owocki

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Author name code: owocki
ADS astronomy entries on 2022-09-14
author:"Owocki, Stanley P."

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Title: Discovery and origin of the radio emission from the multiple
stellar system KQ Vel
Authors: Leto, P.; Oskinova, L. M.; Buemi, C. S.; Shultz, M. E.;
Cavallaro, F.; Trigilio, C.; Umana, G.; Fossati, L.; Pillitteri,
I.; Krtička, J.; Ignace, R.; Bordiu, C.; Bufano, F.; Catanzaro,
G.; Cerrigone, L.; Giarrusso, M.; Ingallinera, A.; Loru, S.; Owocki,
S. P.; Postnov, K. A.; Riggi, S.; Robrade, J.; Leone, F.
2022MNRAS.515.5523L Altcode: 2022MNRAS.tmp.2067L; 2022arXiv220714075L
KQ Vel is a binary system composed of a slowly rotating magnetic Ap
star with a companion of unknown nature. In this paper, we report the
detection of its radio emission. We conducted a multifrequency radio
campaign using the ATCA interferometer (band-names: 16 cm, 4 cm,
and 15 mm). The target was detected in all bands. The most obvious
explanation for the radio emission is that it originates in the
magnetosphere of the Ap star, but this is shown unfeasible. The known
stellar parameters of the Ap star enable us to exploit the scaling
relationship for non-thermal gyro-synchrotron emission from early-type
magnetic stars. This is a general relation demonstrating how radio
emission from stars with centrifugal magnetospheres is supported by
rotation. Using KQ Vel's parameters the predicted radio luminosity is
more than five orders of magnitudes lower than the measured one. The
extremely long rotation period rules out the Ap star as the source
of the observed radio emission. Other possible explanations for the
radio emission from KQ Vel, involving its unknown companion, have been
explored. A scenario that matches the observed features (i.e. radio
luminosity and spectrum, correlation to X-rays) is a hierarchical
stellar system, where the possible companion of the magnetic star is a
close binary (possibly of RS CVn type) with at least one magnetically
active late-type star. To be compatible with the total mass of the
system, the last scenario places strong constraints on the orbital
inclination of the KQ Vel stellar system.

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Title: MOBSTER - VI. The crucial influence of rotation on the radio
magnetospheres of hot stars
Authors: Shultz, M. E.; Owocki, S. P.; ud-Doula, A.; Biswas, A.;
Bohlender, D.; Chandra, P.; Das, B.; David-Uraz, A.; Khalack, V.;
Kochukhov, O.; Landstreet, J. D.; Leto, P.; Monin, D.; Neiner, C.;
Rivinius, Th; Wade, G. A.
2022MNRAS.513.1429S Altcode: 2022arXiv220105512S; 2022MNRAS.tmp.1099S
Numerous magnetic hot stars exhibit gyrosynchrotron radio emission. The
source electrons were previously thought to be accelerated to
relativistic velocities in the current sheet formed in the middle
magnetosphere by the wind opening magnetic field lines. However, a
lack of dependence of radio luminosity on the wind power, and a strong
dependence on rotation, has recently challenged this paradigm. We have
collected all radio measurements of magnetic early-type stars available
in the literature. When constraints on the magnetic field and/or the
rotational period are not available, we have determined these using
previously unpublished spectropolarimetric and photometric data. The
result is the largest sample of magnetic stars with radio observations
that has yet been analysed: 131 stars with rotational and magnetic
constraints, of which 50 are radio-bright. We confirm an obvious
dependence of gyrosynchrotron radiation on rotation, and furthermore
find that accounting for rotation neatly separates stars with and
without detected radio emission. There is a close correlation between
H α emission strength and radio luminosity. These factors suggest that
radio emission may be explained by the same mechanism responsible for H
α emission from centrifugal magnetospheres, i.e. centrifugal breakout
(CBO), however, while the H α-emitting magnetosphere probes the cool
plasma before breakout, radio emission is a consequence of electrons
accelerated in centrifugally driven magnetic reconnection.

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Title: Centrifugal breakout reconnection as the electron acceleration
mechanism powering the radio magnetospheres of early-type stars
Authors: Owocki, S. P.; Shultz, M. E.; ud-Doula, A.; Chandra, P.;
Das, B.; Leto, P.
2022MNRAS.513.1449O Altcode: 2022arXiv220205449O; 2022MNRAS.tmp.1106O
Magnetic B-stars often exhibit circularly polarized radio emission
thought to arise from gyrosynchrotron emission by energetic electrons
trapped in the circumstellar magnetosphere. Recent empirical analyses
show that the onset and strength of the observed radio emission
scale with both the magnetic field strength and the stellar rotation
rate. This challenges the existing paradigm that the energetic electrons
are accelerated in the current sheet between opposite-polarity field
lines in the outer regions of magnetized stellar winds, which includes
no role for stellar rotation. Building on recent success in explaining
a similar rotation-field dependence of H α line emission in terms of
a model in which magnetospheric density is regulated by centrifugal
breakout (CBO), we examine here the potential role of the associated
CBO-driven magnetic reconnection in accelerating the electrons that
emit the observed gyrosynchrotron radio. We show in particular that
the theoretical scalings for energy production by CBO reconnection
match well the empirical trends for observed radio luminosity,
with a suitably small, nearly constant conversion efficiency ϵ ≍
10-8. We summarize the distinct advantages of our CBO
scalings over previous associations with an electromotive force,
and discuss the potential implications of CBO processes for X-rays
and other observed characteristics of rotating magnetic B-stars with
centrifugal magnetospheres.

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Title: VizieR Online Data Catalog: MOBSTER. VI. Radio magnetospheres
of hot stars (Shultz+, 2022)
Authors: Shultz, M. E.; Owocki, S. P.; Ud-Doula, A.; Biswas, A.;
Bohlender, D.; Chandra, P.; Das, B.; David-Uraz, A.; Khalack, V.;
Kochukhov, O.; Landstreet, J. D.; Leto, P.; Monin, D.; Neiner, C.;
Rivinius, T.; Wade, G. A.
2022yCat..75131429S Altcode:
By combining both published and unpublished radio observations,
published rotational and magnetic data, and new determinations
of magnetic models and rotational periods via space photometry and
previously unpublished high- and low-resolution spectropolarimetry, we
have conducted the largest analysis of the gyrosynchrotron emission
properties of magnetic early-type stars undertaken to date. Tables containing the stellar, rotational, and magnetic parameters
and radio luminosities, as well as tables of longitudinal magnetic
field measurements. (5 data files).

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Title: Method and new tabulations for flux-weighted line-opacity
and radiation line-force in supersonic media
Authors: Poniatowski, L. G.; Kee, N. D.; Sundqvist, J. O.; Driessen,
F. A.; Moens, N.; Owocki, S. P.; Gayley, K. G.; Decin, L.; de Koter,
A.; Sana, H.
2022arXiv220409981P Altcode:
In accelerating and supersonic media, the interaction of photons
with spectral lines can be of ultimate importance. However,
fully accounting for such line forces currently can only be done
by specialised codes in 1-D steady-state flows. More general cases
and higher dimensions require alternative approaches. We presented a
comprehensive and fast method for computing the radiation line-force
using tables of spectral line-strength distribution parameters, which
can be applied in arbitrary (multi-D, time-dependent) simulations,
including those accounting for the line-deshadowing instability, to
compute the appropriate opacities. We assumed local thermodynamic
equilibrium to compute a flux-weighted line opacity from $&gt;4$
million spectral lines. We derived the spectral line strength and
tabulated the corresponding line-distribution parameters for a
range of input densities $\rho\in[10^{-20},10^{-10}]gcm^{-3}$ and
temperatures $T\in[10^4,10^{4.7}]K$. We found that the variation of
the line distribution parameters plays an essential role in setting
the wind dynamics in our models. In our benchmark study, we also
found a good overall agreement between the O-star mass-loss rates
of our models and those derived from steady-state studies using more
detailed radiative transfer. Our models reinforce that self-consistent
variation of the line-distribution parameters is important for the
dynamics of line-driven flows. Within a well-calibrated O-star regime,
our results support the proposed methodology. In practice, utilising the
provided tables, yielded a factor $&gt;100$ speed-up in computational
time compared to specialised 1-D model-atmosphere codes of line-driven
winds, which constitutes an important step towards efficient multi-D
simulations. We conclude that our method and tables are ready to be
exploited in various radiation-hydrodynamic simulations where the line
force is important.

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Title: Electron scattering emission in the light curves of stars
with centrifugal magnetospheres
Authors: Berry, I. D.; Owocki, S. P.; Shultz, M. E.; ud-Doula, A.
2022MNRAS.511.4815B Altcode: 2022MNRAS.tmp..327B; 2022arXiv220200615B
Strongly magnetic, rapidly rotating B-type stars with relatively weak
winds form centrifugal magnetospheres (CMs), as the stellar wind becomes
magnetically confined above the Kepler co-rotation radius. Approximating
the magnetic field as a dipole tilted by an angle β with respect
to the rotation axis, the CM plasma is concentrated in clouds at and
above the Kepler radius along the intersection of the rotational and
magnetic equatorial planes. Stellar rotation can bring such clouds in
front of the stellar disc, leading to absorption of the order of 0.1
mag ($\sim 10 {{\ \rm per\ cent}}$ of continuum flux). However, some
stars with prominent CMs, such as σ Ori E, show an emission bump in
addition to absorption dips, which has been so far unexplained. We show
that emission can occur from electron scattering towards the observer
when CM clouds are projected off the stellar limb. Using the rigidly
rotating magnetosphere model, modified with a centrifugal breakout
density scaling, we present a model grid of photometric light curves
spanning parameter space in observer inclination angle i, magnetic
obliquity angle β, critical rotation fraction W, and optical depth
at the Kepler radius τK. We show that τK of
order unity can produce emission bumps of the magnitude ~0.05 seen in
σ Ori E. We discuss the implications for modelling the light curves
of CM stars, as well as future work for applying the radiative transfer
model developed here to 3D magnetohydrodynamic simulations of CMs.

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Title: Detection of an extremely strong magnetic field in the
double-degenerate binary merger product HD 144941
Authors: Shultz, M. E.; Kochukhov, O.; Labadie-Bartz, J.; David-Uraz,
A.; Owocki, S. P.
2021MNRAS.507.1283S Altcode: 2021MNRAS.tmp.1941S; 2021arXiv210711211S
HD 144941 is an extreme He (EHe) star, a rare class of subdwarf OB
star formed from the merger of two white dwarf (WD) stars. Uniquely
amongst EHe stars, its light curve has been reported to be modulated
entirely by rotation, suggesting the presence of a magnetic field. Here,
we report the first high-resolution spectropolarimetric observations
of HD 144941, in which we detect an extremely strong magnetic field
both in circular polarization (with a line-of-sight magnetic field
averaged over the stellar disc ⟨Bz⟩ ~-8 kG) and in
Zeeman splitting of spectral lines (yielding a magnetic modulus of
⟨B⟩ ~17 kG). We also report for the first time weak H α emission
consistent with an origin and a centrifugal magnetosphere. HD 144941's
atmospheric parameters could be consistent with either a subdwarf or a
main-sequence (MS) star, and its surface abundances are neither similar
to other EHe stars nor to He-strong magnetic stars. However, its H α
emission properties can only be reproduced if its mass is around 1
M⊙, indicating that it must be a post-MS object. Since
there is no indication of binarity, it is unlikely to be a stripped
star, and was therefore most likely produced in a WD merger. HD 144941
is therefore further evidence that mergers are a viable pathway for
the generation of fossil magnetic fields.

