A at 9.2 A Full Width at Half Maximum (FWHM) resolution)
of the white dwarf, the accretion disk, and the bright spot. The
white dwarf spectrum has a Stark-broadened photospheric L(alpha)
absorption, but is veiled by a forest of blended Fe II features that
we attribute to absorption by intervening disk material. A fit gives
Tw approx. = 16.5 x 103 K for the white dwarf
with a solar-abundance, log g = 8 model atmosphere, and T approx. =
104 K, ne approx. = 1013/cu cm,
NH approx. = 1022 sq cm, and velocity dispersion
delta V approx. = 60 km/s for the veil of homogeneous solar-abundance
local thermodynamic equilibrium (LTE) gas. The veil parameters probably
measure characteristic physical conditions in the quiescent accretion
disk or its chromosphere. The large velocity dispersion is essential
for a good fit; it lowers (chi square)/778 from 22 to 4. Keplerian shear
can produce the velocity dispersion if the veiling gas is located at R
approx. = 5 RW with (delta R)/R approx. = 0.3, but this model
leaves an unobscured view to the upper hemisphere of the white dwarf,
incompatible with absorptions that are up to 80% deep. The veiling
gas may be in the upper atmosphere of the disk near its outer rim,
but we then require supersonic (Mach approx. = 6) but sub-Keplerian
(delta V/VKep approx. = 0.07) velocity disturbances in
this region to produce both the observed radial velocity dispersion
and vertical motions sufficient to elevate the gas to z/R = cos i
= 0.12. Such motions might be driven by the gas stream, since it
may take several Kepler periods to reestablish the disk's vertical
hydrostatic equilibrium. The temperature and column density of the
gas we see as Fe II absorption in the ultraviolet are similar to what
is required to produce the strong Balmer jump and line emissions seen
in optical spectra of OY Car and similar quiescent dwarf novae. The
outer accretion disk is detected at mid-eclipse with a spectrum that
rises from 0.05 to 0.3 mJy between 2000 and 2500 A, consistent with
combinations of cool blackbodies, blended Fe II emission lines,
and Balmer continuum emission. The total disk flux density is 0.5
mJy at 2500 A, and this shallow disk eclipse implies a roughly flat
surface brightness distribution. The bright spot, somewhat bluer than
the disk, has a flux density rising from 0.05 to 0.15 mJy between
1600 and 2500 A. The C IV emission line has a broad shallow eclipse,
but the radial velocity variations observed during the eclipse do not
clearly distinguish between a disk or wind origin. The only possible
indications of boundary layer emission are fast UV flares that appear
to arise from near the central object -- not from the bright spot.

---------------------------------------------------------
Title: Observations of the Bright Novalike Variable IX Velorum with
the Hopkins Ultraviolet Telescope
Authors: Long, Knox S.; Wade, Richard A.; Blair, William P.; Davidsen,
Arthur F.; Hubeny, Ivan
1994ApJ...426..704L Altcode:
The Hopkins Ultraviolet Telescope, an experiment flown on the Space
Shuttle as part of the Astro-1 mission, was used to obtain a spectrum
of the novalike variable IX Vel (= CPD -48 deg 1577) in the wavelength
range 830-1860 A. The observation revealed a rich absorption-line
and continuum spectrum that peaks near 1050 A at a flux of 1.6 x
10-11 ergs/sq cm/s/A. In the sub-Lyman-alpha region, some
of the more prominent absorption lines are S VI lambda lambda-933, 945,
C III lambda-977, Lyman-beta, O VI lambda lambda-1032, 1038, P V lambda
lambda-1118, 1128, and C III lambda-1176. No emission was detected below
the Lyman limit. The overall continuum shape of IX Vel in the FUV can be
approximated using models of an optically thick accretion disk in which
the integrated spectrum has been constructed by summing model stellar
atmospheres or proper disk model spectra. However, if the distance to
IX Vel is approximately 95 pc, standard disk models without reddening
cannot simultaneously reproduce the color and flux in the UV. While
interstellar reddening can reconcile this difference, the amount of
reddening appears inconsistent with the absence of a 2200 A bump in
the spectrum and the very low H I column density measured along the
line of sight. Improved fits to the data can be obtained by modifying
the accretion disk stucture within three white dwarf radii. None
of the models reproduces the profiles of the Li- and Na-like ions,
which are observed as strong but relatively narrow absorption lines,
and which are almost surely due to a wind above the disk.

---------------------------------------------------------
Title: The Iron Curtain Effect in the Ultraviolet Spectra of Dwarf
Novae Z Cha and OY CAR
Authors: Wade, R. A.; Cheng, F. H.; Hubeny, I.
1994AAS...184.4605W Altcode: 1994BAAS...26Q.936W
We model the ultraviolet spectra of the quiescent dwarf novae Z Cha
and OY Car, obtained in 1986 with the SWP camera of the International
Ultraviolet Explorer, on the assumption that the flux is due to the
central white dwarf seen through a haze of absorbing material. This
material is optically thin in the continuum but thick in the lines
of many atomic species, chiefly of iron-peak elements. These IUE
observations are long exposures covering several orbits, and so they
do not unambiguously refer to the white dwarf alone. This differs
from the 1991 observation of OY Car's white dwarf by the Faint
Object Spectrograph on HST, which was made using time-tagged eclipse
spectrophotometry as discussed in Horne et al. (1994). The HST data
also have the advantage of a photon counting detector. Nevertheless,
there is interest in using the IUE data to observe the so-called
“Iron Curtain” of OY Car at a different epoch, and to extend the
study of the Iron Curtain to another dwarf nova in quiescence, Z
Cha. Results in terms of multiple-parameter fits for the white dwarf
temperature and solid angle, and the column density, temperature, number
density, and turbulent broadening of the Iron Curtain of each star,
are discussed. An attempt is made to be consistent with the optical
photometry of the white dwarfs, as determined by Wood et al. (1986,
1989). Supported by NASA grant NAG 5-703 and NASA/STScI grant GO-2380.

---------------------------------------------------------
Title: NLTE Model Stellar Atmospheres: The Hybrid CL/ALI Method
Authors: Lanz, T.; Hubeny, I.
1994AAS...184.4011L Altcode: 1994BAAS...26..924L
A new numerical method for computing sophisticated NLTE model
stellar atmospheres is presented. The method, called the hybrid
Complete Linearization/Accelerated Lambda Iteration (CL/ALI) method,
combines advantages of both its constituents. Its rate of convergence
is virtually as high as for the standard CL method, while the computer
time per iteration is almost as low as for the standard ALI method. The
method is formulated as the standard complete linearization; the only
difference being that the radiation intensity at selected frequency
points is not explicitly linearized; instead, it is treated by means
of the ALI approach. The scheme offers a wide spectrum of options,
ranging from the full CL to the full ALI method. We demonstrate
that the method works optimally if the majority of frequency points
are treated in the ALI mode, while the radiation intensity at a few
(typically 2 - 30) frequency points is explicitly linearized. We show
how this method can be applied to calculate metal line blanketed NLTE
model atmospheres of hot stars. We use the idea of “superlevels” and
“superlines” introduced originally by Anderson (1989). We calculate
several illustrative models taking into account several tens of thousand
of lines of Fe iii to Fe vi, and show that the hybrid CL/ALI method
provides a robust method for calculating NLTE line blanketed model
atmospheres for a wide range of stellar parameters. This work was
supported in part by NASA grants NAGW-3025 and NAGW-3834.

---------------------------------------------------------
Title: GHRS observations and theoretical modeling of early type
stars in R136a
Authors: de Koter, A.; Heap, S.; Hubeny, I.; Lanz, T.; Hutchings,
J.; Lamers, H. J. G. L. M.; Maran, S.; Schmutz, W.
1994AAS...184.3106D Altcode: 1994BAAS...26..909D
We present the first spectroscopic observations of individual stars in
R136a, the most dense part of the starburst cluster 30 Doradus in the
LMC. Spectra of two stars are scheduled to be obtained with the GHRS
on board the HST: R136a5, the brightest of the complex and R136a2,
a Wolf-Rayet star of type WN. The 30 Doradus cluster is the only
starburst region in which individual stars can be studied. Therefore,
quantitative knowledge of the basic stellar parameters will yield
valuable insight into the formation of massive stars in starbursts and
into their subsequent evolution. Detailed modeling of the structure
of the atmosphere and wind of these stars will also lead to a better
understanding of the mechanism(s) that govern their dynamics. We
present the first results of our detailed quantitative spectral
analysis using state-of-the-art non-LTE model atmospheres for stars
with extended and expanding atmospheres. The models are computed using
the Improved-Sobolev Approximation wind code (ISA-WIND) of de Koter,
Schmutz &amp; Lamers (1993, A&amp;A 277, 561), which has been extended
to include C, N and Si. Our model computations are not based on the
core-halo approximation, but use a unified treatment of the photosphere
and wind. This approach is essential for Wolf-Rayet stars. Our synthetic
spectra, dominated by the P Cygni profiles of the UV resonance lines,
also account for the numerous weak metal lines of photospheric origin.

