Author name code: hubeny ADS astronomy entries on 2022-09-14 author:"Hubeny, Ivan" ------------------------------------------------------------------------ Title: Properties of Primitive Galaxies Authors: Heap, Sara R.; Hubeny, I.; Bouret, J. -C.; Lanz, T.; Brinchmann, J. Bibcode: 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. Title: How Do Spitzer IRAC Fluxes Compare to HST CALSPEC? Authors: Bohlin, Ralph C.; Krick, Jessica E.; Gordon, Karl D.; Hubeny, Ivan Bibcode: 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. Title: WD 1145+017: Alternative models of the atmosphere, dust clouds, and gas rings Authors: Budaj, J.; Maliuk, A.; Hubeny, I. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: An improved model for the spectra of discs of nova-like variables Authors: Hubeny, Ivan; Long, Knox S. Bibcode: 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. Title: TLUSTY and SYNSPEC Users's Guide IV: Upgraded Versions 208 and 54 Authors: Hubeny, Ivan; Allende Prieto, Carlos; Osorio, Yeisson; Lanz, Thierry Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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). 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 Bibcode: 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. 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 Bibcode: 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. Title: NLTE for APOGEE: simultaneous multi-element NLTE radiative transfer Authors: Osorio, Y.; Allende Prieto, C.; Hubeny, I.; Mészáros, Sz.; Shetrone, M. Bibcode: 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. Title: Supernovae Chemical Yields in Magellanic Cloud Environments Authors: Peters, G. J.; Lanz, T.; Bouret, J.; Proffitt, C. R.; Adelman, S. J.; Hubeny, I. Bibcode: 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, & 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&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 Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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 > 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 & 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 2019ApJS..241...20N Altcode: 2018arXiv181112534N We have established a network of 19 faint (16.5 mag < V < 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/618/A25 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. Bibcode: 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<= Teff<=6000 10000 128868874 f_nsc2.dat.bz2 5750<= Teff<=8000 102539424 f_nsc3.dat.bz2 7000<= Teff<=12000 77786534 f_nsc4.dat.bz2 10000<=Teff<=20000 26089034 f_nsc5.dat.bz2 20000<=Teff<=30000

1430284037 f_hnsc1.dat.bz2 3500<= Teff<=6000 100000 1159906320 f_hnsc2.dat.bz2 5750<= Teff<=8000 913976329 f_hnsc3.dat.bz2 7000<= Teff<=12000 695827559 f_hnsc4.dat.bz2 10000<=Teff<=20000 234598400 f_hnsc5.dat.bz2 20000<=Teff<=30000

2596690648 f_uhnsc1.dat.bz2 3500<= Teff<=6000 300000 2067630039 f_uhnsc2.dat.bz2 5750<= Teff<=8000 1605227763 f_uhnsc3.dat.bz2 7000<= Teff<=12000 1221970143 f_uhnsc4.dat.bz2 10000<=Teff<=20000 411096450 f_uhnsc5.dat.bz2 20000<=Teff<=30000