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Title: H-alpha Emission as a Diagnostic of Plasma Transport Mechanics
in Centrifugal Magnetospheres
Authors: Shultz, M. E.; Owocki, S.; Rivinius, Th.; Wade, G. A.;
Neiner, C.; Alecian, E.; Kochukhov, O.; Bohlender, D.; ud-Doula, A.;
Landstreet, J. D.; Sikora, J.; David-Uraz, A.; Petit, V.; Cerrahoğlu,
P.; Fine, R.; Henson, G.; MiMeS Collaboration; BinaMIcS Collaboration
2021mobs.confE..29S Altcode:
Approximately one quarter of magnetic early (B5-B0) B-type type stars
display Balmer line emission originating in centrifugal magnetospheres
(CMs). Indeed, the CMs of these stars are uniquely detectable in
all available magnetospheric diagnostics (radio synchrotron, NIR,
Balmer emission, UV emission, and X-rays), and therefore represent an
excellent opportunity for detailed study of magnetospheric plasmas. All
CM host-stars are young, possess strong magnetic fields, and are rapid
rotators, properties consistent with expectations from the Rigidly
Rotating Magnetosphere model and evolutionary models incorporating
magnetic braking. The nature of mass transport within CMs has been
debated, with two competing scenarios: centrifugally driven magnetic
reconnection due to overloading of the magnetic field by the stellar
wind, and leakage arising from diffusion and drift of ions across
magnetic field lines. We have conducted the first detailed study of the
Halpha emission properties of the population of CM host stars. The
results demonstrate that mass balancing must be accomplished by
breakout. We find that emission strength is independent of mass-loss
rate and can, to first order, be predicted simply by the strength of
the equatorial magnetic field at the Kepler corotation radius. However,
an apparent cutoff in the presence of emission lines at low luminosities
may be indicative of diffusion/drift mechanisms becoming dominant in
the weak-wind regime. We briefly point to potential implications of
these results for X-ray and radio synchrotron emission.

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Title: Can Magnetospheric Scattering Explain Inferred Emission in
Photometric Light Curves of σ Ori E and Magnetic Stars Observed
with TESS?
Authors: Berry, Ian; Owocki, Stanley; Shultz, Matt; ud-Doula, Asif
2021mobs.confE..43B Altcode:
σ Ori E is a prototypical magnetic B star with a rapid 1.2 day rotation
period. Two dips in brightness can be seen in its photometric light
curve, which was well fit by the Rigidly Rotating Magnetosphere (RRM)
model. This model shows that if a star is rotating rapidly enough,
then material will become trapped in the centrifugal magnetosphere
(CM). However, this model only considers absorption and as such does
not fully explain the light curve of σ Ori E. We must take emission
into account as well. To do this, we examine the possibility of electron
scattering from the CM being responsible for extra emission seen in σ
Ori E's photometric light curve. These initial results could provide
insight into explaining photometric light curves of other magnetic
stars measured by TESS.

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Title: Simulating the formation of η Carinae's surrounding nebula
through unstable triple evolution and stellar merger-induced eruption
Authors: Hirai, Ryosuke; Podsiadlowski, Philipp; Owocki, Stanley P.;
Schneider, Fabian R. N.; Smith, Nathan
2021MNRAS.503.4276H Altcode: 2021MNRAS.tmp..595H; 2020arXiv201112434H
η Carinae is an extraordinary massive star famous for its 19th
century Great Eruption and the surrounding Homunculus nebula ejected
in that event. The cause of this eruption has been the centre of
a long-standing mystery. Recent observations, including light-echo
spectra of the eruption, suggest that it most likely resulted from
a stellar merger in an unstable triple system. Here we present a
detailed set of theoretical calculations for this scenario; from the
dynamics of unstable triple systems and the mass ejection from close
binary encounters, to the mass outflow from the eruption caused by the
stellar merger and the post-merger wind phase. In our model the bipolar
post-merger wind is the primary agent for creating the Homunculus, as it
sweeps up external eruption ejecta into a thin shell. Our simulations
reproduce many of the key aspects of the shape and kinematics of both
the Homunculus nebula and its complex surrounding structure, providing
strong support for the merger-in-a-triple scenario.

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Title: Dynamically inflated wind models of classical Wolf-Rayet stars
Authors: Poniatowski, L. G.; Sundqvist, J. O.; Kee, N. D.; Owocki,
S. P.; Marchant, P.; Decin, L.; de Koter, A.; Mahy, L.; Sana, H.
2021A&A...647A.151P Altcode: 2020arXiv201205823P
Context. Vigorous mass loss in the classical Wolf-Rayet (WR)
phase is important for the late evolution and final fate of massive
stars. Aims: We develop spherically symmetric time-dependent
and steady-state hydrodynamical models of the radiation-driven wind
outflows and associated mass loss from classical WR stars. Methods: The simulations are based on combining the opacities
typically used in static stellar structure and evolution models with
a simple parametrised form for the enhanced line opacity expected
within a supersonic outflow. Results: Our simulations reveal
high mass-loss rates initiated in deep and hot, optically thick
layers around T ≈ 200 kK. The resulting velocity structure is
non-monotonic and can be separated into three phases: (i) an initial
acceleration to supersonic speeds (caused by the static opacity), (ii)
stagnation and even deceleration, and (iii) an outer region of rapid
re-acceleration (by line opacity). The characteristic structures seen
in converged steady-state simulations agree well with the outflow
properties of our time-dependent models. Conclusions: By
directly comparing our dynamic simulations to corresponding hydrostatic
models, we explicitly demonstrate that the need to invoke extra energy
transport in convectively inefficient regions of stellar structure and
evolution models, in order to prevent drastic inflation of static
WR envelopes, is merely an artefact of enforcing a hydrostatic
outer boundary. Moreover, the dynamically inflated inner regions
of our simulations provide a natural explanation for the often-found
mismatch between predicted hydrostatic WR radii and those inferred from
spectroscopy; by extrapolating a monotonic β-type velocity law from
the observable supersonic regions to the invisible hydrostatic core,
spectroscopic models likely overestimate the core radius by a factor
of a few. Finally, we contrast our simulations with alternative recent
WR wind models based on co-moving frame (CMF) radiative transfer to
compute the radiation force. Since CMF transfer currently cannot handle
non-monotonic velocity fields, the characteristic deceleration regions
found here are avoided in such simulations by invoking an ad hoc very
high degree of clumping.

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Title: Line-drag damping of Alfvén waves in radiatively driven
winds of magnetic massive stars
Authors: Driessen, F. A.; Kee, N. D.; Sundqvist, J. O.; Owocki, S. P.
2020MNRAS.499.4282D Altcode: 2020MNRAS.tmp.2979D; 2020arXiv201005650D
Line-driven stellar winds from massive (OB) stars are subject to a
strong line-deshadowing instability. Recently, spectropolarimetric
surveys have collected ample evidence that a subset of Galactic
massive stars hosts strong surface magnetic fields. We investigate
here the propagation and stability of magnetoradiative waves in such a
magnetized, line-driven wind. Our analytic, linear stability analysis
includes line-scattering from the stellar radiation, and accounts for
both radial and non-radial perturbations. We establish a bridging law
for arbitrary perturbation wavelength after which we analyse separately
the long- and short-wavelength limits. While long-wavelength radiative
and magnetic waves are found to be completely decoupled, a key result is
that short-wavelength, radially propagating Alfvén waves couple to the
scattered radiation field and are strongly damped due to the line-drag
effect. This damping of magnetic waves in a scattering-line-driven flow
could have important effects on regulating the non-linear wind dynamics,
and so might also have strong influence on observational diagnostics
of the wind structure and clumping of magnetic line-driven winds.

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Title: The magnetic early B-type stars - IV. Breakout or leakage? H
α emission as a diagnostic of plasma transport in centrifugal
magnetospheres
Authors: Shultz, M. E.; Owocki, S.; Rivinius, Th; Wade, G. A.;
Neiner, C.; Alecian, E.; Kochukhov, O.; Bohlender, D.; ud-Doula, A.;
Landstreet, J. D.; Sikora, J.; David-Uraz, A.; Petit, V.; Cerrahoğlu,
P.; Fine, R.; Henson, G.; Henson, G.; MiMeS Collaboratio; BinaMIcS
Collaboration
2020MNRAS.499.5379S Altcode: 2020MNRAS.tmp.3072S; 2020arXiv200912336S
Rapidly rotating early-type stars with strong magnetic fields
frequently show H α emission originating in centrifugal magnetospheres
(CMs), circumstellar structures in which centrifugal support due to
magnetically enforced corotation of the magnetically confined plasma
enables it to accumulate to high densities. It is not currently
known whether the CM plasma escapes via centrifugal breakout (CB),
or by an unidentified leakage mechanism. We have conducted the first
comprehensive examination of the H α emission properties of all stars
currently known to display CM-pattern emission. We find that the onset
of emission is dependent primarily on the area of the CM, which can
be predicted simply by the value BK of the magnetic field
at the Kepler corotation radius RK. Emission strength is
strongly sensitive to both CM area and BK. Emission onset
and strength are not dependent on effective temperature, luminosity,
or mass-loss rate. These results all favour a CB scenario; however, the
lack of intrinsic variability in any CM diagnostics indicates that CB
must be an essentially continuous process, i.e. it effectively acts as
a leakage mechanism. We also show that the emission profile shapes are
approximately scale-invariant, i.e. they are broadly similar across
a wide range of emission strengths and stellar parameters. While
the radius of maximum emission correlates closely as expected to
RK, it is always larger, contradicting models that predict
that emission should peak at RK.