---------------------------------------------------------
Title: Physics of Massive Hot Stars
Authors: Hubeny, I.
1994AAS...184.3401H Altcode: 1994BAAS...26..913H
We discuss the impact of HST on our understanding of physics of hot
massive stars. Since these stars are very luminous, they can be studied
as individual stars in several parent galaxies. In order to take full
advantage of the high spectral resolution and high signal-to-noise
ratio offered by the HST spectra, new highly accurate and reliable
methods of spectroscopic analysis have to be developed. We show that
thanks to the recent dramatic advances in fast numerical methods it
is now possible to construct model atmospheres of an unprecedented
degree or realism and accuracy. We concentrate on two basic issues
of the modern stellar atmospheres models of hot stars. First, we
discuss the effects of departures from local thermodynamic equilibrium
(referred to as non-LTE or NLTE). We show that model atmospheres are
now becoming avaliable that include the opacity of tens to hundred
thousand metal lines in full NLTE (the so-called NLTE line-blanketed
model atmospheres). It is now possible to derive the basic stellar
parameters (effective temperature, luminosity, chemical composition)
to a high degree of accuracy. Second, we discuss models which do not
employ the traditional core-halo approximation, i.e. an arbitrary
separation of the static photosphere and the dynamic stellar wind --
the so-called unified model atmospheres. We review the recent research
effort in this direction, and show several interpretational consequences
of this methodological improvement.

---------------------------------------------------------
Title: The Discovery of NI V in the Photospheres of the Hot DA White
Dwarfs RE 2214-492 and G191-B2B
Authors: Holberg, J. B.; Hubeny, I.; Barstow, M. A.; Lanz, T.; Sion,
E. M.; Tweedy, R. W.
1994ApJ...425L.105H Altcode:
We have co-added six recently obtained International Ultraviolet
Explorer (IUE) echelle spectra of the hot DA white dwarf RE 2214-492
and 10 existing archive spectra of the well-known hot DA, G191-B2B. We
find that both stars contain numerous weak features due to Ni V. Nickel
is thus the second iron-group element to be found in the spectra of the
very hottest DA white dwarfs. In addition to Ni V, we also observe Al
III in both stars and present evidence for the possible presence of
Ni IV and Fe IV in RE 2214-492. The presence of Ni and Al, together
with previously reported elements, will contribute significantly to
both the EUV opacity and to the apparent complexity of the UV spectra
of these stars. Using Non-Local Thermodynamic Equilibrium (NLTE) model
atmospheres we estimate the Ni abundances in RE 2214-492 the G191-B2B
to be log(Ni/H) = -5.5 +/- 0.3 and -6.0 +/- 0.3, respectively.

---------------------------------------------------------
Title: NLTE model stellar atmospheres with line blanketing near the
series limits.
Authors: Hubeny, I.; Hummer, D. G.; Lanz, T.
1994A&A...282..151H Altcode:
In this paper we study the influence of line-merging regions at the
intermediate long-wavelength side of a continuum threshold on the
computed model atmosphere structure and predicted spectrum. In order
to model these regions sufficiently accurately, we have developed two
concepts. First, we have extended the occupation probability formalism
of Hummer and Mihalas to non-local thermodynamic equilibrium (non-LTE)
plasmas. Second, in order to treat the very complicated opacity in
the line merging region, we have generalized the concept of opacity
distribution functions to treat non-LTE situations. All Rydberg states
are consistently included within this framework, so that no arbitrary
cutoff of high (LTE) levels is made. We have calculated several
pure hydrogen models atmospheres for two effective temperatures,
Teff = 20000 and 35000 K, and discussed the differences
between models calculated with various treatments of the line
merging. In particular, we have shown that the error in the predicted
profiles of Balmer lines resulting from the neglect of line merging
is typically of the order of 3-4%, while the errors in the far-UV
portion of the Balmer continuum reaches 15-35%. The errors generally
decrease with increasing effective temperature. At the same time,
the internal accuracy of the models is shown to be about or below 0.5%
for all predicted spectral features. We conclude that for interpreting
current high-accuracy spectrophotometric observations models including
the line merging are necessary, and that the formalism developed in
this paper is capable of providing a sufficiently accurate and robust
modeling technique.

---------------------------------------------------------
Title: Puzzling problems of He I line formation in early B stars
Authors: Smith, M. A.; Hubeny, I.; Lanz, T.
1994IAUS..162..273S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Lyman-Line Region in Models of Dwarf Nova Accretion Disks
Authors: Wade, R. A.; Hubeny, I.; Polidan, R. S.
1994ASPC...56..319W Altcode: 1994ibs..conf..319W
No abstract at ADS

---------------------------------------------------------
Title: HST Eclipse Mapping of the Dwarf Nova OY Carinae in
Quiescence: an "Fe II Curtain" with Mach approximately equals 6
Velocity Dispersion Veils the White Dwarf
Authors: Horne, K.; Cheng, F. H.; Marsh, T.; Hubeny, I.; Lanz, T.
1994ASPC...56..283H Altcode: 1994ibs..conf..283H
No abstract at ADS

---------------------------------------------------------
Title: The Cooling of the White Dwarf in OY CAR After 1992
Superoutburst
Authors: Cheng, F. H.; Marsh, T. R.; Horne, K.; Hubeny, I.
1994AIPC..308..197C Altcode: 1994exrb.conf..197C
HST observations of the eclipsing dwarf nova OY Car after its 1992
April superoutburst are used to isolate ultraviolet spectra (1150-2500
Å at 9.2 Å FWHM resolution) of the white dwarf, the accretion disk,
and the bright spot. The white dwarf spectra have a Stark-broadened
photospheric Lα absorption, but are veiled by a forest of blended
Fe II features that we attribute to absorption by intervening disk
material. Spectral fits give white dwarf temperatures changing from
∼19500 K just after outburst ∼17400 K around three months after
outburst. The temperature of intervening disk material is ∼8600
K-9800 K; the velocity dispersion of the intervening disk material is
∼60-70 km/s. Fitting results also shows that the decay time of white
dwarf temperature is ∼27 days, that is much shorter than ∼687 days
in dwarf nova WZ Sge.

---------------------------------------------------------
Title: Accretion in "Ordinary" Interacting Binaries
Authors: Plavec, M. J.; Hubeny, I.
1994ASPC...56...87P Altcode: 1994ibs..conf...87P
No abstract at ADS

---------------------------------------------------------
Title: Vertical Structure and Theoretical Spectra of Accretion Disks
Authors: Hubeny, I.
1994ASPC...56....3H Altcode: 1994ibs..conf....3H
No abstract at ADS

---------------------------------------------------------
Title: An Atlas of the Lyman-alpha Region for a Grid of Dwarf Nova
Accretion Disk Models
Authors: Wade, R. A.; Hubeny, I.; Polidan, R. S.
1993AAS...182.6312W Altcode: 1993BAAS...25Q.910W
We use the accretion disk modeling code TLUSDISK and spectrum synthesis
tools to compute a grid of spectra between 900 and 1500 Angstroms for
accretion disks of cataclysmic variables in quiescence. The disk models
and spectra are calculated in LTE. A contribution from the white dwarf
is included, for a variety of white dwarf effective temperatures. The
occultation of part of the inner disk by the white dwarf is also
taken into account. The contribution of a boundary layer, assumed
to be optically thin and emitting primarily at x-ray temperatures,
is neglected. The mass and radius of the accreting white dwarf are
varied. The mass accretion rate, assumed to be steady, is chosen to
reproduce the estimated luminosities of quiescent dwarf novae. The
emergent spectra are presented for a variety of inclination angles, to
show the effect of Doppler broadening at a resolution similar to that
of the Hopkins Ultraviolet Telescope and somewhat better than that of
low-dispersion IUE spectra. Comparisons of the Lyman-alpha region with
some IUE spectra of dwarf novae in quiescence are attempted. Supported
by NASA grant NAG 5-2125.

---------------------------------------------------------
Title: NLTE Line Blanketed Model Atmospheres for Hot, Metal-rich
White Dwarfs
Authors: Hubeny, I.; Lanz, T.
1993AAS...182.4701H Altcode: 1993BAAS...25..878H
Recent observations of some hot DA white dwarfs (Feige 24, G191 B2B -
Sion et al. 1992, Ap.J. 391, L29; Vennes et al. 1992, Ap.J. 392, L27)
and subsequent analyses have demonstrated that their atmospheres are
contaminated with heavy metal species with appreciable abundances. So
far, modeling was mostly limited to calculating synthetic spectra
(usually in LTE), based on previously calculated model atmospheres that
were constructed assuming a simplified chemical composition. However,
in reality the metal lines may also significantly influence the
temperature structure, and consequently the ionization balance of some
important species. Since the effective temperature is rather high
(55000 to 60000 K), the NLTE effects may be important despite the
high gravity. Therefore, in order to settle the question on metal
abundances of hot DA white dwarfs, we need to calculate NLTE fully
line blanketed model atmospheres. Using our previously developed
hybrid complete linearization/accelerated lambda iteration method,
we have calculated a set of NLTE models including H, He, C, N, O, and
Fe, with some 13000 lines of Fe IV, Fe V and Fe VI taken into account
explicitly in model construction. The NLTE departure coefficients
for all levels of Fe IV, Fe V, and Fe VI are then employed in the
spectrum synthesis program SYNSPEC. In the first part of this study,
we present a theoretical comparison between models computed with adding
more and more opacity sources, and show how the temperature structure
and synthetic spectra develop. We also discuss possible errors arising
from using an inconsistent spectrum synthesis (the metals considered
only in the spectrum synthesis, not in the model construction). In the
second part, we compare the computed UV spectra with with available
observations of Feige 24 and G191 B2B, and deduce limits for the iron
abundance of these stars.