ns ('large' grids): [Fe/H], [alpha/Fe], log(micro), Teff, logg

30936708717 f_ns1.dat.bz2 3500<= Teff<=6000 10000 22957349531 f_ns2.dat.bz2 5750<= Teff<=8000 19640724566 f_ns3.dat.bz2 7000<= Teff<=12000 14267448478 f_ns4.dat.bz2 10000<=Teff<=20000 8622793847 f_ns5.dat.bz2 20000<=Teff<=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 Bibcode: 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, & 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 & 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 Bibcode: 2018AAS...23135417N Altcode: We have established a network of faint (16.5 < V < 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 (<Teff> ≃ 23 000 K) and exhibit much more scatter compared to those below the gap (<Teff> ≃ 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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>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 & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 http://www.aanda.org Title: Chemical enrichment and physical conditions in I Zw 18 Authors: Lebouteiller, V.; Heap, S.; Hubeny, I.; Kunth, D. Bibcode: 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 http://www.aanda.org 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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 & 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 & 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. Bibcode: 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). Title: The Hot Components of AM CVn Helium Cataclysmics Authors: Sion, Edward M.; Linnell, A. P.; Godon, P.; Hubeny, I. Bibcode: 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& 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. Bibcode: 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. Title: Hot, Massive Stars in I Zw 18 Authors: Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry; Landsman, Wayne; Lindler, Don; Malumuth, Eliot Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 >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 Bibcode: 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. Bibcode: 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. &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. &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. &nbspBy coupling new stellar evolution models to appropriate non-LTE synthetic spectra, we investigate various theoretical explanations for these stars. &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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 <~ Minit <~ 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 Bibcode: 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 <2 per cent, as candidates for spectrophotometric standards in the range 14 < g < 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 <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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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<=Teff<=30,000 K with 1000 K steps, 13 surface gravities, 1.75<=logg<=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<=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 Bibcode: 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. Bibcode: 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 <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<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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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&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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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-->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-->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 Bibcode: 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 Bibcode: 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. Bibcode: 2006cmt..conf...15H Altcode: No abstract at ADS Title: Non-LTE Line-Blanketed Model Atmospheres of B-type Stars Authors: Lanz, T.; Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 RE J0503-285 Authors: Dobbie, P. D.; Burleigh, M. R.; Levan, A. J.; Barstow, M. A.; Napiwotzki, R.; Hubeny, I. Bibcode: 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<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. Bibcode: 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. Bibcode: 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&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. Bibcode: 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&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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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&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. Title: A Study of the Near-Ultraviolet Spectrum of Vega Authors: García-Gil, Alejandro; García López, Ramón J.; Allende Prieto, Carlos; Hubeny, Ivan Bibcode: 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. Bibcode: 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. Bibcode: 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 < 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 < 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<~ 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. Bibcode: 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). 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 > 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 Bibcode: 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>~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. Bibcode: 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. Bibcode: 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. Bibcode: 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 <=M<=McritMsolar and 2.4 <=M<= 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. Bibcode: 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. Bibcode: 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. Title: X-ray Heated Accretion Discs around Stellar Mass Black Holes Authors: Hubeny, I.; Wickramasinghe, D. T. Bibcode: 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. Bibcode: 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 Bibcode: 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. Title: Evidence for Flash Mixing in He-rich sdB Stars Authors: Sweigart, Allen V.; Lanz, Thierry; Brown, Thomas M.; Hubeny, Ivan; Landsman, Wayne B. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. <J/A+A/319/867>) 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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'' (ApJS, 146, 417 [2003]) Authors: Lanz, Thierry; Hubeny, Ivan Bibcode: 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<=Teff<=55,000 K with 2500 K steps, eight surface gravities, 3.0<=logg<=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 ApJS, 146, 417 [2003].