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Title: How the breakout-limited mass in B-star centrifugal
magnetospheres controls their circumstellar H α emission
Authors: Owocki, Stanley P.; Shultz, Matt E.; ud-Doula, Asif;
Sundqvist, Jon O.; Townsend, Richard H. D.; Cranmer, Steven R.
2020MNRAS.499.5366O Altcode: 2020MNRAS.tmp.3094O; 2020arXiv200912359O
Strongly magnetic B-type stars with moderately rapid rotation form
'centrifugal magnetospheres' (CMs) from the magnetic trapping
of stellar wind material in a region above the Kepler co-rotation
radius. A long-standing question is whether the eventual loss of such
trapped material occurs from gradual drift and/or diffusive leakage, or
through sporadic 'centrifugal breakout' (CBO) events, wherein magnetic
tension can no longer contain the built-up mass. We argue here that
recent empirical results for Balmer-α emission from such B-star CMs
strongly favour the CBO mechanism. Most notably, the fact that the
onset of such emission depends mainly on the field strength at the
Kepler radius, and is largely independent of the stellar luminosity,
strongly disfavours any drift/diffusion process, for which the net mass
balance would depend on the luminosity-dependent wind feeding rate. In
contrast, we show that in a CBO model, the maximum confined mass in
the magnetosphere is independent of this wind feeding rate and has a
dependence on field strength and Kepler radius that naturally explains
the empirical scalings for the onset of H α emission, its associated
equivalent width, and even its line profile shapes. However, the general
lack of observed Balmer emission in late-B and A-type stars could still
be attributed to a residual level of diffusive or drift leakage that
does not allow their much weaker winds to fill their CMs to the breakout
level needed for such emission; alternatively, this might result from
a transition to a metal-ion wind that lacks the requisite hydrogen.

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Title: How Centrifugal Breakout from Magnetic B-stars Controls the
Onset of Their H\alpha Emission
Authors: Owocki, S.; Shultz, M. E.; ud-Doula, A.; Sundqvist, J.
2020pase.conf..131O Altcode:
Strongly magnetic B-type stars with moderately rapid rotation form
`centrifugal magnetospheres' (CMs), from the magnetic trapping
of stellar wind material in a region above the Kepler co-rotation
radius. (where outward centrifugal forces exceed the inward pull of
gravity). A longstanding question is whether the eventual loss of
such trapped material occurs from gradual drift/diffusive leakage,
or through sporadic 'centrifugal break out' (CBO) events, wherein
magnetic tension can no longer contain the built-up mass. We argue
here that recent empirical results for Balmer-$\alpha$ emission from
such B-star CMs strongly favor the CBO mechanism. Most notably, the
fact that the onset of such emission depends mainly on the location and
field strength at the Kepler radius, and is largely independent of the
stellar luminosity strongly disfavors any drift/diffusion process, for
which the net mass balance would depend on the luminosity-dependent wind
feeding rate. In contrast, we show that in a CBO model the equilibrium
mass in the magnetosphere is indeed independent of this wind feeding
rate, and has a dependence on field strength and Kepler radius that
naturally explains the empirical scalings for the onset of H$\alpha$
emission. its associated equivalent width, and even its line profile
shapes. However, the general lack of observed Balmer emission in
late-B and A-type stars could still be attributed to a residual level
of diffusive or drift leakage that does not allow their much weaker
winds to fill their CMs to the breakout level needed for such emission.

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Title: The effects of surface fossil magnetic fields on massive
star evolution - II. Implementation of magnetic braking in MESA and
implications for the evolution of surface rotation in OB stars
Authors: Keszthelyi, Z.; Meynet, G.; Shultz, M. E.; David-Uraz, A.;
ud-Doula, A.; Townsend, R. H. D.; Wade, G. A.; Georgy, C.; Petit,
V.; Owocki, S. P.
2020MNRAS.493..518K Altcode: 2020arXiv200106239K; 2020MNRAS.tmp..227K
The time evolution of angular momentum and surface rotation of massive
stars are strongly influenced by fossil magnetic fields via magnetic
braking. We present a new module containing a simple, comprehensive
implementation of such a field at the surface of a massive star within
the Modules for Experiments in Stellar Astrophysics (MESA) software
instrument. We test two limiting scenarios for magnetic braking:
distributing the angular momentum loss throughout the star in the
first case, and restricting the angular momentum loss to a surface
reservoir in the second case. We perform a systematic investigation
of the rotational evolution using a grid of OB star models with
surface magnetic fields (M⋆ = 5-60 M⊙,
Ω/Ωcrit = 0.2-1.0, Bp = 1-20 kG). We then employ
a representative grid of B-type star models (M⋆ = 5, 10,
15 M⊙, Ω/Ωcrit = 0.2, 0.5, 0.8, Bp
= 1, 3, 10, 30 kG) to compare to the results of a recent self-consistent
analysis of the sample of known magnetic B-type stars. We infer that
magnetic massive stars arrive at the zero-age main sequence (ZAMS)
with a range of rotation rates, rather than with one common value. In
particular, some stars are required to have close-to-critical rotation
at the ZAMS. However, magnetic braking yields surface rotation rates
converging to a common low value, making it difficult to infer the
initial rotation rates of evolved, slowly rotating stars.

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Title: On the Dynamical Requirements for Slow, Extended Acceleration
in Massive-Star Winds
Authors: Hilligoss, D.; Owocki, S.
2020AAS...23511029H Altcode:
Hot, luminous, massive stars have winds that are driven by the
line scattering of the star's radiation by minor ions. For a star
with radius , the wind velocity v at radius r is commonly fit by
a "beta velocity law", β. The original point-star CAK (Castor,
Abbott, &amp; Klein) model for line-driving gives β, corresponding
to an outward acceleration proportional to the local inward pull of
gravity. But empirical data often suggests a much more gradual, extended
acceleration, fit by approximately β = 2 or higher. We show here that
this requires an enhanced line opacity in the both the inner and outer
wind, with the minimum in-between defining a critical point that sets
the maximum allowed mass-loss rate. We then present time-dependent
hydrodynamic simulations that show the associated overloading of
the inner-wind mass flux leads to cycles of stagnation, infall and
recovery. While the resulting time-averaged velocity roughly fits the
beta velocity form, this suggests such high-beta outflows should be
intrinsically variable.

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Title: The magnetic early B-type stars - III. A main-sequence
magnetic, rotational, and magnetospheric biography
Authors: Shultz, M. E.; Wade, G. A.; Rivinius, Th; Alecian, E.; Neiner,
C.; Petit, V.; Owocki, S.; ud-Doula, A.; Kochukhov, O.; Bohlender,
D.; Keszthelyi, Z.; MiMeS Collaboration; BinaMIcS Collaboration
2019MNRAS.490..274S Altcode: 2019MNRAS.tmp.2196S; 2019arXiv190902530S
Magnetic confinement of stellar winds leads to the formation of
magnetospheres, which can be sculpted into centrifugal magnetospheres
(CMs) by rotational support of the corotating plasma. The conditions
required for the CMs of magnetic early B-type stars to yield detectable
emission in H α - the principal diagnostic of these structures -
are poorly constrained. A key reason is that no detailed study of
the magnetic and rotational evolution of this population has yet
been performed. Using newly determined rotational periods, modern
magnetic measurements, and atmospheric parameters determined via
spectroscopic modelling, we have derived fundamental parameters,
dipolar oblique rotator models, and magnetospheric parameters for 56
early B-type stars. Comparison to magnetic A- and O-type stars shows
that the range of surface magnetic field strength is essentially
constant with stellar mass, but that the unsigned surface magnetic
flux increases with mass. Both the surface magnetic dipole strength
and the total magnetic flux decrease with stellar age, with the rate of
flux decay apparently increasing with stellar mass. We find tentative
evidence that multipolar magnetic fields may decay more rapidly than
dipoles. Rotational periods increase with stellar age, as expected
for a magnetic braking scenario. Without exception, all stars with H
α emission originating in a CM are (1) rapid rotators, (2) strongly
magnetic, and (3) young, with the latter property consistent with the
observation that magnetic fields and rotation both decrease over time.

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Title: Magnetic OB[A] Stars with TESS: probing their Evolutionary
and Rotational properties (MOBSTER) - I. First-light observations
of known magnetic B and A stars
Authors: David-Uraz, A.; Neiner, C.; Sikora, J.; Bowman, D. M.; Petit,
V.; Chowdhury, S.; Handler, G.; Pergeorelis, M.; Cantiello, M.; Cohen,
D. H.; Erba, C.; Keszthelyi, Z.; Khalack, V.; Kobzar, O.; Kochukhov,
O.; Labadie-Bartz, J.; Lovekin, C. C.; MacInnis, R.; Owocki, S. P.;
Pablo, H.; Shultz, M. E.; ud-Doula, A.; Wade, G. A.
2019MNRAS.487..304D Altcode: 2019MNRAS.tmp.1133D; 2019arXiv190411539D
In this paper we introduce the MOBSTER collaboration and lay out its
scientific goals. We present first results based on the analysis of 19
previously known magnetic O, B, and A stars observed in 2-min cadence
in sectors 1 and 2 of the Transiting Exoplanet Survey Satellite (TESS)
mission. We derive precise rotational periods from the newly obtained
light curves and compare them to previously published values. We also
discuss the overall photometric phenomenology of the known magnetic
massive and intermediate-mass stars and propose an observational
strategy to augment this population by taking advantage of the
high-quality observations produced by TESS.

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Title: The crucial role of high resolution X-ray spectroscopy in
studies of massive stars and their winds
Authors: Leutenegger, Maurice; Corcoran, Michael; David-Uraz,
Alexandre; Gosset, Eric; Hamaguchi, Kenji; Huenemoerder, David;
Mossoux, Enmanuelle; Nazé, Yaël; Owocki, Stanley; Petit, Véronique;
Puls, Joachim; Rauw, Gregor; Sugawara, Yasuharu; ud-Doula, Asif
2019BAAS...51c.512L Altcode: 2019astro2020T.512L
High-resolution X-ray spectroscopy has proven to be a crucial tool
for addressing a wide range of problems relating to massive stars. New
X-ray observatories featuring high resolution spectroscopy with square
meter collecting area are required.

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Title: High Angular Resolution Astrophysics: Evolutionary Impact of
Stellar Mass Loss
Authors: Gies, Douglas; Owocki, S. P.; Ridgway, S.; ten Brummelaar, T.
2019BAAS...51c.171G Altcode: 2019astro2020T.171G
This is a discussion of how high angular resolution investigations will
explore the processes of mass ejection at unprecedented photospheric
scales.

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Title: Hydrodynamical simulations and similarity relations for
eruptive mass-loss from massive stars
Authors: Owocki, Stanley P.; Hirai, Ryosuke; Podsiadlowski, Philipp;
Schneider, Fabian R. N.
2019MNRAS.485..988O Altcode: 2019MNRAS.tmp..470O; 2019arXiv190206220O
Motivated by the eruptive mass-loss inferred from Luminous Blue Variable
(LBV) stars, we present 1D hydrodynamical simulations of the response
from sudden energy injection into the interior of a very massive (100
M_⊙) star. For a fiducial case with total energy addition set to a
factor f = 0.5 of the net stellar binding energy, and applied within
the stellar envelope, we detail the dynamical response that leads
to ejection of the outermost 7.2 M_⊙. We find that the ejecta's
variations in time t and radius r for the velocity v, density ρ, and
temperature T are quite well fit by similarity forms in the variable
r/t ≈ v. Specifically the scaled density follows a simple exponential
decline ρt3 ∼ exp (- r/vot). This `exponential
similarity' leads to analytic scaling relations for total ejecta mass
ΔM and kinetic energy ΔK that agree well with the hydrodynamical
simulations, with the specific-energy-averaged speed related to the
exponential scale speed vo through \bar{v} ≡ √{2 Δ K/Δ
M} = √{12} v_o, and a value comparable to the star's surface escape
speed, vesc. Models with energy added in the core develop
a surface shock breakout that propels an initial, higher speed ejecta
(&gt;5000 km s-1), but the bulk of the ejected material
still follows the same exponential similarity scalings with {\bar{v}}
≈ v_esc. A broader parameter study examines how the ejected mass
and energy depends on the energy-addition factor f, for three distinct
model series that locate the added energy in either the core, envelope,
or near-surface. We conclude by discussing the relevance of these
results for understanding LBV outbursts and other eruptive phenomena,
such as failed supernovae and pulsational pair instability events.