---------------------------------------------------------
Title: Accretion on Non-Degenerate Binary Components
Authors: Plavec, M. J.; Hubeny, I.
1993AAS...182.4505P Altcode: 1993BAAS...25Q.871P
We are studying accretion disks in a variety of binary systems with
non-degenerate components. Disks are modeled by program TLUSDISK, their
detailed spectra are calculated for a range of inclination angles,
and differential Doppler broadening due to the Keplerian rotation in
the disk is duly included. Model spectra are compared with observed IUE
and CCD optical spectra. We find that various combinations of the rate
of mass transfer, mass and size of the gainer, and degree of viscosity
in the disk can produce a wide variety of observable phenomena. Here
we report on two cases. (1) The Algol-type eclipsing binary RY Persei
with a relatively large gainer mass and low mass transfer rate shows
a primary spectrum of B6 IV but with deep and strong UV circumstellar
absorptions, corresponding to strong emissions seen during total
eclipse. These lines are obviously formed in a layer or column
associated with the boundary layer between the disk and the star. (2)
The question if a symbiotic spectrum can be produced by an accretion
disk around a small red dwarf is studied (case of CI Cygni). The
vertical thickness of the disk strongly depends on the Reynolds number
describing viscosity, and can be substantial for a conventional value
of Re=5000. Since observations of this eclipsing system suggest i =
73(deg) , Re would have to be much smaller (1000). This provides an
interesting constraint on Re which may be exploited in future studies
of disk viscosity. Our models predict the spectra of the various
types of binary systems if viewed at moderate or low inclinations,
thus offering hints for systematic searches.

---------------------------------------------------------
Title: Observations of the White Dwarf in the U Geminorum System
with the Hopkins Ultraviolet Telescope
Authors: Long, Knox S.; Blair, William P.; Bowers, Charles W.;
Davidsen, Arthur F.; Kriss, Gerard A.; Sion, Edward M.; Hubeny, Ivan
1993ApJ...405..327L Altcode:
The Hopkins Ultraviolet Telescope was used to obtain an FUV spectrum
of U Gem about 10 d after the end of an outburst. Most of the FUV
emission from U Gem appears to rise from the white dwarf (WD) in the
system. Various possibilities to explain the hot component in the
present spectrum are explored. It is most likely due to radiation
from the boundary-layer region of the WD surface rather than from the
accretion disk which exists in U Gem in quiescence. If the standard
picture of boundary-layer emission is correct, then the hot component
is not due to ongoing accretion onto the WD because the temperature
observed is too low. Delayed emission from a viscously heated,
differentially rotating atmosphere is argued to be a more valid
explanation.

---------------------------------------------------------
Title: Modelling A-Type Atmospheres - NLTE Models
Authors: Hubeny, I.; Lanz, T.
1993ASPC...44...98H Altcode: 1993IAUCo.138...98H; 1993pvnp.conf...98H
No abstract at ADS

---------------------------------------------------------
Title: Spectroscopic Properties of the Nucleus of NGC 6826
Authors: Altner, B.; Heap, S. R.; Hubeny, I.
1993IAUS..155...86A Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Spectral Diagnoses of Chromospheres and Winds in A-Type Stars
Authors: Lanz, T.; Hubeny, I.
1993ASPC...44..517L Altcode: 1993IAUCo.138..517L; 1993pvnp.conf..517L
No abstract at ADS

---------------------------------------------------------
Title: Line Blanketed Model Atmospheres for Metal-rich White Dwarfs
Authors: Lanz, T.; Hubeny, I.; Cheng, F. -H.; Horne, K.
1992AAS...181.5004L Altcode: 1992BAAS...24.1203L
In many cases of a close binary system composed of a white dwarf, a
companion main-sequence or giant star, and an accretion disk around the
white dwarf, the UV spectrum of the system contains contributions from
both the white dwarf and the disk. In order to derive information about
the system from its UV spectrum, the white dwarf atmosphere has to be
accurately modeled. The chemical composition of the white dwarf is very
likely to be quite unusual, however, because it may have nearly solar
composition due to accretion. Due to its high surface gravity, the white
dwarf's lines are generally very broad, and thus metal line blanketing
may play a very important role. We have therefore begun a project of
calculating a grid of non-LTE, fully line-blanketed model atmospheres
for white dwarfs, for a large range of metal abundances, from zero (pure
DA) to solar composition. The method is described in a companion paper
(Hubeny and Lanz) at this meeting. We consider a representative model
atmosphere with T_eff=15000 K and log g = 8. In this preliminary study,
only hydrogen and iron (Fe II and Fe III) opacity was considered. The
models take into account 45815 Fe II and 23059 Fe III lines (all lines
between the levels with measured energies). As expected, the NLTE
effects are rather small for this temperature, but the effect of metal
line blanketing is very important. In particular, we demonstrate how
the metal line blanketing influences the UV spectrum and the hydrogen
Lyman and Balmer lines profiles. The work on the project was supported
in part by the NASA grant No.65 under program NRA 91-OSSA-12.

---------------------------------------------------------
Title: Non-LTE Line Blanketed Model Atmospheres: Application to
A-stars
Authors: Hubeny, I.; Lanz, T.
1992AAS...181.1911H Altcode: 1992BAAS...24.1152H
We present a new method for calculating realistic line blanketed model
atmospheres without the assumption of local thermodynamic equilibrium
(LTE). The method is based on complete linearization, with the
following two important modifications with respect to the standard
variant. i) Instead of dealing separately with individual energy
levels of a complicated metal species (e.g. Fe II), several levels
with the same parity and close enough energy are grouped together to
form a “superlevel". Transitions between superlevels, the so-called
“superlines", are treated by resampling the detailed absorption
cross-section and forming a partial “NLTE opacity distribution
function", which is representable by a relatively small number of
frequency points (typically 15 - 30 points per superline). ii) The
radiative rates (together with the heating/cooling rates, and the
contributions to the rates in other important transitions - e.g. the
hydrogen and carbon continua) are not fully linearized. However,
in contrast to the earlier approaches, they are not held fixed. The
only fixed quantity here is the approximate lambda operator. The new
method is therefore a hybrid combining the complete linearization
and the class of modern methods called accelerated lambda iteration
(ALI). We have contructed several non-LTE model atmospheres for T_eff
= 10000 K, log g = 4, with H, C I, C II, Mg II, and Fe II treated in
NLTE. A variable number of Fe II lines, up to 45815 (i.e. all lines
originating between the levels with measured energies), have been
included, to form 221 superlines. The most interesting preliminary
result is that the first 8 Fe II superlevels (about 13000 lines)
produce most of the total blanketing effect. The work on the project
was supported by the NASA grant No.65 under program NRA 91-OSSA-12.

---------------------------------------------------------
Title: Far Ultraviolet Observations of IX Vel with the Hopkins
Ultraviolet Telescope
Authors: Long, K. S.; Blair, W. P.; Davidsen, A. F.; Hubeny, I.;
Wade, R. A.
1992AAS...18110208L Altcode: 1992BAAS...24.1285L
No abstract at ADS

---------------------------------------------------------
Title: Non-LTE Analysis of the Hot Subdwarf: BD 75 325
Authors: Heap, S. R.; Hubeny, I.; Lanz, T.; Altner, B.
1992AAS...181.5003H Altcode: 1992BAAS...24.1203H
No abstract at ADS

---------------------------------------------------------
Title: Accelerated complete-linearization method for calculating
NLTE model stellar atmospheres
Authors: Hubeny, I.; Lanz, T.
1992A&A...262..501H Altcode:
Two approaches to accelerating the method of complete linearization
for calculating NLTE model stellar atmospheres are suggested. The
first one, the so-called Kantorovich variant of the Newton-Raphson
method, consists of keeping the Jacobi matrix of the system fixed,
which allows us to calculate the costly matrix inversions only a few
times and then keep them fixed during the subsequent computations. The
second method is an application of the Ng acceleration. Both methods
are extremely easy to implement with any model atmosphere code based
on complete linearization. It is demonstrated that both methods, and
especially their combination, yield a rapidly and globally convergent
algorithm, which takes 2 to 5 times less computer time, depending
on the model at hand and the required accuracy, than the ordinary
complete linearization. Generally, the time gain is more significant
for more complicated models. The methods were tested for a broad range
of atmospheric parameters, and in all cases they exhibited similar
behavior. Ng acceleration applied on the Kantorovich variant thus offers
a significant improvement of the standard complete-linearization method,
and may now be used for calculating relatively involved NLTE model
stellar atmospheres.