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. Bibcode: 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. Bibcode: 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 Bibcode: 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<=Teff<=55,000 K with 2500 K steps, eight surface gravities, 3.0<=logg<=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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 2003IAUS..210P.E44B Altcode: No abstract at ADS Title: Atomic Data in Non-LTE Model Stellar Atmospheres Authors: Lanz, T.; Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 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'' (ApJ, 579, L75 [2002]) Authors: Brown, Thomas M.; Heap, Sara R.; Hubeny, Ivan; Lanz, Thierry; Lindler, Don Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 > 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<= kT<= 100 eV; 8<= log g<= 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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>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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 2001ASPC..247..351L Altcode: 2001scpp.conf..351L No abstract at ADS Title: From Escape Probabilities to Exact Radiative Transfer Authors: Hubeny, Ivan Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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 <= T eff <= 55000 K, 3.0 <= log g <= 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 (<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 Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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>=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<2.87, the He II Lyα opacity detected by STIS, τ=1.88, is significantly lower than at z>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 Bibcode: 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>~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 & 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. 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. Bibcode: 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. Title: The He II Gunn-Peterson Effect Authors: Heap, Sara R.; Hubeny, Ivan; Smette, Alain; Williger, Gerard Bibcode: 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 > 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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>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. Bibcode: 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. 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 Bibcode: 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 RE J0720-318 Authors: Dobbie, P. D.; Barstow, M. A.; Burleigh, M. R.; Hubeny, I. Bibcode: 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 RE J0720-318. 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 ~> 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 & 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 & 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. Bibcode: 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. Bibcode: 1999IAUS..193..485D Altcode: No abstract at ADS Title: Evidence for the stratification of Fe in the photosphere of G 191-B2B Authors: Barstow, M. A.; Hubeny, I.; Holberg, J. B. Bibcode: 1999ASPC..169..479B Altcode: 1999ewwd.conf..479B 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 & 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 & 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. Bibcode: 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. Bibcode: 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 >= 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<2.87, the HeII Lya opacity detected by STIS, tau=1.88, is significantly lower than at z>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 Bibcode: 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 Bibcode: 1998ApJ...509..879D Altcode: We report on a detailed spectroscopic study of 12 very massive and luminous stars (M >~ 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, & Hubeny. Low-resolution spectra (<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. Bibcode: 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 Bibcode: 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<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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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<=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. Bibcode: 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. Bibcode: 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 >~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 & 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. Bibcode: 1998AAS...192.2602L Altcode: 1998BAAS...30..850L We have used the program suite BINSYN (Linnell & 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 & 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 & Lapasset (1981,WL). WL modeled the mass gainer as a greatly flattened oblate spheroid. A standard model accretion disk representation was developed by Hubeny & 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1998ASPC..131..108H Altcode: 1998phls.conf..108H No abstract at ADS Title: Non-LTE line-blanketed model atmospheres of hot stars Authors: Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 > 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. Bibcode: 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 & 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. Bibcode: 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 >=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. Bibcode: 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 Bibcode: 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 Bibcode: 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 (<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. Bibcode: 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 --> Of --> H-rich WNL --> 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1997ASSL..214..237B Altcode: 1997whdw.work..237B No abstract at ADS Title: Stellar Atmosphers Theory: an Introduction Authors: Hubeny, I. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 1996ApJ...459..721L Altcode: A self-consistent physical model, described in an earlier paper by Linnell & 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 & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & Hubeny 1994, ApJ, 434, 738). The existing standard light curve solution is by Lacy & 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. Bibcode: 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. Bibcode: 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 > 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. Bibcode: 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 <= Mwd/Msun <= 1.21 and 1.14 >= Rwd/(10(9) cm) >= 0.38 respectively. The mass accretion rate is also varied, on the range -10.5 <= log dM/dt <= -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. Bibcode: 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 <=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 Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Title: Non-LTE Line-blanketed Model Atmospheres of Hot Stars. I. Hybrid Complete Linearization/Accelerated Lambda Iteration Method Authors: Hubeny, I.; Lanz, T. Bibcode: 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. Bibcode: 1995LNP...443...98H Altcode: 1995whdw.conf...98H No abstract at ADS Title: Hydrogene line profiles Authors: Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 1994AAS...185.8505L Altcode: 1994BAAS...26.1460L We recently reported a spectrum synthesis program for binary stars (Linnell & 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&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. Bibcode: 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. Bibcode: 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&A, 247, L25). Title: HST/FOS Spectroscopy of the Central Star of NGC 7027 Authors: Hubeny, I.; Heap, S. R.; Lanz, T. Bibcode: 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 < T_eff < 250,000 K, 5.8 < log g < 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. Bibcode: 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 < 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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 & 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & Lamers (1993, A&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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1994ASPC...56...87P Altcode: 1994ibs..conf...87P No abstract at ADS Title: Vertical Structure and Theoretical Spectra of Accretion Disks Authors: Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 1993IAUS..155...86A Altcode: No abstract at ADS Title: Spectral Diagnoses of Chromospheres and Winds in A-Type Stars Authors: Lanz, T.; Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1991BAAS...23R1381L Altcode: No abstract at ADS Title: Can A Disk Explain Beta Lyrae? Authors: Hubeny, Ivan; Plavec, Mirek J. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 1991ASIC..341.....C Altcode: 1991QB809.N38......; 1991sabc.conf.....C No abstract at ADS Title: Model Atmospheres for Accretion Disks. Authors: Hubeny, I. Bibcode: 1991sepa.conf..227H Altcode: 1991IAUCo.129..227H No abstract at ADS Title: Computer Codes for Stellar Atmospheric Modeling Authors: Hummer, D. G.; Hubeny, I. Bibcode: 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 Bibcode: 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. Bibcode: 1990ESASP.310..397H Altcode: 1990eaia.conf..397H No abstract at ADS Title: Vertical Structure of Accretion Disks: A Simplified Analytical Model Authors: Hubeny, I. Bibcode: 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. Bibcode: 1990BAAS...22..848H Altcode: No abstract at ADS Title: Model atmospheres of hot stars: a rescaling method. Authors: Hubeny, Ivan Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 1985Ap&SS.115..199H Altcode: No abstract at ADS Title: Book-Review - Methods in Radiative Transfer Authors: Kalkofen, W.; Hubeny, I. Bibcode: 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. Bibcode: 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. Title: A modified Rybicki method and the partial coherent scattering approximation Authors: Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1985BIEBe..11....9H Altcode: No abstract at ADS Title: Partial redistribution interlocking in the solar chromosphere. Authors: Heinzel, P.; Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1983BAAS...15Q.879H Altcode: No abstract at ADS Title: Non-coherent scattering in subordinate lines: III. Generalized redistribution functions. Authors: Hubený, I. Bibcode: 1982JQSRT..27..593H Altcode: No abstract at ADS Title: Non-coherent scattering in subordinate lines: II. Collisional redistribution. Authors: Heinzel, P.; Hubený, I. Bibcode: 1982JQSRT..27....1H Altcode: No abstract at ADS Title: On the importance of convective transport of excited atoms in stellar atmospheres Authors: Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 1981PAICz..57...32H Altcode: No abstract at ADS Title: Radiative transfer and model atmospheres of A and Ap stars. Authors: Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 1981BAICz..32..255G Altcode: No abstract at ADS Title: Non-LTE Line Transfer with Partial Redistribution. I. General Emission Profile Authors: Hubený, I. Bibcode: 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. Bibcode: 1981sspi.book.....H Altcode: No abstract at ADS Title: Line blanketing in the Lyman-alpha wings Authors: Hubeny, I. Bibcode: 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. Bibcode: 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. Bibcode: 1976CoSka...6..383H Altcode: No abstract at ADS Title: Multidimensional radiative transfer in ultraviolet resonance lines of the chromospheric flash spectrum. Authors: Hubený, I. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1972BAICz..23..331H Altcode: No abstract at ADS