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Title: Extreme resonance line profile variations in the ultraviolet
spectra of NGC 1624-2: probing the giant magnetosphere of the most
strongly magnetized known O-type star
Authors: David-Uraz, A.; Erba, C.; Petit, V.; Fullerton, A. W.;
Martins, F.; Walborn, N. R.; MacInnis, R.; Barbá, R. H.; Cohen,
D. H.; Maíz Apellániz, J.; Nazé, Y.; Owocki, S. P.; Sundqvist,
J. O.; ud-Doula, A.; Wade, G. A.
2019MNRAS.483.2814D Altcode: 2018MNRAS.tmp.3069D; 2018arXiv181110113D
In this paper, we present high-resolution HST/COS observations
of the extreme magnetic O star NGC 1624-2. These represent the
first ultraviolet spectra of this archetypal object. We examine the
variability of its wind-sensitive resonance lines, comparing it to
that of other known magnetic O stars. In particular, the observed
variations in the profiles of the C IV and Si IV doublets between
low state and high state are the largest observed in any magnetic
O-type star, consistent with the expected properties of NGC 1624-2's
magnetosphere. We also observe a redshifted absorption component in the
low state, a feature not seen in most stars. We present preliminary
modelling efforts based on the Analytic Dynamical Magnetosphere
(ADM) formalism, demonstrating the necessity of using non-spherically
symmetric models to determine wind/magnetospheric properties of magnetic
O stars.

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Title: Line-driven ablation of circumstellar discs - III. Accounting
for and analysing the effects of continuum optical depth
Authors: Kee, Nathaniel Dylan; Owocki, Stanley; Kuiper, Rolf
2018MNRAS.479.4633K Altcode: 2018MNRAS.tmp.1637K; 2018arXiv180608753K
In studying the formation of massive stars, it is
essential to consider the strong radiative feedback on
the stars' natal environments from their high luminosities
(104-106L⊙). Given that massive stars
contract to main-sequence-like radii before accretion finishes, one
form this feedback takes is UV line-acceleration, resulting in outflows
much like those expected from main-sequence massive stars. As shown by
the prior papers in this series, in addition to driving stellar winds,
such line forces also ablate the surface layers off of circumstellar
discs within a few stellar radii of the stellar photosphere. This
removal of material from an accretion disc in turn results in a
decreased accretion rate onto the forming star. Quantifying this,
however, requires accounting for the continuum optical depth of the
disc along the non-radial rays required for the three-dimensional
line-acceleration prescription used in this paper series. We introduce
the `thin-disc approximation', allowing these continuum optical
depths arising from an optically thick but geometrically thin disc
to be dynamically treated in the context of radiation-hydrodynamics
simulations. Using this approximation in full dynamical simulations,
we show that such continuum optical depth effects only reduce the disc
ablation by 30 per cent or less relative to previous simulations that
ignored continuum absorption.

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Title: Disruption of circumstellar discs by large-scale stellar
magnetic fields
Authors: ud-Doula, Asif; Owocki, Stanley P.; Kee, Nathaniel Dylan
2018MNRAS.478.3049U Altcode: 2018MNRAS.tmp.1171U; 2018arXiv180503001U
Spectropolarimetric surveys reveal that 8-10 per cent of OBA stars
harbor large-scale magnetic fields, but thus far no such fields have
been detected in any classical Be stars. Motivated by this, we present
here magnetohydrodynamical simulations for how a pre-existing Keplerian
disc - like that inferred to form from decretion of material from
rapidly rotating Be stars - can be disrupted by a rotation-aligned
stellar dipole field. For characteristic stellar and disc parameters
of a near critically rotating B2e star, we find that a polar surface
field strength of just 10 G can significantly disrupt the disc, while
a field of 100 G, near the observational upper limit inferred for
most Be stars, completely destroys the disc over just a few days. Our
parameter study shows that the efficacy of this magnetic disruption
of a disc scales with the characteristic plasma beta (defined as
the ratio between thermal and magnetic pressure) in the disc, but is
surprisingly insensitive to other variations, e.g. in stellar rotation
speed, or the mass-loss rate of the star's radiatively driven wind. The
disc disruption seen here for even a modest field strength suggests
that the presumed formation of such Be discs by decretion of material
from the star would likely be strongly inhibited by such fields; this
provides an attractive explanation for why no large-scale fields are
detected from such Be stars.

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Title: Radiation Transport Through Super-Eddington Stellar Winds
Authors: Guzik, Joyce A.; Fryer, Chris; Urbatsch, Todd J.; Owocki,
Stanley P.
2018pas8.conf...33G Altcode: 2018arXiv180600691G
We present results of simulations to assess the feasibility of
modeling outflows from massive stars using the Los Alamos 3-D radiation
hydrodynamics code Cassio developed for inertial confinement fusion
(ICF) applications. We find that a 1-D stellar envelope simulation
relaxes into hydrostatic equilibrium using computing resources that
would make the simulation tractable in 2-D. We summarize next steps
to include more physics fidelity and model the response to a large
and abrupt energy deposition at the base of the envelope.

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Title: Hot-Star Winds: CIRs, DACs &amp; BRITE Spots
Authors: Owocki, Stanley P.
2018pas8.conf...48O Altcode:
The high luminosities of massive stars drive strong stellar winds,
through line scattering of the star's continuum radiation. After
summarizing the basic, steady-state CAK theory for wind driving by a
power-law ensemble of lines, the discussion here %reviews how bright
spots on the stellar surface can induce co-rotating interaction regions
(CIR's) %with velocity law plateaus that lead to formation of discrete
absorption components (DAC's) in UV wind lines. examines the origin
of migrating discrete absorption components (DAC's) commonly seen in
UV wind lines, with focus on the bright spot model proposed more than
20 years ago by Cranmer &amp; Owocki. Within modern constraints of
hot-star photometric variability observed by the BRITE satellites,
we present a semi-analytic analysis for the spot size and amplitude
needed to produce an overloaded wind that develops a kink transition
to a slowly decelerating velocity plateau that form the DAC.

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Title: 2D wind clumping in hot, massive stars from hydrodynamical
line-driven instability simulations using a pseudo-planar approach
Authors: Sundqvist, J. O.; Owocki, S. P.; Puls, J.
2018A&A...611A..17S Altcode: 2017arXiv171007780S
Context. Clumping in the radiation-driven winds of hot, massive
stars arises naturally due to the strong, intrinsic instability of
line-driving (the line-deshadowing instability, hereafter LDI). But
LDI wind models have so far mostly been limited to 1D, mainly because
of the severe computational challenges regarding calculation of the
multi-dimensional radiation force. Aim. In this paper we simulate
and examine the dynamics and multi-dimensional nature of wind
structure resulting from the LDI. Methods: We introduce a
pseudo-planar, box-in-a-wind method that allows us to efficiently
compute the line force in the radial and lateral directions, and
then use this approach to carry out 2D radiation-hydrodynamical
simulations of the time-dependent wind. Results: Our 2D
simulations show that the LDI first manifests itself by mimicking
the typical shell structure seen in 1D models, but that these shells
quickly break up into complex 2D density and velocity structures,
characterized by small-scale density "clumps" embedded in larger
regions of fast and rarefied gas. Key results of the simulations are
that density variations in the well-developed wind are statistically
quite isotropic and that characteristic length scales are small;
a typical clump size is ℓcl/R* 0.01 at
2R*, thus also resulting in rather low typical clump
masses mcl 1017 g. Overall, our results agree
well with the theoretical expectation that the characteristic scale
for LDI generated wind-structure is on the order of the Sobolev length
ℓSob. We further confirm some earlier results that lateral
"filling in" of radially compressed gas leads to somewhat lower
clumping factors in 2D simulations than in comparable 1D models. We
conclude by discussing an extension of our method toward rotating
LDI wind models that exhibit an intriguing combination of large-
and small-scale structures extending down to the wind base.

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Title: Characterizing the turbulent porosity of stellar wind structure
generated by the line-deshadowing instability
Authors: Owocki, Stanley P.; Sundqvist, Jon O.
2018MNRAS.475..814O Altcode: 2017arXiv171203457O
We analyse recent 2D simulations of the non-linear evolution of the
line-deshadowing instability (LDI) in hot-star winds, to quantify
how the associated highly clumped density structure can lead to
a `turbulent porosity' reduction in continuum absorption and/or
scattering. The basic method is to examine the statistical variations
of mass column as a function of path length, and fit these to analytic
forms that lead to simple statistical scalings for the associated
mean extinction. A key result is that one can characterize porosity
effects on continuum transport in terms of a single `turbulent porosity
length', found here to scale as H ≈ (fcl - 1)a, where
fcl ≡ 〈ρ2〉/〈ρ〉2 is the
clumping factor in density ρ, and a is the density autocorrelation
length. For continuum absorption or scattering in an optically thick
layer, we find the associated effective reduction in opacity scales
as ∼ 1/√{1+τ_H}, where τH ≡ κρH is the local
optical thickness of this porosity length. For these LDI simulations,
the inferred porosity lengths are small, only about a couple per cent
of the stellar radius, H ≈ 0.02R*. For continuum processes
like bound-free absorption of X-rays that are only marginally optically
thick throughout the full stellar wind, this implies τH
≪ 1, and thus that LDI-generated porosity should have little effect
on X-ray transport in such winds. The formalism developed here could
however be important for understanding the porous regulation of
continuum-driven, super-Eddington outflows from luminous blue variables.

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Title: Diffusion-plus-drift models for the mass leakage from
centrifugal magnetospheres of magnetic hot-stars
Authors: Owocki, Stanley P.; Cranmer, Steven R.
2018MNRAS.474.3090O Altcode: 2017arXiv171105414O
In the subset of luminous, early-type stars with strong, large-scale
magnetic fields and moderate to rapid rotation, material from the
star's radiatively driven stellar wind outflow becomes trapped by
closed magnetic loops, forming a centrifugally supported, corotating
magnetosphere. We present here a semi-analytic analysis of how this
quasi-steady accumulation of wind mass can be balanced by losses
associated with a combination of an outward, centrifugally driven
drift in the region beyond the Kepler co-rotation radius, and an
inward/outward diffusion near this radius. We thereby derive scaling
relations for the equilibrium spatial distribution of mass, and the
associated emission measure for observational diagnostics like Balmer
line emission. We discuss the potential application of these relations
for interpreting surveys of the emission line diagnostics for OB stars
with centrifugally supported magnetospheres. For a specific model of
turbulent field-line-wandering rooted in surface motions associated
with the iron opacity bump, we estimate values for the associated
diffusion and drift coefficients.