---------------------------------------------------------
Title: Detection of High-Degree Nonradial Pulsations in Gamma Bootis
Authors: Kennelly, E. J.; Yang, S.; Walker, G. A. H.; Hubeny, I.
1992PASP..104...15K Altcode:
The line-profile variations of the rapidly rotating delta Scuti
star gamma Boo can be explained by high-degree (|m| /= 10) nonradial
pulsations (NRPs) with an apparent period of Delta t /= 0.047 days. This
same period was derived from two data sets taken three months apart
where-in the amplitude increased by 30%. Such high-degree NRP cannot
explain the apparent reversals previously observed (Auvergne, Le
Contel &amp; Baglin 1979) for this star in the cores of the hydrogen
Balmer lines and CaII K line. Our radial-velocity variations can be
reconciled with their 0.25 day spectroscopic period if an amplitude
of 1 km s^-1 is adopted, an order of magnitude less than previous
measurements. We demonstrate that the presence of line-profile
variations from high-degree modes probably limits the accuracy
of radial-velocity measurements and may appear as bumps in the
radial-velocity curve. (SECTION: Stars)

---------------------------------------------------------
Title: Accelerated Lambda Iteration
Authors: Hubeny, Ivan
1992LNP...401..375H Altcode: 1992LNP...401..377H
Accelerated Lambda Iteration, or ALI, methods are reviewed. An emphasis
is given to the critical evaluation of various methods, analysing
their physical and mathematical meaning, and recommending the most
advantageous methods to interested non-specialists who consider
applying these methods to solving actual line formation and model
stellar atmosphere problems.

---------------------------------------------------------
Title: Accelerated Lambda iteration (review)
Authors: Hubeny, I.
1992LNP...401..377H Altcode: 1992aets.conf..377H
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Stellar Atmospheres Beyond Classical Models
Authors: Crivellari, L.; Hubeny, I.; Hummer, D. G.
1991JBAA..101..364C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Nonradial Pulsation Model for the Rapidly Rotating (Delta)
Scuti Star k^2 Bootis
Authors: Kennelly, E. J.; Walker, G. A. H.; Hubeny, I.
1991PASP..103.1250K Altcode:
A sectorial nonradial pulsation model is used to construct
theoretical line profiles which mimic the variations for Kappa(2)
Boo. Synthetic spectra generated with the appropriate Teff and log
g are used as input. It is found that the data can be reproduced by
the combination of a high-degree l is approximately equal to 12 mode
with P(osc) aproximately equal to 0.071 d, and a low-degree mode,
l is approximately equal to 0-2 with P(osc) approximately equal to
0.071-0.079 d. The projected rotational velocity (v sin i - 115 +/-5
km/s) was determined by fitting synthetic line profiles to the observed
spectra. The velocity amplitude of the high-degree oscillations is
estimated to be about 3.5 km/s. It is found that the ratio of the
horizontal and radial pulsation amplitudes is small (about 0.02) and
consistent with p-mode oscillations. Comparisons are made with models
invoking starspots, and it is impossible to fit the observations of
Kappa(2) Boo by a starspot model without assuming unrealistic values
of radius or equatorial velocity.

---------------------------------------------------------
Title: A New Look at the Standard Teff = 10000K, log g=4, Non-LTE
Model Atmosphere
Authors: Lanz, T.; Hubeny, I.; Altner, B.
1991BAAS...23R1381L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Can A Disk Explain Beta Lyrae?
Authors: Hubeny, Ivan; Plavec, Mirek J.
1991AJ....102.1156H Altcode:
The Beta Lyrae eclipsing binary system is interpreted in terms of a
model where the primary, i.e., spectroscopically observable, B star
transfers matter to a 'canonical' accretion disk surrounding a 'gainer'
which may be a rather ordinary B0.5 V star. Disk models are calculated
using the TLUSDISK program. Most of the optical radiation from the
secondary object is found to come from the disk rim, provided that the
rate of mass transfer is not much lower than about 0.0001 solar mass
per year. The rather high disk rim then introduces severe constraints on
the orbital inclination; these constraints are discussed in detail. It
is concluded that the disk plays an essentially passive role, and the
observable radiation from its face is insignificant. A model in which
most ultraviolet radiation would be coming from the face of the disk
is found unacceptable both because of these constraints and because
it would require an unrealistically small radius for the accreting
star. It is proposed that the bulk of the ultraviolet radiation comes
from a small unocculted segment of the gainer.

---------------------------------------------------------
Title: Line Broadening in O-Type Stars: Microturbulence or an Outflow
Velocity Gradient?
Authors: Hubeny, I.; Altner, B.; Heap, S. R.
1991BAAS...23.1382H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: First Results from the Goddard High-Resolution Spectrograph:
Evidence for Photospheric Microturbulence in Early O Stars: Are
Surface Gravities Systematically Underestimated?
Authors: Hubeny, I.; Heap, S. R.; Altner, B.
1991ApJ...377L..33H Altcode:
GHRS spectra of two very hot stars provide evidence for the presence
of microturbulence in their photospheres. In attempting to reproduce
the observed spectra, theoretical models have been built in which
the microturbulence is allowed to modify not only the Doppler line
widths (classical 'spectroscopic' microturbulence), but also the
turbulent pressure (thus mimicking a 'physical' turbulence). It is
found that a corresponding modification of the temperature-pressure
stratification influences the hydrogen and helium line profiles to the
extent that the surface gravities of early O stars determined without
considering microturbulence are too low by 0.1-0.15 dex. Thus, including
microturbulence would reduce, or resolve completely, a long-standing
discrepancy between evolutionary and spectroscopic stellar masses.

---------------------------------------------------------
Title: First Results from the Goddard High-Resolution Spectrograph:
Spectroscopic Determination of Stellar Parameters of Melnick 42,
an O3f Star in the Large Magellanic Cloud
Authors: Heap, S. R.; Altner, B.; Ebbets, D.; Hubeny, I.; Hutchings,
J. B.; Kudritzki, R. P.; Voels, S. A.; Haser, S.; Pauldrach, A.;
Puls, J.; Butler, K.
1991ApJ...377L..29H Altcode:
GHRS and optical (ESO 3.6 m) observations of the O3f star Melnick 42 in
the 30 Doradus complex are reported. A first analysis reveals that with
a luminosity of 2.3 x 10^6^ L_sun_ and a present mass of 100 M_sun_,
Melnick 42 is one of the most luminous and massive stars known. An
estimate of abundances indicates that iron and oxygen are very likely
reduced by a factor of 4 relative to the Sun, whereas carbon is more
strongly depleted and nitrogen is approximately solar. The terminal
velocity of the stellar wind is 3000 km s^-1^. The mass-loss rate is 4
x 10^-6^ M_sun_ yr^-1^, with a large uncertainty. The excellent quality
GHRS spectrum taken in a crowded region of the LMC demonstrates the
superiority of the HST for quantitative ultraviolet spectroscopy of
hot stars in other galaxies.

---------------------------------------------------------
Title: Non--LTE Analysis of the Ofpe/WN9 Star HDE 269227 (R84)
Authors: Schmutz, Werner; Leitherer, Claus; Hubeny, Ivan; Vogel,
Manfred; Hamann, Wolf-Rainer; Wessolowski, Ulf
1991ApJ...372..664S Altcode:
The paper presents the results of a spectral analysis of the Ofpe/WN9
star HD 269227 (R84), which assumes a spherically expanding atmosphere
to find solutions for equations of radiative transfer. The spectra of
hydrogen and helium were predicted with a non-LTE model. Six stellar
parameters were determined for R84. The shape of the velocity law is
empirically found, since it can be probed from the terminal velocity
of the wind. The six stellar parameters are further employed in a
hydrodynamic model where stellar wind is assumed to be directed by
radiation pressure, duplicating the mass-loss rate and the terminal
wind velocity. The velocity laws found by computation and analysis
are found to agree, supporting the theory of radiation-driven stellar
wind. R84 is surmised to be a post-red supergiant which lost half of
its initial mass, possibly during the red-supergiant phase. This mass
loss is also suggested by its spectroscopic similarity to S Doradus.