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Title: Line-driven ablation of circumstellar discs - II. Analysing
the role of multiple resonances
Authors: Kee, Nathaniel Dylan; Owocki, Stanley; Kuiper, Rolf
2018MNRAS.474..847K Altcode:
We extend our previous study of radiative ablation of circumstellar
discs by line-scattering of the star's radiation, accounting now for
the effect of multiple line resonances off the stellar limb. For an
analytic, three-dimensional model of the velocity structure of an
equatorial Keplerian disc bounded at higher latitudes by a radially
accelerating stellar wind outflow, we use root-finding methods to
identify multiple resonances from a near-disc circumstellar location
along starward rays both on and off the stellar core. Compared to our
previous study that accounted only for the effect of on-core resonances
in reducing the radiative driving through the scattering of radiation
away from a near-disc circumstellar location, including off-limb
resonances leads to additional radiative driving from scattering
towards this location. Instead of the up to 50 per cent reduction in
line-acceleration previously inferred from multiple resonance effects,
we now find a more modest 15-20 per cent net reduction.

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Title: On the optically thick winds of Wolf-Rayet stars
Authors: Gräfener, G.; Owocki, S. P.; Grassitelli, L.; Langer, N.
2017A&A...608A..34G Altcode: 2017arXiv171004543G
Context. The classical Wolf-Rayet (WR) phase is believed to mark the end
stage of the evolution of massive stars with initial masses higher than
25M⊙. Stars in this phase expose their stripped cores with
the products of H- or He-burning at their surface. They develop strong,
optically thick stellar winds that are important for the mechanical
and chemical feedback of massive stars, and that determine whether the
most massive stars end their lives as neutron stars or black holes. The
winds of WR stars are currently not well understood, and their inclusion
in stellar evolution models relies on uncertain empirical mass-loss
relations. Aims: We investigate theoretically the mass-loss
properties of H-free WR stars of the nitrogen sequence (WN stars). Methods: We connected stellar structure models for He stars with
wind models for optically thick winds and assessed the degree to which
these two types of models can simultaneously fulfil their respective
sonic-point conditions. Results: Fixing the outer wind law and
terminal wind velocity ν∞, we obtain unique solutions
for the mass-loss rates of optically thick, radiation-driven winds
of WR stars in the phase of core He-burning. The resulting mass-loss
relations as a function of stellar parameters agree well with previous
empirical relations. Furthermore, we encounter stellar mass limits below
which no continuous solutions exist. While these mass limits agree with
observations of WR stars in the Galaxy, they contradict observations in
the LMC. Conclusions: While our results in particular confirm the
slope of often-used empirical mass-loss relations, they imply that only
part of the observed WN population can be understood in the framework
of the standard assumptions of a smooth transonic flow and compact
stellar core. This means that alternative approaches such as a clumped
and inflated wind structure or deviations from the diffusion limit at
the sonic point may have to be invoked. Qualitatively, the existence
of mass limits for the formation of WR-type winds may be relevant for
the non-detection of low-mass WR stars in binary systems, which are
believed to be progenitors of Type Ib/c supernovae. The sonic-point
conditions derived in this work may provide a possibility to include
optically thick winds in stellar evolution models in a more physically
motivated form than in current models.

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Title: Super-Eddington stellar winds: unifying radiative-enthalpy
versus flux-driven models
Authors: Owocki, Stanley P.; Townsend, Richard H. D.; Quataert, Eliot
2017MNRAS.472.3749O Altcode: 2017arXiv170807790O
We derive semi-analytic solutions for optically thick, super-Eddington
stellar winds, induced by an assumed steady energy addition Δ
{\dot{E}} concentrated around a near-surface heating radius R in
a massive star of central luminosity L*. We show that
obtaining steady wind solutions requires both that the resulting total
luminosity L_o = L_\ast + Δ {\dot{E}} exceed the Eddington luminosity,
Γo ≡ Lo/LEdd &gt; 1, and that the
induced mass-loss rate be such that the 'photon-tiring' parameter,
m ≡ {\dot{M}} GM/R L_o ≤ 1-1/Γ _o, ensuring the luminosity is
sufficient to overcome the gravitational potential GM/R. Our analysis
unifies previous super-Eddington wind models that either: (1) assumed
a direct radiative flux-driving without accounting for the advection of
radiative enthalpy that can become important in such an optically thick
flow; or (2) assumed that such super-Eddington outflows are adiabatic,
neglecting the effects of the diffusive radiative flux. We show that
these distinct models become applicable in the asymptotic limits of
small versus large values of mΓo, respectively. By solving
the coupled differential equations for radiative diffusion and wind
momentum, we obtain general solutions that effectively bridge the
behaviours of these limiting models. Two key scaling results are for
the terminal wind speed to escape speed, which is found to vary as
v_∞^2/v_esc^2 = Γ _o/(1+m Γ _o) -1, and for the final observed
luminosity Lobs, which for all allowed steady-solutions
with m &lt; 1 - 1/Γo exceeds the Eddington luminosity,
Lobs &gt; LEdd. Our super-Eddington wind
solutions have potential applicability for modelling phases of eruptive
mass-loss from massive stars, classical novae, and the remnants of
stellar mergers.

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Title: Destruction of Be star disk by large scale magnetic fields
Authors: Ud-Doula, Asif; Owocki, Stanley; Kee, Nathaniel Dylan;
Vanyo, Michael
2017IAUS..329..453U Altcode:
Classical Be stars are rapidly rotating stars with circumstellar
disks that come and go on time scale of years. Recent observational
data strongly suggests that these stars lack the 10% incidence of
global magnetic fields observed in other main-sequence B stars. Such
an apparent lack of magnetic fields may indicate that Be disks are
fundamentally incompatible with a significant large scale magnetic
field. In this work, using numerical magnetohydrodynamics (MHD)
simulations, we show that a dipole field of only 100G can lead to
the quick disruption of a Be disk. Such a limit is in line with the
observational upper limits for these objects.

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Title: New Insights into the Puzzling P-Cygni Profiles of Magnetic
Massive Stars
Authors: Erba, Christiana; David-Uraz, Alexandre; Petit, Véronique;
Owocki, Stanley P.
2017IAUS..329..246E Altcode: 2017arXiv170208535E
Magnetic massive stars comprise approximately 10% of the total OB star
population. Modern spectropolarimetry shows these stars host strong,
stable, large-scale, often nearly dipolar surface magnetic fields of 1
kG or more. These global magnetic fields trap and deflect outflowing
stellar wind material, forming an anisotropic magnetosphere that
can be probed with wind-sensitive UV resonance lines. Recent HST UV
spectra of NGC 1624-2, the most magnetic O star observed to date, show
atypically unsaturated P-Cygni profiles in the Civ resonant doublet,
as well as a distinct variation with rotational phase. We examine the
effect of non-radial, magnetically-channeled wind outflow on P-Cygni
line formation, using a Sobolev Exact Integration (SEI) approach for
direct comparison with HST UV spectra of NGC 1624-2. We demonstrate
that the addition of a magnetic field desaturates the absorption
trough of the P-Cygni profiles, but further efforts are needed to
fully account for the observed line profile variation. Our study thus
provides a first step toward a broader understanding of how strong
magnetic fields affect mass loss diagnostics from UV lines.

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Title: Investigating the Magnetospheres of Rapidly Rotating B-type
Stars
Authors: Fletcher, C. L.; Petit, V.; Nazé, Y.; Wade, G. A.; Townsend,
R. H.; Owocki, S. P.; Cohen, D. H.; David-Uraz, A.; Shultz, M.
2017IAUS..329..369F Altcode: 2017arXiv170206500F
Recent spectropolarimetric surveys of bright, hot stars have found that
~10% of OB-type stars contain strong (mostly dipolar) surface magnetic
fields (~kG). The prominent paradigm describing the interaction between
the stellar winds and the surface magnetic field is the magnetically
confined wind shock (MCWS) model. In this model, the stellar wind
plasma is forced to move along the closed field loops of the magnetic
field, colliding at the magnetic equator, and creating a shock. As the
shocked material cools radiatively it will emit X-rays. Therefore, X-ray
spectroscopy is a key tool in detecting and characterizing the hot wind
material confined by the magnetic fields of these stars. Some B-type
stars are found to have very short rotational periods. The effects of
the rapid rotation on the X-ray production within the magnetosphere have
yet to be explored in detail. The added centrifugal force due to rapid
rotation is predicted to cause faster wind outflows along the field
lines, leading to higher shock temperatures and harder X-rays. However,
this is not observed in all rapidly rotating magnetic B-type stars. In
order to address this from a theoretical point of view, we use the
X-ray Analytical Dynamical Magnetosphere (XADM) model, originally
developed for slow rotators, with an implementation of new rapid
rotational physics. Using X-ray spectroscopy from ESA's XMM-Newton
space telescope, we observed 5 rapidly rotating B-types stars to add
to the previous list of observations. Comparing the observed X-ray
luminosity and hardness ratio to that predicted by the XADM allows
us to determine the role the added centrifugal force plays in the
magnetospheric X-ray emission of these stars.

---------------------------------------------------------
Title: Investigating the origin of cyclical wind variability in
hot massive stars - II. Hydrodynamical simulations of corotating
interaction regions using realistic spot parameters for the O giant
ξ Persei
Authors: David-Uraz, A.; Owocki, S. P.; Wade, G. A.; Sundqvist, J. O.;
Kee, N. D.
2017MNRAS.470.3672D Altcode: 2017arXiv170603647D
OB stars exhibit various types of spectral variability historically
associated with wind structures, including the apparently ubiquitous
discrete absorption components (DACs). These features have been
proposed to be caused either by magnetic fields or non-radial
pulsations. In this second paper of this series, we revisit the
canonical phenomenological hydrodynamical modelling used to explain
the formation of DACs by taking into account modern observations and
more realistic theoretical predictions. Using constraints on putative
bright spots located on the surface of the O giant ξ Persei derived
from high precision space-based broad-band optical photometry obtained
with the Microvariability and Oscillations of Stars (MOST) space
telescope, we generate 2D hydrodynamical simulations of corotating
interaction regions in its wind. We then compute synthetic ultraviolet
(UV) resonance line profiles using Sobolev Exact Integration and
compare them with historical timeseries obtained by the International
Ultraviolet Explorer (IUE) to evaluate if the observed behaviour of ξ
Persei's DACs is reproduced. Testing three different models of spot
size and strength, we find that the classical pattern of variability
can be successfully reproduced for two of them: the model with the
smallest spots yields absorption features that are incompatible with
observations. Furthermore, we test the effect of the radial dependence
of ionization levels on line driving, but cannot conclusively assess the
importance of this factor. In conclusion, this study self-consistently
links optical photometry and UV spectroscopy, paving the way to a
better understanding of cyclical wind variability in massive stars in
the context of the bright spot paradigm.

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Title: A multi-wavelength view of NGC 1624-2
Authors: David-Uraz, A.; Petit, V.; MacInnis, R.; Erba, C.; Owocki,
S. P.; Fullerton, A. W.; Walborn, N. R.; Cohen, D. H.
2017IAUS..329..394D Altcode:
Large magnetometric surveys have contributed to the detection of an
increasing number of magnetic massive stars, and to the recognition
of a population of magnetic massive stellar objects with distinct
properties. Among these, NGC 1624-2 possesses the largest magnetic
field of any O-type star; such a field confines the stellar wind into
a circumstellar magnetosphere, which can be probed using observations
at different wavelength regimes. Recent optical and X-ray observations
suggest that NGC 1624-2's magnetosphere is much larger than that of
any other magnetic O star. By modeling the variations of UV resonance
lines, we can constrain its velocity structure. Furthermore, recent
spectropolarimetric observations raise the possibility of a more
complex field topology than previously expected. Putting all of
these multi-wavelength constraints together will allow us to paint
a consistent picture of NGC 1624-2 and its surprising behavior,
giving us valuable insight into the ve! ry nature of massive
star magnetospheres.

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Title: Magnetic massive stars as progenitors of `heavy' stellar-mass
black holes
Authors: Petit, V.; Keszthelyi, Z.; MacInnis, R.; Cohen, D. H.;
Townsend, R. H. D.; Wade, G. A.; Thomas, S. L.; Owocki, S. P.; Puls,
J.; ud-Doula, A.
2017MNRAS.466.1052P Altcode: 2016arXiv161108964P
The groundbreaking detection of gravitational waves produced by the
inspiralling and coalescence of the black hole (BH) binary GW150914
confirms the existence of 'heavy' stellar-mass BHs with masses &gt;25
M⊙. Initial characterization of the system by Abbott et
al. supposes that the formation of BHs with such large masses from
the evolution of single massive stars is only feasible if the wind
mass-loss rates of the progenitors were greatly reduced relative to
the mass-loss rates of massive stars in the Galaxy, concluding that
heavy BHs must form in low-metallicity (Z ≲ 0.25-0.5 Z⊙)
environments. However, strong surface magnetic fields also provide a
powerful mechanism for modifying mass-loss and rotation of massive
stars, independent of environmental metallicity. In this paper,
we explore the hypothesis that some heavy BHs, with masses &gt;25
M⊙ such as those inferred to compose GW150914, could
be the natural end-point of evolution of magnetic massive stars in a
solar-metallicity environment. Using the MESA code, we developed a new
grid of single, non-rotating, solar-metallicity evolutionary models for
initial zero-age main sequence masses from 40 to 80 M⊙
that include, for the first time, the quenching of the mass-loss due
to a realistic dipolar surface magnetic field. The new models predict
terminal-age main-sequence (TAMS) masses that are significantly greater
than those from equivalent non-magnetic models, reducing the total
mass lost by a strongly magnetized 80 M⊙ star during
its main-sequence evolution by 20 M⊙. This corresponds
approximately to the mass-loss reduction expected from an environment
with metallicity Z = 1/30 Z⊙.

---------------------------------------------------------
Title: Mass and Angular Momentum Loss of B[e] Stars via Decretion
Disks
Authors: Krtička, J. K.; Owocki, S. P.; Kurfürst, P.
2017ASPC..508...73K Altcode:
We study the disks of B[e] stars assuming that the disks stem from the
angular momentum loss from the central object. The angular momentum
loss may be induced either by evolution of the stellar interior of
critically rotating star or by merger event in a binary. In contrast
to the usual stellar wind mass loss set by driving from the stellar
luminosity, such decretion-disk mass loss is determined by the angular
momentum loss needed to keep the central object in equilibrium. The
angular momentum loss is given either by the interior evolution and
decline in the star's moment of inertia, or by excess angular momentum
present in a merging binary. Because the specific angular momentum
in a Keplerian disk increases with the square root of the radius, the
decretion mass loss associated with a required level of angular momentum
loss depends crucially on the outer radius for viscous coupling of the
disk. The magnetorotational instability can be the source of anomalous
viscosity in decretion disks. The instability operates close to the
star and disappears in the region where the disk orbital velocity is
roughly equal to the sound speed. We study the differences between Be
and B[e] star disks and discuss the reasons why stars of the stellar
type B have disks, while other stars do not.

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Title: Destruction of Be star disk by large scale magnetic fields
Authors: Ud-Doula, Asif; Owocki, Stanley P.; Kee, Nathaniel; Vanyo,
Michael
2017AAS...22915109U Altcode:
Classical Be stars are rapidly rotating stars with circumstellar
disks that come and go on time scale of years. Recent observational
data strongly suggests that these stars lack the ~10% incidence of
global magnetic fields observed in other main-sequence B stars. Such
an apparent lack of magnetic fields may indicate that Be disks are
fundamentally incompatible with a significant large scale magnetic
field. In this work, using numerical magnetohydrodynamics (MHD)
simulations, we show that a dipole field of only 100G can lead to
the quick disruption of a Be disk. Such a limit is in line with the
observational upper limits for these objects.

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Title: Line-driven ablation of circumstellar disks .
Authors: Kee, N. Dylan; Owocki, S.; Kuiper, R.; Sundqvist, J.
2017MmSAI..88..781K Altcode:
Evolved hot, luminous stars are known to drive strong mass
loss (10-10 to 10-5 M_⊙/yr) through UV
line-scattering. High-mass stars already drive such strong winds while
still in their accretion epoch. This means stellar line-scattering
forces can efficiently ablate material off the surface of a massive
protostar's disk, and perhaps even shut off the final accretion onto
the protostar. By using a fully three-dimensional line-scattering
prescription, we quantify this effect, and it's potential role in
setting the stellar upper mass limit.

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Title: Pulsational Mass Ejection in Be Star Disks
Authors: Kee, N. D.; Owocki, S.; Townsend, R.; Müller, H. -R.
2016ASPC..506...47K Altcode: 2014arXiv1412.8511D; 2014arXiv1412.8511K
This work explores a Pulsationally Driven Orbital Mass Ejection
(PDOME) model for the launching of Classical Be star disks. Under
this model, a combination of rapid rotation and non-radial pulsation
modes, characterized by their propagation direction and the relative
phase of their velocity and density perturbations, are considered. As
well, the orbital stability of material launched by such a mechanism
is investigated.

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Title: An `analytic dynamical magnetosphere' formalism for X-ray
and optical emission from slowly rotating magnetic massive stars
Authors: Owocki, Stanley P.; ud-Doula, Asif; Sundqvist, Jon O.; Petit,
Veronique; Cohen, David H.; Townsend, Richard H. D.
2016MNRAS.462.3830O Altcode: 2016arXiv160708568O
Slowly rotating magnetic massive stars develop `dynamical
magnetospheres' (DMs), characterized by trapping of stellar wind outflow
in closed magnetic loops, shock heating from collision of the upflow
from opposite loop footpoints, and subsequent gravitational infall
of radiatively cooled material. In 2D and 3D magnetohydrodynamic
(MHD) simulations, the interplay among these three components is
spatially complex and temporally variable, making it difficult to
derive observational signatures and discern their overall scaling
trends. Within a simplified, steady-state analysis based on overall
conservation principles, we present here an `analytic dynamical
magnetosphere' (ADM) model that provides explicit formulae for density,
temperature, and flow speed in each of these three components - wind
outflow, hot post-shock gas, and cooled inflow - as a function of
colatitude and radius within the closed (presumed dipole) field lines of
the magnetosphere. We compare these scalings with time-averaged results
from MHD simulations, and provide initial examples of application
of this ADM model for deriving two key observational diagnostics,
namely hydrogen H α emission line profiles from the cooled infall,
and X-ray emission from the hot post-shock gas. We conclude with a
discussion of key issues and advantages in applying this ADM formalism
towards derivation of a broader set of observational diagnostics and
scaling trends for massive stars with such dynamical magnetospheres.

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Title: The spectral temperature of optically thick outflows with
application to light echo spectra from η Carinae's giant eruption
Authors: Owocki, Stanley P.; Shaviv, Nir J.
2016MNRAS.462..345O Altcode: 2016arXiv160603681O
The detection by Rest et al. of light echoes from η Carinae has
provided important new observational constraints on the nature of its
1840s era giant eruption. Spectra of the echoes suggest a relatively
cool spectral temperature of about 5500 K, lower than the lower limit
of about 7000 K suggested in the optically thick wind-outflow analysis
of Davidson. This has led to a debate about the viability of this
steady wind model relative to alternative, explosive scenarios. Here
we present an updated analysis of the wind-outflow model using newer
low-temperature opacity tabulations and accounting for the stronger
mass-loss implied by the &gt;10 M⊙ mass now inferred for
the Homunculus. A major conclusion is that, because of the sharp drop
in opacity due to recombination loss of free electrons for T &lt;
6500 K, a low temperature of about 5000 K is compatible with, and
indeed expected from, a wind with the extreme mass-loss inferred for
the eruption. Within a spherical grey model in radiative equilibrium,
we derive spectral energy distributions for various assumptions for
the opacity variation of the wind, providing a basis for comparisons
with observed light echo spectra. The scaling results here are also
potentially relevant for other highly optically thick outflows,
including those from classical novae, giant eruptions of luminous
blue variables and supernovae Type IIn precursors. A broader issue
therefore remains whether the complex, variable features observed from
such eruptions are better understood in terms of a steady or explosive
paradigm, or perhaps a balance of these idealizations.

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Title: VizieR Online Data Catalog: MiMeS survey of magnetism in
massive stars (Wade+, 2016)
Authors: Wade, G. A.; Neiner, C.; Alecian, E.; Grunhut, J. H.; Petit,
V.; de Batz, B.; Bohlender, D. A.; Cohen, D. H.; Henrichs, H. F.;
Kochukhov, O.; Landstreet, J. D.; Manset, N.; Martins, F.; Mathis, S.;
Oksala, M. E.; Owocki, S. P.; Rivinius, T.; Shultz, M. E.; Sundqvist,
J. O.; Townsend, R. H. D.; Ud-Doula, A.; Bouret, J. -C.; Braithwaite,
J.; Briquet, M.; Carciofi, A. C.; David-Uraz, A.; Folsom, C. P.;
Fullerton, A. W.; Leroy, B.; Marcolino, W. L. F.; Moffat, A. F. J.;
Naze, Y.; St, Louis N.; Auriere, M.; Bagnulo, S.; Bailey, J. D.;
Barba, R. H.; Blazere, A.; Bohm, T.; Catala, C.; Donati, J. -F.;
Ferrario, L.; Harrington, D.; Howarth, I. D.; Ignace, R.; Kaper, L.;
Luftinger, T.; Prinja, R.; Vink, J. S.; Weiss, W. W.; Yakunin, I.
2016yCat..74560002W Altcode:
To initiate the observational component of the MiMeS project,
the collaboration was awarded a 640 hour Large Program (LP) with
ESPaDOnS. This award was followed by LP allocations with Narval
(137 nights, or 1213 hours), and with HARPSpol (30 nights , or 280
hours). The TC sample (summarized in Table 4) consists of stars
that were established or suspected to be magnetic at the beginning
of the project. The majority of these stars are confirmed period ic
variables with periods ranging from approximately 1 d to 1.5 years,
with the majority having a period of less than 10 days so that they
are suitable candidates for observational monitoring and mapping. (1 data file).