---------------------------------------------------------
Title: Stellar atmospheres. Beyond classical models
Authors: Crivellari, Lucio; Hubeny, I.; Hummer, D. G.
1991ASIC..341.....C Altcode: 1991QB809.N38......; 1991sabc.conf.....C
No abstract at ADS

---------------------------------------------------------
Title: Model Atmospheres for Accretion Disks.
Authors: Hubeny, I.
1991sepa.conf..227H Altcode: 1991IAUCo.129..227H
No abstract at ADS

---------------------------------------------------------
Title: Computer Codes for Stellar Atmospheric Modeling
Authors: Hummer, D. G.; Hubeny, I.
1991ASIC..341..119H Altcode: 1991sabc.conf..119H
No abstract at ADS

---------------------------------------------------------
Title: Stringent Limits on the Ionized Mass Loss from A and F Dwarfs
Authors: Brown, Alexander; Veale, Anthony; Judge, Philip; Bookbinder,
Jay A.; Hubeny, Ivan
1990ApJ...361..220B Altcode:
Following the suggestion of Willson et al. (1987) that A- and
F-type main-sequence stars might undergo significant mass loss due to
pulsationally driven winds, upper limits to the ionized mass loss from A
and F dwarfs have been obtained using VLA observations. These stringent
upper limits show that the level of ionized mass loss would have at
most only a small effect on stellar evolution. Radiative-equilibrium
atmospheric and wind models for early A dwarfs indicate that it is
highly likely that a wind flowing from such stars would be significantly
ionized. In addition, late A and early F dwarfs exhibit chromospheric
emission indicative of significant nonradiative heating. The present
mass-loss limits are thus representative of the total mass-loss rates
for these stars. It is concluded that A and F dwarfs are not losing
sufficient mass to cause A dwarfs to evolve into G dwarfs.

---------------------------------------------------------
Title: Properties of the central star of NGC 6826
Authors: Heap, S.; Altner, B.; Hubeny, I.; Bohannan, B.
1990ESASP.310..397H Altcode: 1990eaia.conf..397H
No abstract at ADS

---------------------------------------------------------
Title: Vertical Structure of Accretion Disks: A Simplified Analytical
Model
Authors: Hubeny, I.
1990ApJ...351..632H Altcode:
A simplified model of the vertical structure of accretion disks
is derived. Analytical expressions for the temperature and density
structure, which represent a generalization of the gray model long known
in the theory of classical stellar atmospheres, are presented. The
formalism naturally explains similarities and differences between
the structure of a disk and a stellar atmosphere. In particular, the
influence of viscous dissipation and external irradiation of the disk
by the central star, as well as of the finite optical thickness of the
disk, may be easily accounted for and explained by the present model.

---------------------------------------------------------
Title: Spectroscopic Determinations of the Properties of Planetary
Nuclei
Authors: Heap, S. R.; Altner, B.; Hubeny, I.; Bohannan, B.
1990BAAS...22..848H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Model atmospheres of hot stars: a rescaling method.
Authors: Hubeny, Ivan
1990ASPC....7...93H Altcode: 1990phls.work...93H
A simple method is suggested that overcomes convergence difficulties
encountered in calculating non-LTE model atmospheres of hot stars. The
method is based on a rescaling of the radiative as well as heating
/ cooling rates by removing analytically the overwhelming number of
scatterings and retaining only terms of the order of thermal absorptions
and emissions.

---------------------------------------------------------
Title: Mass Loss Upper Limits for a and F Dwarfs
Authors: Brown, A.; Veale, A.; Judge, P.; Bookbinder, J.; Hubeny, I.
1990ASPC....9..183B Altcode: 1990csss....6..183B
The upper limits of the ionized mass losses of A- and F-type main
sequence stars are obtained with the VLA to investigate the theory that
pulsationally driven winds contribute to substantial mass loss in the
stars. The upper limits are found to be at least one order of magnitude
lower than the mass-loss loci proposed by Willson et al. (1987). Because
any wind flowing from the stars should be detectable, the notion that
A dwarfs are evolving into G dwarfs cannot be supported by the amount
of mass that A and F dwarfs are shown to be losing.

---------------------------------------------------------
Title: Approximate Formulation of Redistribution in the LY alpha ,
LY beta , H alpha System
Authors: Cooper, J.; Ballagh, R. J.; Hubeny, I.
1989ApJ...344..949C Altcode:
Simple approximate formulas are given for the coupled redistribution of
Ly-alpha, Ly-beta, and H-alpha, by using well-defined approximations to
an essentially exact formulation. These formulas incorporate all the
essential physics including Raman scattering, lower state radiative
decay, and correlated terms representing emission during a collision
which must be retained in order that the emission coefficients are
properly behaved in the line wings. Approximate expressions for
the appropriate line broadening parameters are collected. Finally,
practical expressions for the source functions are given. These are
formulated through newly introduced nonimpact redistribution functions,
which are shown to be reasonably approximated by existing (ordinary
and generalized) redistribution functions.

---------------------------------------------------------
Title: Theoretical Modelling of Algol Disks
Authors: Hubeny, I.
1989SSRv...50..117H Altcode: 1989IAUCo.107..117H
A brief review of various theoretical approaches to model accretion
disks is presented. Emphasis is given to models that determine
self-consistently the structure of a disk together with the radiation
field. It is argued that a proper treatment of the vertical structure
is essential for calculating theoretical spectra to be compared
with observations. In particular, it is shown that hot layers above
an accretion disk (sometimes called disk “chromospheres” or
“coronae”), whose presence is indicated by recent UV observations
of strong emission lines of highly ionized species, may be explained
using simple energy balance arguments.

---------------------------------------------------------
Title: Redistribution in astrophysically important hydrogen lines
Authors: Cooper, J.; Ballagh, R. J.; Hubeny, I.
1989STIN...8924251C Altcode:
Under typical solar chromospheric conditions for hydrogen radiators,
strong collisions due to both electrons and ions are well separated in
time, so that a binary collision theory for collisional redistribution
is applicable. However, a simple impact approximation may not be
used, but rather a unified type theory is required in which frequency
dependent line shape parameters are used to describe both impact and
quasi-static regions of the spectrum. In addition, correlated terms
which describe absorption and emission during a collision are important,
and, in fact, without correlated terms describing both transfer of
excitation and emission during the same collision unphysical predictions
(such as negative intensities) would be obtained. In this paper theory
is specifically developed for the coupled Lyman-alpha, Lyman-beta,
Hydrogen-alpha system, and equations of statistical equilibrium and
absorption and emission coefficients are given. All correlated events
are examined and emission during a collision is found to be important
in the line wings. Stimulated emission and absorption is also included
within a broadband approximation. The major approximation is to ignore
lower state interaction. It is found that for Lyman-beta Raman-coupling
with Hydrogen-alpha occurs and the overall scattering of radiation in
the line wings is mostly coherent. In contrast, for Hydrogen-alpha,
incoherent redistribution due to lower state radiative decay (which
occurs even in the absence of collisions) is found to dominate
the coherent scattering. Finally, in the Lyman series the dominant
incoherent contribution is associated with cascade transitions and
inelastic collisions between different principal quantum states.

---------------------------------------------------------
Title: On the Interpretation of Emission Wings of Balmer Lines
in Lbv's
Authors: Hubeny, I.; Leitherer, C.
1989ASSL..157..283H Altcode: 1989plbv.coll..283H; 1989IAUCo.113..283H
No abstract at ADS

---------------------------------------------------------
Title: Influence of Radiative Transfer on the Vertical Structure of
Accretion Disks
Authors: Hubeny, I.
1989ASIC..290..445H Altcode: 1989tad..conf..445H
No abstract at ADS

---------------------------------------------------------
Title: On the Interpretation of Emission Wings of Balmer Lines in
Luminous Blue Variables
Authors: Hubeny, Ivan; Leitherer, Claus
1989PASP..101..114H Altcode:
H-alpha line profiles calculated with plane-parallel, hydrostatic
non-LTE model atmospheres are discussed. In the lowest log g models
the profiles show extended emission wings. Qualitatively, these wings
are similar to the extended wings generated by electron scattering of
line photons in the stellar wind. It is proposed that the line wings
observed in luminous blue variables may be due to a combination of the
non-LTE effect discussed here and the traditional scattering mechanism.

---------------------------------------------------------
Title: Radiation transfer with partial frequency redistribution and
generalized redistribution functions.
Authors: Hubeny, I.
1989fapi.conf..242H Altcode:
The author attempted to analyse the available astrophysical partial
redistribution studies. He introduced the term quasi-Markovian,
classical view, where the basic physical concepts of the current
astrophysical approach are summarized. Its physical uncertainties, and
even inconsistencies, are discussed in detail. The quasi-Markovian,
classical treatment has been used to generalize the Oxenius
(1965) approach. The reformulation of the Oxenius' approach, in the
two-level-atom case, to a form similar to that of Milkey and Mihalas
(1973) and Heasley and Kneer (1976) showed, that both formulations yield
almost identical results. Using the same approach as in reformulating
the two-level-atom case, the author derived a suitable form of the
emission coefficient in the case of the multilevel atom. Comparing
its form to that following from a heuristic derivation, two points
appeared to be different.

---------------------------------------------------------
Title: A computer program for calculating non-LTE model stellar
atmospheres
Authors: Hubeny, I.
1988CoPhC..52..103H Altcode:
The program calculates model stellar atmospheres, assuming
plane-parallel, horizontally homogeneous atmosphere in radiative
and hydrostatic equilibrium, and allowing for departures from local
thermodynamic equilibrium (LTE) for a set of occupation numbers of
selected atomic and ionic energy levels. The program is very flexible
as to the choice of chemical.

---------------------------------------------------------
Title: Scattering of Polarized Light in Spectral Lines with Partial
Frequency Redistribution: General Redistribution Matrix
Authors: Domke, H.; Hubeny, I.
1988ApJ...334..527D Altcode:
The redistribution matrix for resonance scattering of arbitrarily
polarized light, described by a vector of Stokes parameters, is derived
assuming that the ground state is isotropic. When specified in the
atomic rest frame, the redistribution matrix is found to be composed
of several terms with individually separate frequency and angular
dependence. The laboratory frame redistribution matrix exhibits an
analogous structure, but the angular and frequency dependences are
intermingled. Two possibilities for treating the angular dependence
in practical applications are considered: an expansion in a series of
Legendre polynomials and an azimuthal expansion. Finally, the concept
of aximuthally average redistribution matrix is examined, and explicit
expressions for resonance lines are given.

---------------------------------------------------------
Title: Non-LTE Effects in the Atmospheres of F-Type Supergiants -
Part Two - Analysis of NAI Lines (The Method of Computations)
Authors: Boyarchuk, A. A.; Hubeny, I.; Kubat, J.; Lyubimkov, L. S.;
Sakhibullin, N. A.
1988Afz....28..335B Altcode:
The method of non-LTE computations of Na I lines is described. The
calculations were aimed to reveal a probable connection between the
observed sodium overabundance in F-K supergiants and departures from LTE
in Na I lines is used for the analysis. Special attention is given to
subordinate lines, therefore the accepted model of Na I atom includes
19 levels instead of 7-9 levels as in the previous investigations
[3-6]. A list of stars for which the lines are computed compiles six
F-supergiants and three dwarfs (table 1). The data on the studied Na
I lines are adduced (table 2).

---------------------------------------------------------
Title: Non-LTE effects in the atmospheres of F-type
supergiants. III. Analysis of NA I lines (results of computations).
Authors: Boyarchuk, A. A.; Hubeny, I.; Kubat, J.; Lyubimkov, L. S.;
Sakhibullin, N. A.
1988Afz....28..343B Altcode: 1988Afz....28..342B
Non-LTE populations of the Na I atomic levels are calculated for six
F-supergiants and three dwarfs. The profiles and equivalent widths
W_lambda of some Na I lines of interest are computed. It is shown
thatordinary changes in widths W_lambda of subrodinate lines relative
to LTE are not more than 10% and they do not lead to appreciable
correction in the sodium abundance (corrections are less than 0.1
dex). Only for most massive supergiants with log g ~ 0 the derived
abundance might be reduced by about 0.2 dex. The Na overabundance found
for yellow supergiants from LTE analysis is confirmed. The correlation
between the Na excess and log g obtained earlier is confirmed, too;
probably it is a result of the analogous correlation between the Na
excess and masses of supergiants (fig.3). It is noticed that the modern
calculations confirm the supposition that the source of enhanced sodium
abundance is the NeNa-cycle.

---------------------------------------------------------
Title: NLTE Analysis of the Luminous Blue Variable R71
Authors: Leitherer, C.; Schmutz, W.; Hubeny, I.
1988BAAS...20.1012L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Overabundance of Sodium in the Atmospheres of Massive
Supergiants as a Possible Manifestation of Nena Cycle
Authors: Boyarchuk, A. A.; Denisenkov, P. A.; Hubeny, I.; Ivanov,
V. V.; Kubat, I.; Lyubimkov, L. S.; Sakhibullin, N. A.
1988LNP...305...94B Altcode: 1988IAUCo.108...94B; 1988adse.conf...94B
No abstract at ADS

---------------------------------------------------------
Title: Probabilistic interpretation of radiative transfer. I -
The square root of epsilon law. II - Rybicki equation
Authors: Hubeny, I.
1987A&A...185..332H Altcode:
A simple physical explanation is given for the standard problem of line
formation theory, stating that the source function at the surface
is given by (the square root of epsilon)B. The present approach
assumes that the frequency averaged mean intensity of radiation
and the source function, at a given depth, are proportional to the
probability of ultimate thermalization of a photon emitted or absorbed
at this depth. In the second part, a physical explanation of the Rybcki
(1977) generalization of the square root of epsilon law is given to all
depths of an atmosphere. Various probabilistic concepts such as the
frequency-dependent and angle-dependent thermalization probabilities
are introduced.

---------------------------------------------------------
Title: Probabilistic Interpretation of Radiative Transfer - Part Two -
Rybicki Equation
Authors: Hubeny, I.
1987A&A...185..336H Altcode:
By extending the approach developed in the previous part of this paper,
we present a physical explanation of the Rybicki generalization of
the √ ɛ-law to all depths of an atmosphere. To this end, various
probabilistic concepts, in particular the frequency- and angle-dependent
thermalization probabilities, are introduced, and relations between
them and the specific intensity of radiation are studied in detail.

---------------------------------------------------------
Title: Absorption and emission line profile coefficients of multilevel
atoms - IV. Velocity-averaged generalized redistribution functions
for three-photon processes.
Authors: Hubeny, I.; Oxenius, J.
1987JQSRT..37..397H Altcode:
The atomic generalized redistribution functions for three-photon
processes, derived in the previous paper of this series, are formulated
in terms of linear superpositions of newly introduced auxiliary
functions qI - qVI, thus extending the traditional
formalism of redistribution functions for two-photon processes. The
corresponding velocity-averaged laboratory functions QI -
QVI of these auxiliary functions are derived in both their
angle-dependent and angle-averaged forms. Since the expressions found
for QI - QVI are quite complicated, the so-called
disentangled approximation is employed that uses the representative
values of the generalized redistribution function at an orthogonal
triad of photon directions rather than the angle-averaged function
itself. This approximation yields relatively simple expressions which
can be used in radiative transfer calculations.

---------------------------------------------------------
Title: Absorption and emission line profile coefficients of
multilevel atoms - III. Generalized atomic redistribution functions
for three-photon processes.
Authors: Hubeny, I.; Oxenius, J.
1987JQSRT..37...65H Altcode:
A previously published semiclassical formulation of the atomic line
profile coefficients in terms of generalized redistribution functions
is made more explicit by deriving atomic redistribution functions for
three-photon processes. Quantum mechanical calculations are carried
out for a general three-photon process corresponding to the atomic
transition sequence i → j → k → f, for nondegenerate states
i, j, k, f and within the framework of the impact approximation. For
comparison, analogous calculations are also performed using the substate
(Weisskopf-Woolley) picture. It is found that the quantum mechanical
and the substate forms of generalized redistribution functions agree
in the limit of no collisions, whereas in the presence of collisions
they agree only if level interference phenomena of the collisional
line broadening are negligible. The physical assumptions underlying
the substate picture are discussed in detail.

---------------------------------------------------------
Title: How Strong is the Evidence of Superionization and Large Mass
Outflows in B/Be Stars? 2. The C IV and SI IV Lines in the UV Spectra
of V 767 Cen, ο And, Θ CrB, λ Eri, and 59 CYG
Authors: Hubeny, I.; Harmanec, P.; Stefl, S.
1986BAICz..37..370H Altcode:
The theoretical spectra in the neighborhood of the C IV and Si IV
resonance lines have been compared with the published observed spectra
of five Be stars, V 767 Cen, omicron And, Theta CrB, Lambda Eri,
and 59 Cyg. It is shown that the published determinations of large
outflow velocities evidenced by a violet asymmetry of the C IV and
Si IV lines, as well as the reports of superionization based on the
observations of the C IV and Si IV lines, should be reconsidered in
view of considerable line blending effects. In general, it is suggested
that many of the puzzling features observed in the UV spectra of B/Be
stars may be explained in terms of heavy blending of lines originating
in the photospheres and/or subionized, slowly rotating envelopes of
the underlying stars.

---------------------------------------------------------
Title: Redistribution of Radiation in the Presence of
Velocity-changing Collisions
Authors: Hubeny, I.; Cooper, J.
1986ApJ...305..852H Altcode:
Starting with the recent work of Cooper et al., a suitable form
of the normalized redistribution function relevant for a treatment
of redistribution in the presence of velocity-changing collisions
is presented. Attention is devoted to provide practical estimates
of the ratio between the usual line-broadening parameters and the
velocity-changing rate. It is shown that for virtually all cases of
astrophysical interest the effect of velocity-changing collisions is
quite negligible.