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Title: Constraining the Extremely Hard X-ray Excess of Eta Carinae
using XMM-Newton and NuSTAR
Authors: Sharma, Neetika; Hamaguchi, Kenji; Gull, Theodore R.;
Corcoran, Michael F.; Madura, Thomas; Russell, Christopher Michael
Post; Takahashi, Hiromitsu; Grefenstette, Brian; Yuasa, Tadayuki;
Moffat, Anthony F. J.; Richardson, Noel; Groh, Jose H.; Pittard,
Julian M.; Owocki, Stanley P.
2016AAS...22821806S Altcode:
Eta Carinae (η Car), the most luminous (L∼106.7
L⊙), evolved, supermassive star (M≥100 M⊙)
in our Galaxy, has been extensively studied over the entire range of
wavelengths of the electromagnetic spectrum, and yet it remains to be
intriguingly mysterious. η Car is a binary system with an orbital
period of 2024 days (5.53 years). The collision of the slow (∼500
km s-1), dense winds from the primary star with the fast
(∼3000 km s-1), thin winds from the companion, produces
very hot plasma with temperatures of severals of millions of Kelvin
via shock heating. Previously, the INTEGRAL and Suzaku observatories
have suggested extremely high energy (15-100 keV) emission from η Car,
which may arise from inverse Compton scattering of UV/optical photons
by high-energy electrons accelerated in the wind colliding regions,
or from the super hot plasma at the head-on collision. Recently,
within a span of about 1.4 years (March 2014-July 2015), η Car
was observed a total of 13 times with NuSTAR. The spectrum from the
2015 July observation, shows a hard X-ray excess above ∼ 17 keV,
which can be constrained with a flat power-law (Γ∼0.5) or very
hot bremsstrahlung (kT∼10 keV) component. This hard X-ray excess is
significantly above the background level below 25 keV and therefore
should not be instrumental. The light curves of the narrow sections
of energy bands above 10 keV do not show significant variation. We
discuss the origin of this extremely hard excess component from combined
analysis of the XMM-Newton and NuSTAR data.

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Title: Modelling the Central Constant Emission X-ray component of
η Carinae
Authors: Russell, Christopher M. P.; Corcoran, Michael F.; Hamaguchi,
Kenji; Madura, Thomas I.; Owocki, Stanley P.; Hillier, D. John
2016MNRAS.458.2275R Altcode: 2016MNRAS.tmp..119R; 2016arXiv160301629R
The X-ray emission of η Carinae shows multiple features at various
spatial and temporal scales. The central constant emission (CCE)
component is centred on the binary and arises from spatial scales much
smaller than the bipolar Homunculus nebula, but likely larger than the
central wind-wind collision region between the stars as it does not
vary over the ∼2-3 month X-ray minimum when it can be observed. Using
large-scale 3D smoothed particle hydrodynamics (SPH) simulations, we
model both the colliding-wind region between the stars, and the region
where the secondary wind collides with primary wind ejected from the
previous periastron passage. The simulations extend out to one hundred
semimajor axes and make two limiting assumptions (strong coupling
and no coupling) about the influence of the primary radiation field
on the secondary wind. We perform 3D radiative transfer calculations
on the SPH output to synthesize the X-ray emission, with the aim of
reproducing the CCE spectrum. For the preferred primary mass-loss rate
dot{M}_A≈ 8.5× 10^{-4} M_{⊙} yr-1, the model spectra
well reproduce the observation as the strong- and no-coupling spectra
bound the CCE observation for longitude of periastron ω ≈ 252°,
and bound/converge on the observation for ω ≈ 90°. This suggests
that η Carinae has moderate coupling between the primary radiation
and secondary wind, that both the region between the stars and the
comoving collision on the backside of the secondary generate the CCE,
and that the CCE cannot place constraints on the binary's line of
sight. We also discuss comparisons with common X-ray fitting parameters.

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Title: Line-driven ablation of circumstellar discs - I. Optically
thin decretion discs of classical Oe/Be stars
Authors: Kee, Nathaniel Dylan; Owocki, Stanley; Sundqvist, J. O.
2016MNRAS.458.2323K Altcode: 2016arXiv160207874K; 2016MNRAS.tmp..245K
The extreme luminosities of massive, hot OB stars drive strong stellar
winds through line-scattering of the star's UV continuum radiation. For
OB stars with an orbiting circumstellar disc, we explore here the
effect of such line-scattering in driving an ablation of material
from the disc's surface layers, with initial focus on the marginally
optically thin decretion discs of classical Oe and Be stars. For this
we apply a multidimensional radiation-hydrodynamics code that assumes
simple optically thin ray tracing for the stellar continuum, but uses
a multiray Sobolev treatment of the line transfer; this fully accounts
for the efficient driving by non-radial rays, due to desaturation of
line-absorption by velocity gradients associated with the Keplerian
shear in the disc. Results show a dense, intermediate-speed surface
ablation, consistent with the strong, blueshifted absorption of UV
wind lines seen in Be shell stars that are observed from near the disc
plane. A key overall result is that, after an initial adjustment to
the introduction of the disc, the asymptotic disc destruction rate is
typically just an order-unity factor times the stellar wind mass-loss
rate. For optically thin Be discs, this leads to a disc destruction
time of order months to years, consistent with observationally
inferred disc decay times. The much stronger radiative forces of O
stars reduce this time to order days, making it more difficult for
decretion processes to sustain a disc in earlier spectral types,
and so providing a natural explanation for the relative rarity of Oe
stars in the Galaxy. Moreover, the decrease in line-driving at lower
metallicity implies both a reduction in the winds that help spin-down
stars from near-critical rotation, and a reduction in the ablation of
any decretion disc; together these provide a natural explanation for
the higher fraction of classical Be stars, as well as the presence of
Oe stars, in the lower metallicity Magellanic Clouds. We conclude with a
discussion of future extensions to study line-driven ablation of denser,
optically thick, accretion discs of pre-main-sequence massive stars.

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Title: The MiMeS survey of magnetism in massive stars: introduction
and overview
Authors: Wade, G. A.; Neiner, C.; Alecian, E.; Grunhut, J. H.; Petit,
V.; Batz, B. de; Bohlender, D. A.; Cohen, D. H.; Henrichs, H. F.;
Kochukhov, O.; Landstreet, J. D.; Manset, N.; Martins, F.; Mathis, S.;
Oksala, M. E.; Owocki, S. P.; Rivinius, Th.; Shultz, M. E.; Sundqvist,
J. O.; Townsend, R. H. D.; ud-Doula, A.; Bouret, J. -C.; Braithwaite,
J.; Briquet, M.; Carciofi, A. C.; David-Uraz, A.; Folsom, C. P.;
Fullerton, A. W.; Leroy, B.; Marcolino, W. L. F.; Moffat, A. F. J.;
Nazé, Y.; Louis, N. St; Aurière, M.; Bagnulo, S.; Bailey, J. D.;
Barbá, R. H.; Blazère, A.; Böhm, T.; Catala, C.; Donati, J. -F.;
Ferrario, L.; Harrington, D.; Howarth, I. D.; Ignace, R.; Kaper, L.;
Lüftinger, T.; Prinja, R.; Vink, J. S.; Weiss, W. W.; Yakunin, I.
2016MNRAS.456....2W Altcode: 2015arXiv151108425W
The MiMeS (Magnetism in Massive Stars) project is a large-scale,
high-resolution, sensitive spectropolarimetric investigation of the
magnetic properties of O- and early B-type stars. Initiated in 2008 and
completed in 2013, the project was supported by three Large Program
allocations, as well as various programmes initiated by independent
principal investigators, and archival resources. Ultimately, over 4800
circularly polarized spectra of 560 O and B stars were collected with
the instruments ESPaDOnS (Echelle SpectroPolarimetric Device for the
Observation of Stars) at the Canada-France-Hawaii Telescope, Narval
at the Télescope Bernard Lyot and HARPSpol at the European Southern
Observatory La Silla 3.6 m telescope, making MiMeS by far the largest
systematic investigation of massive star magnetism ever undertaken. In
this paper, the first in a series reporting the general results of the
survey, we introduce the scientific motivation and goals, describe
the sample of targets, review the instrumentation and observational
techniques used, explain the exposure time calculation designed to
provide sensitivity to surface dipole fields above approximately 100
G, discuss the polarimetric performance, stability and uncertainty
of the instrumentation, and summarize the previous and forthcoming
publications.

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Title: Constraining the evolutionary state of the hot, massive
companion star and the wind-wind collision region in Eta Carinae
Authors: Gull, Theodore Raymond; Corcoran, Michael F.; Damineli,
Augusto; Groh, Jose; Hamaguchi, Kenji; Hillier, D. John; Madura,
Thomas I.; Owocki, Stanley P.; Richardson, Noel D.; Weigelt, Gerd
2016hst..prop13395G Altcode:
Our primary goal is to constrain the physical properties of Eta
Car's wind-wind collision region and the properties of Eta Car B,
the hot, very massive secondary companion that is hidden from our
view. We propose to do this by measuring the forbidden emission lines
in the extended, interacting wind structures resolved in the inner
1.5 arcseconds region. As the mass-loss structures evolve across the
5.54-year orbital period, we will selectively map their spatial and
velocity changes at critical phases using the spatial resolution of
HST and moderate spectral resolving power of the STIS to generate
spatial (2-D), velocity (1-D) data cubes of regions of critical
collisional densities. We will use these spatial-velocity data cubes
to drive ongoing 3-D models of the interacting winds, adding radiative
transfer. We will (A) strongly constrain the 3-D mass loss from the
system and (B) determine the properties of Eta Car B, the source of
FUV radiation and the driving wind creating the X-ray emitting cavity
out of the dominating wind of Eta Car A.