---------------------------------------------------------
Title: Models and theoretical spectra of accretion discs in dwarf
novae.
Authors: Kriz, S.; Hubeny, I.
1986BAICz..37..129K Altcode:
Attention is given to a new technique for modelling dwarf novae
accretion disks which permits the consistent determination of
disk structure together with the radiation field. A cylindrically
symmetric disk is divided into a set of concentric rings, each of them
behaving independently, like a plane-parallel radiating slab. The
vertical structure of each ring is determined by the joint solution
of hydrostatic equilibrium, energy balance and radiative transfer
equations. Numerical computations were carried out for a stationary
disk with a central star having a mass equal to 1 solar mass and a
radius equal to 5 x 10 to the 8th cm; the mass flux through the disk
is taken to be 10 to the -8th, 10 to the -10th, and 10 to the -11th
the solar mass per yr. Comparison of the theoretical radiative flux
distribution with the observed distribution of the white dwarf WX Hyi
reveals that the quiescent state of Wx Hyi corresponds to a mass flux
of about 10 to the -11th the solar mass per yr.

---------------------------------------------------------
Title: Comments on "The angle-dependent redistribution functions
RIII and RIV" by S. J. McKenna.
Authors: Hubeny, I.; Heinzel, P.
1986Ap&SS.119..409H Altcode:
It is shown that the complicated form of the line absorption probability
function for scattering in subordinate lines, derived by McKenna (1984),
is an artifact of adopting a mathematically inconvenient expression
for the corresponding redistribution function. It is demonstrated that
the absorption probability must be given by the Voigt function.

---------------------------------------------------------
Title: Model Atmospheres and Radiative Transfer in Chemically Peculiar
Stars: Interpretational Significance of Non-Lte (invited Review)
Authors: Hubeny, I.
1986ASSL..125...57H Altcode: 1986umss.conf...57H; 1986umss.proc...57H; 1986IAUCo..90...57H
Local-thermodynamic equilibrium (LTE) and more general non-TEL (NLTE)
approaches are investigated in the study of chemically peculiar (CP)
stars, and it is found that while LTE model atmospheres appear to be
a satisfactory diagnostic tool for continua of A and late B stars,
more detailed observations and/or the far UV region require an NLTE
treatment. Detailed comparison of the observed and predicted profiles of
resonance lines, in addition to an account for partial redistribution
effects through calculation of the NLTE theoretical profiles, are
suggested for accurate abundance determinations. Line blanketing is
also considered, and it is found that some important lines and continua
must be treated in NLTE, while others may be treated in LTE.

---------------------------------------------------------
Title: Methods in radiative transfer
Authors: Hubený, Ivan
1985Ap&SS.115..199H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Methods in Radiative Transfer
Authors: Kalkofen, W.; Hubeny, I.
1985Ap&SS.115..199K Altcode:
No abstract at ADS

---------------------------------------------------------
Title: How Strong is the Evidence of Superionization and Large Mass
Outflows in B/Be Stars?
Authors: Hubeny, I.; Stefl, S.; Harmanec, P.
1985BAICz..36..214H Altcode:
Consideration is given to the spectroscopic diagnostics of
superionization and large mass outflows in B stars according to the UV
resonance lines of C IV, SiIV, and N V. The effects of line blending in
the vicinity of these lines were estimated by means of a theoretical
spectral grid which was computed for main sequence and supergiant
model atmospheres in the range of effective temperatures 8000-40,000
K. It is argued that the supergiant spectra may simulate the spectra
of Be envelopes well enough to serve as a model. The results are used
to demonstrate the observability of shell lines in the Be envelope
in the ultraviolet spectrum, even when they are not observable in
optical spectra.

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Title: A modified Rybicki method and the partial coherent scattering
approximation
Authors: Hubeny, I.
1985A&A...145..461H Altcode:
It is the purpose of the paper to demonstrate that it is possible
to combine the advantages of the partial coherent scattering (PCS)
approximation with the favorable numerical properties of the Rybicki
method. To accomplish this goal, the author carries out an analysis
which indicates the way of accounting for Doppler diffusion within the
framework of the PCS approximation. The essence of the present method
consists in considering the frequency that sets the boundary between
the complete redistribution core region and the coherent wing region to
be depth-dependent. From the mathematical point of view, the present
modification is purely at the computational level and thus retains
the ease of formulation and programming of the original Rybicki method.

---------------------------------------------------------
Title: Non-LTE line transfer with partial redistribution. II -
an equivalent-two-level-atom approach
Authors: Hubeny, I.
1985BAICz..36....1H Altcode:
A formulation of equations for radiative transfer in a gas of
multilevel atoms, taking into account recent developments of theoretical
description, is presented. It is shown that for a simple case where one
chosen transition is allowed to depart from complete redistribution,
the global multilevel problem may be solved by suitably modified
complete-redistribution numerical techniques. In particular, the author
has formulated a modification of the equivalent-two-level-atom approach
that enables a multilevel transfer to be solved by a simple iteration
scheme. Various approximate forms of the line source function are
also discussed.

---------------------------------------------------------
Title: General aspects of partial redistribution and its astrophysical
importance.
Authors: Hubeny, I.
1985ASIC..152...27H Altcode: 1985pssl.proc...27H
A review is given of new developments in the theory of partial
redistribution in radiative transfer problems. Emphasis is on the
transfer of unpolarized radiation in plane parallel static media,
but effects of velocity fields and of geometrical structure are also
briefly discussed. Applications to solar and stellar spectral line
formation are outlined.

---------------------------------------------------------
Title: How strong is the evidence of superionization and large mass
outflowsin B/Be stars?
Authors: Hubeny, I.; Stefl, S.; Harmanec, P.
1985BIEBe..11....9H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Partial redistribution interlocking in the solar chromosphere.
Authors: Heinzel, P.; Hubeny, I.
1985ASIC..152..137H Altcode: 1985pssl.proc..137H
Starting with the model of a quiet solar chromosphere, the authors
have calculated the relative probabilities of radiative and natural
population of the second and third hydrogen levels, pertinent to
various population processes. The analysis indicates that, while the Lα
line is formed by resonance scattering between the first two levels,
the third hydrogen level, from which Lβ and Hα are generated,
is populated partly by direct photoexcitation 1→3 (about 55%),
and partly by two-photon absorption 1→2→3 (about 45%).

---------------------------------------------------------
Title: A modified Rybicki method with partial redistribution.
Authors: Hubený, I.
1985ASIC..152..101F Altcode: 1985pssl.proc..101F; 1985pssl.proc..109H
The mathematical expression of the emission profile Ψν
is given in the case of a two-level atom plus continuum: the result
is that Ψν is independent of populations and abundances,
depending only on Te, Ne and Jν.

---------------------------------------------------------
Title: A modified Rybicki method with partial redistribution.
Authors: Hubeny, I.
1985ASIC..152..109H Altcode:
A new approximate numerical method is presented that retains the basic
computational advantages of the Rybicki method while still being
capable of handling partial redistribution transfer problems. The
crucial point of this method is to consider the frequency which
separates the complete redistribution core region from the coherent
wing region, to be depth-dependent. The present method yields excellent
agreement with exact calculations and gives much better results than
any depth-independent version of the partial coherent scattering
approximation.

---------------------------------------------------------
Title: Non-coherent scattering in subordinate lines - V. Solutions
of the transfer problem.
Authors: Hubeny, I.; Heinzel, P.
1984JQSRT..32..159H Altcode:
Redistribution functions are defined for resonance scattering and shown
to be of use in astrophysics problems. An isotropic approximation is
included in a definition of a two-level atom line source function and
the redistribution function is angle-averaged for scattered photons,
which are treated as negative absorption. Various redistribution
functions, either partial or complete, are then applied to slab,
isothermal, finite, and semi-infinite atmospheres for a given absorption
profile and the line source function to examine the resulting transfer
functions. The ratios of the upper/lower level damping are found to
have a significant impact on the type of redistribution expected. An
approximate form of the redistribution function is determined as
adequate for calculating line profiles, especially in the wings.

---------------------------------------------------------
Title: The far-ultraviolet energy distribution of Sirius B from
Voyager 2.
Authors: Holberg, J. B.; Wesemael, F.; Hubeny, I.
1984ApJ...280..679H Altcode:
Observations of Sirius obtained with the Voyager 2 ultraviolet
spectrometer clearly reveal the presence of flux from the white dwarf
Sirius B at wavelengths between 950 and 1100 A. These observations are
in good agreement with all previous ultraviolet observations of Sirius
B, and in particular with the IUE observations of Boehm-Vitense,
Dettmann, and Kapranidis. A joint analysis of the Voyager 2 and
IUE observations yields a temperature range of 26,000-28,000 K. A
reexamination of current ultraviolet, visible, and X-ray observations
produces good general agreement, but no single, mutually consistent,
temperature for Sirius B. The Voyager 2 observations can be used to
place a firm upper limit of 28,000 K on the temperature of Sirius B.

---------------------------------------------------------
Title: On the line profile coefficient for stimulated emission
Authors: Cooper, J.; Hubeny, I.; Oxenius, J.
1983A&A...127..224C Altcode:
It is pointed out that the line profile coefficients for spontaneous and
stimulated emission are identical in low-intensity radiation fields. In
more intense radiation fields, however, the stimulated emission profiles
in the radiative transfer equation and in the rate equations for the
atomic level populations may differ from each other, owing to their
different physical nature. A seeming discrepancy between the stimulated
emission profiles of the usual semi-classical approach and a recent
quantum mechanical approach by Cooper et al. (1982), which should
also be valid for intense "broadband" fields, is discussed and shown
to have negligible consequences for low-intensity radiation fields.

---------------------------------------------------------
Title: Non-coherent scattering in subordinate lines:
IV. Angle-averaged redistribution functions.
Authors: Heinzel, P.; Hubený, I.
1983JQSRT..30...77H Altcode:
It is demonstrated that a simple Gaussian quadrature over the
scattering angles provides a sufficiently accurate and stable method
for evaluating all the angle-averaged redistribution functions
Ri(xarcmin,x) (i = I-V). The authors display graphically
the functions RII,III,V and discuss in detail the behaviour
of the newly calculated redistribution RV(xarcmin,x).

---------------------------------------------------------
Title: Voyager 2 far-ultraviolet observations of Sirius B.
Authors: Holberg, J. B.; Wesemael, F.; Hubeny, I.; Forrester, W. T.;
Barry, D. C.
1983BAAS...15..879H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Absorption and emission line profile coefficients of multilevel
atoms - II. Velocity-averaged profile coefficients.
Authors: Hubený, I.; Oxenius, J.; Simonneau, E.
1983JQSRT..29..495H Altcode:
Starting from the atomic profile coefficients of a multilevel atom
derived in the previous first part of this paper, the authors consider
the velocity-averaged line profile coefficients appearing in the
radiative transfer equation for the important special case that the
velocity distribution of atoms in the ground state is Maxwellian and
that the streaming of excited atoms is negligible.

---------------------------------------------------------
Title: Absorption and emission line profile coefficients of multilevel
atoms - I. Atomic profile coefficients.
Authors: Hubeny, I.; Oxenius, J.; Simonneau, E.
1983JQSRT..29..477H Altcode:
The line profile coefficients for absorption and emission appearing
in the radiative transfer equation are formulated in terms of atomic
line profile coefficients and velocity distribution functions.

---------------------------------------------------------
Title: Voyager 2 far-ultraviolet observations of Sirius B.
Authors: Holberg, J. B.; Wesemael, F.; Hubený, I.; Forrester, W. T.;
Barry, D. C.
1983BAAS...15Q.879H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Non-coherent scattering in subordinate lines: III. Generalized
redistribution functions.
Authors: Hubený, I.
1982JQSRT..27..593H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Non-coherent scattering in subordinate lines: II. Collisional
redistribution.
Authors: Heinzel, P.; Hubený, I.
1982JQSRT..27....1H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the importance of convective transport of excited atoms
in stellar atmospheres
Authors: Hubeny, I.
1981A&A...100..314H Altcode:
The question of the importance of the convective or diffusion transport
of excited atoms dur to their density gradients generated by non-LTE
line transfer, is reconsidered. It is shown that such kinetic effects
are practically never important under stellar atmospheric conditions,
contrary to the previous study of Oxenius (1979). This different
conclusion follows from the discussion of an explicit form of the
coupled set of kinetic equation for excited atoms and the radiative
transfer equation, as well as from the proper parameterization of the
physical state of stellar atmospheres.

---------------------------------------------------------
Title: Non-LTE analysis of the ultraviolet spectrum of A type
stars. II Theoretical considerations and interpretation of the VEGA
Lyman-alpha region
Authors: Hubeny, I.
1981A&A....98...96H Altcode:
Main-sequence early A-type model stellar atmospheres are
investigated. The intention is to clarify the interpretational
significance of the various assumptions used in calculating model
stellar atmospheres and the emergent radiative flux. Several non-LTE
model atmospheres in radiative and hydrostatic equilibria for early
A-type stars are constructed. The effects of departures from LTE
for early A-type stars are found to be of primary importance for
the far ultraviolet spectrum and significant in the ultraviolet
spectrum. It is also found that the properties of the computed NLTE
model atmospheres are sensitive to the assumptions of the treatment
of the far ultraviolet opacity sources. A qualitative estimate reveals
that the run of physical parameters of a realistic NLTE line-blanketed
model atmosphere is bracketed by that of the NLTE model without
line-blanketing from the one side and that of the LTE line-blanketed
model from the other side. It is noted that at present there does not
exist any theoretical model yielding a 'correct' ultraviolet flux,
particularly that below 1520 A for A stars.

---------------------------------------------------------
Title: A computer program for calculating non-LTE model stellar
atmospheres.
Authors: Hubený, I.
1981PAICz..57...32H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Radiative transfer and model atmospheres of A and Ap stars.
Authors: Hubeny, I.
1981SoSAO..32...23H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Stellar spectra and their interpretation. Proceedings of
the First Conference of the Subcommission No. 2 of the Committee
of Multilateral Cooperation of the Academies of Sciences of the
Socialist Countries "Physics and the Evolution of Stars", held at
Brno, Czechoslovakia, June 8 - 12, 1981.
Authors: Hubený, I.; Onderlička, B.
1981PAICz..57.....H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book-Review - Stellar Turbulence - I.A.U. Colloquium 51 -
London Ontario, Canada - 1979AUG27-30
Authors: Gray, D. F.; Linsky, J. L.; Hubeny, I.
1981BAICz..32..255G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Non-LTE Line Transfer with Partial Redistribution. I. General
Emission Profile
Authors: Hubený, I.
1981BAICz..32..271H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Stellar Spectra and Their Interpretation. 1st conference of
the Subcommission No. 2
Authors: Hubeny, I.; Onderlicka, B.
1981sspi.book.....H Altcode:
No abstract at ADS

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Title: Line blanketing in the Lyman-alpha wings
Authors: Hubeny, I.
1981LIACo..23..373H Altcode: 1981cpsu.conf..373H
A schematic study of the formation of lines in the Lyman-alpha wings
in A and late B stars is presented. It is demonstrated that lines
situated in the Lyman-alpha wings have very complex profiles, and in
several cases they can appear strongly in emission. fitting the observed
fluxes to those predicted by the model atmosphere calculation of Kurucz
(1979). This emission tends to increase with increasing abundance and
with decreasing distance from the Lyman-alpha center. It is tentatively
suggested that this effect can explain the anomalous brightness
variations in the Lyman-alpha wings of alpha-2 CVn and similar stars.

---------------------------------------------------------
Title: The effect of Lyman-alpha on the non-LTE model atmospheres
of A type stars
Authors: Hubeny, I.
1980A&A....86..225H Altcode:
The importance of the Ly-alpha line in the calculation of non-LTE model
atmospheres of A type stars is pointed out. It is shown that due to
the specific conditions in the atmospheres of A type stars, even very
far wings of Ly-alpha are important and produce considerable effects
upon the atmospheric structure. The importance of a proper treatment
of Stark broadening as well as redistribution in the Ly-alpha line is
briefly discussed.

---------------------------------------------------------
Title: Multidimensional radiative transfer in ultraviolet resonance
lines of the chromospheric flash spectrum
Authors: Hubený, I.
1976CoSka...6..383H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Multidimensional radiative transfer in ultraviolet resonance
lines of the chromospheric flash spectrum.
Authors: Hubený, I.
1976str..book..383H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Improved Complete-Linearization Method for the Solution of
the Non-LTE Line Transfer Problem
Authors: Hubeny, I.
1975BAICz..26...38H Altcode:
A numerical method is presented for the simultaneous solution of
the radiative transport equation and the equations of statistical
equilibrium of level populations in the investigation of non-LTE
line blanketing in the atmospheres of early-type stars. This method
allows integrals over frequencies in radiation rates to be replaced
by quadrature sums of a significantly lower order than previously
used. Errors in radiation rates are estimated explicitly by introducing
an error matrix. The obtained estimates of the matrix elements are
used for calculating corrections to the radiation field during each
iteration of the complete linearization procedure. The basic matrix size
for this procedures significantly reduced, and more atomic transitions
can be treated explicitly.

---------------------------------------------------------
Title: Diagnostic methods in astrophysics.
Authors: Hekela, J.; Hubeny, I.
1974CCpFS..24..477H Altcode:
Two basic ways of the spectroscopic diagnostics of the astrophysical
plasma are briefly described. In the synthetic approach the problems
are illustrated namely on the stellar atmospheres theory. In the
analytical approach we deal especially with the methods of solution
and the problems of mathematical stability.

---------------------------------------------------------
Title: Optically thick lines in an expanding medium - synthetic
approach formulation.
Authors: Hubený, I.
1973saa..conf...65H Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Spatial spectroscopic diagnostic of planetary
nebulae. III. Numerical investigation of local absolute monochromatic
energies and local absolute energies in spherically symmetric models
Authors: Hekela, J.; Hubený, I.
1972BAICz..23..331H Altcode:
No abstract at ADS