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Title: θ Car: X-ray Emission from Low Density Radiation-Driven Winds
Authors: Doyle Mizusawa, Trisha; Petit, Veronique; Held Cohen, David;
Fullerton, Alexander W.; Gagne, Marc; Leutenegger, Maurice A.; Li,
Zequn; Owocki, Stanley P.; Sundqvist, Jon; Wade, Gregg
2016AAS...22712901D Altcode:
We present Chandra X-ray grating spectroscopy (and IUE spectroscopy)
of the B0.2 V star, θ Carina. θ Car is in a critical transition
region between the earliest B stars and the latest O stars, where the
density of the wind is observed to decrease more than theoretically
expected. In general, X-ray emission in this low-density wind regime
should be less prominent, but observations have shown that there is
a higher than expected production of X-ray emission from the winds
of these stars; this severely challenges predictions of radiatively
driven wind theory. We measure the f/i ratio, widths, and velocities
of several Helium-like lines in the X-ray spectrum. The f/i ratio is a
diagnostic of the radial location of the X-ray emitting plasma, which
is sensitive to the specific transition of each He-like ion. We use
θ Car to study the radiatively-driven mass-loss of early B-type stars.

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Title: Eta Carinae's Thermal X-Ray Tail Measured with XMM-Newton
and NuSTAR
Authors: Hamaguchi, Kenji; Corcoran, Michael F.; Gull, Theodore
R.; Takahashi, Hiromitsu; Grefenstette, Brian W.; Yuasa, Takayuki;
Stuhlinger, Martin; Russell, Christopher M. P.; Moffat, Anthony F. J.;
Sharma, Neetika; Madura, Thomas I.; Richardson, Noel D.; Groh, Jose;
Pittard, Julian M.; Owocki, Stanley
2016ApJ...817...23H Altcode: 2016arXiv160201148H
The evolved, massive highly eccentric binary system, η Car, underwent a
periastron passage in the summer of 2014. We obtained two coordinated
X-ray observations with XMM-Newton and NuSTAR during the elevated
X-ray flux state and just before the X-ray minimum flux state around
this passage. These NuSTAR observations clearly detected X-ray emission
associated with η Car extending up to ∼50 keV for the first time. The
NuSTAR spectrum above 10 keV can be fit with the bremsstrahlung tail
from a kT ∼ 6 keV plasma. This temperature is ΔkT ∼ 2 keV higher
than those measured from the iron K emission line complex, if the
shocked gas is in collisional ionization equilibrium. This result
may suggest that the companion star's pre-shock wind velocity is
underestimated. The NuSTAR observation near the X-ray minimum state
showed a gradual decline in the X-ray emission by 40% at energies
above 5 keV in a day, the largest rate of change of the X-ray flux
yet observed in individual η Car observations. The column density
to the hardest emission component, NH ∼ 1024
H cm-2, marked one of the highest values ever observed for
η Car, strongly suggesting increased obscuration of the wind-wind
colliding X-ray emission by the thick primary stellar wind prior
to superior conjunction. Neither observation detected the power-law
component in the extremely hard band that INTEGRAL and Suzaku observed
prior to 2011. If the non-detection by NuSTAR is caused by absorption,
the power-law source must be small and located very near the wind-wind
collision apex. Alternatively, it may be that the power-law source is
not related to either η Car or the GeV γ-ray source.

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Title: Constraining the evolutionary state of the hot, massive
companion star and the wind-wind collision region in Eta Carinae
Authors: Gull, Theodore Raymond; Cherchneff, Isabelle; Corcoran,
Michael F.; Damineli, Augusto; Groh, Jose; Hamaguchi, Kenji; Hillier,
D. John; Icke, Vincent; Madura, Thomas I.; Owocki, Stanley P.;
Richardson, Noel D.; Weigelt, Gerd
2016hst..prop12508G Altcode:
Our primary goal is to constrain the physical properties of Eta
Car's wind-wind collision region and the properties of Eta Car B,
the hot, very massive secondary companion that is hidden from our
view. We propose to do this by measuring the forbidden emission lines
in the extended, interacting wind structures resolved in the inner
1.5 arcseconds region. As the mass-loss structures evolve across the
5.54-year orbital period, we will selectively map their spatial and
velocity changes at critical phases using the spatial resolution of
HST and moderate spectral resolving power of the STIS to generate
spatial {2-D}, velocity {1-D} data cubes of regions of critical
collisional densities. We will use these spatial-velocity data cubes
to drive ongoing 3-D models of the interacting winds, adding radiative
transfer. We will {A} strongly constrain the 3-D mass loss from the
system and {B} determine the properties of Eta Car B, the source of
FUV radiation and the driving wind creating the X-ray emitting cavity
out of the dominating wind of Eta Car A.

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Title: Effect of scattering on the transonic solution topology and
intrinsic variability of line-driven stellar winds
Authors: Sundqvist, Jon O.; Owocki, Stanley P.
2015MNRAS.453.3428S Altcode: 2015arXiv150802955S
For line-driven winds from hot, luminous OB stars, we examine the subtle
but important role of diffuse, scattered radiation in determining both
the topology of steady-state solutions and intrinsic variability in
the transonic wind base. We use a smooth source function formalizm
to obtain non-local, integral expressions for the direct and diffuse
components of the line-force that account for deviations from the usual
localized, Sobolev forms. As the scattering source function is reduced,
we find the solution topology in the transonic region transitions from
X-type, with a unique wind solution, to a nodal type, characterized by
a degenerate family of solutions. Specifically, in the idealized case
of an optically thin source function and a uniformly bright stellar
disc, the unique X-type solution proves to be a stable attractor to
which time-dependent numerical radiation-hydrodynamical simulations
relax. But in models where the scattering strength is only modestly
reduced, the topology instead turns nodal, with the associated solution
degeneracy now manifest by intrinsic structure and variability that
persist down to the photospheric wind base. This highlights the
potentially crucial role of diffuse radiation for the dynamics and
variability of line-driven winds, and seriously challenges the use of
Sobolev theory in the transonic wind region. Since such Sobolev-based
models are commonly used in broad applications like stellar evolution
and feedback, this prompts development of new wind models, not only
for further quantifying the intrinsic variability found here, but also
for computing new theoretical predictions of global properties like
velocity laws and mass-loss rates.

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Title: Hydrodynamic and radiative transfer modeling of X-ray emission
from colliding WR winds: WR 140 &amp; the Galactic center
Authors: Russell, Christopher M. P.; Corcoran, Michael F.; Cuadra,
Jorge; Owocki, Stanley P.; Wang, Q. Daniel; Hamaguchi, Kenji; Sugawara,
Yasuharu; Pollock, Andrew M. T.; Kallman, Timothy R.
2015arXiv151101150R Altcode:
Colliding Wolf-Rayet (WR) winds produce thermal X-ray emission widely
observed by X-ray telescopes. In wide WR+O binaries, such as WR 140,
the X-ray flux is tied to the orbital phase, and is a direct probe
of the winds' properties. In the Galactic center, $\sim$30 WRs orbit
the super massive black hole (SMBH) within $\sim$10", leading to a
smorgasbord of wind-wind collisions. To model the X-ray emission of WR
140 and the Galactic center, we perform 3D hydrodynamic simulations
to trace the complex gaseous flows, and then carry out 3D radiative
transfer calculations to compute the variable X-ray spectra. The
model WR 140 RXTE light curve matches the data well for all phases
except the X-ray minimum associated with periastron, while the model
spectra agree with the RXTE hardness ratio and the shape of the Suzaku
observations throughout the orbit. The Galactic center model of the
Chandra flux and spectral shape match well in the region r$&lt;$3",
but the model flux falls off too rapidly beyond this radius.

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Title: X-ray emission from the giant magnetosphere of the magnetic
O-type star NGC 1624-2
Authors: Petit, V.; Cohen, D. H.; Wade, G. A.; Nazé, Y.; Owocki,
S. P.; Sundqvist, J. O.; ud-Doula, A.; Fullerton, A.; Leutenegger,
M.; Gagné, M.
2015MNRAS.453.3288P Altcode: 2015arXiv150708621P
We observed NGC 1624-2, the O-type star with the largest known magnetic
field (Bp ∼ 20 kG), in X-rays with the Advanced CCD Imaging
Spectrometer (ACIS-S) camera on-board the Chandra X-ray Observatory. Our
two observations were obtained at the minimum and maximum of the
periodic Hα emission cycle, corresponding to the rotational phases
where the magnetic field is the closest to equator-on and pole-on,
respectively. With these observations, we aim to characterize the
star's magnetosphere via the X-ray emission produced by magnetically
confined wind shocks. Our main findings are as follows. (i) The observed
spectrum of NGC 1624-2 is hard, similar to the magnetic O-type star
θ1 Ori C, with only a few photons detected below 0.8
keV. The emergent X-ray flux is 30 per cent lower at the Hα minimum
phase. (ii) Our modelling indicated that this seemingly hard spectrum
is in fact a consequence of relatively soft intrinsic emission,
similar to other magnetic Of?p stars, combined with a large amount
of local absorption (∼1-3× 1022 cm-2). This
combination is necessary to reproduce both the prominent Mg and Si
spectral features, and the lack of flux at low energies. NGC 1624-2 is
intrinsically luminous in X-rays (log L^{em}_X∼ 33.4) but 70-95 per
cent of the X-ray emission produced by magnetically confined wind shocks
is absorbed before it escapes the magnetosphere (log L^{ISMcor}_X∼
32.5). (iii) The high X-ray luminosity, its variation with stellar
rotation, and its large attenuation are all consistent with a large
dynamical magnetosphere with magnetically confined wind shocks.

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Title: Accretion, Outflows, and Winds of Magnetized Stars
Authors: Romanova, Marina M.; Owocki, Stanley P.
2015SSRv..191..339R Altcode: 2016arXiv160504979R; 2015SSRv..tmp...97R
Many types of stars have strong magnetic fields that can dynamically
influence the flow of circumstellar matter. In stars with accretion
disks, the stellar magnetic field can truncate the inner disk and
determine the paths that matter can take to flow onto the star. These
paths are different in stars with different magnetospheres and periods
of rotation. External field lines of the magnetosphere may inflate and
produce favorable conditions for outflows from the disk-magnetosphere
boundary. Outflows can be particularly strong in the propeller regime,
wherein a star rotates more rapidly than the inner disk. Outflows may
also form at the disk-magnetosphere boundary of slowly rotating stars,
if the magnetosphere is compressed by the accreting matter. In isolated,
strongly magnetized stars, the magnetic field can influence formation
and/or propagation of stellar wind outflows. Winds from low-mass,
solar-type stars may be either thermally or magnetically driven,
while winds from massive, luminous O and B type stars are radiatively
driven. In all of these cases, the magnetic field influences matter flow
from the stars and determines many observational properties. In this
chapter we review recent studies of accretion, outflows, and winds of
magnetized stars with a focus on three main topics: (1) accretion onto
magnetized stars; (2) outflows from the disk-magnetosphere boundary;
and (3) winds from isolated massive magnetized stars. We show results
obtained from global magnetohydrodynamic simulations and, in a number
of cases compare global simulations with observations.

---------------------------------------------------------
Title: Confirming HD 23478 as a new magnetic B star hosting an
Hα-bright centrifugal magnetosphere
Authors: Sikora, J.; Wade, G. A.; Bohlender, D. A.; Neiner, C.;
Oksala, M. E.; Shultz, M.; Cohen, D. H.; ud-Doula, A.; Grunhut, J.;
Monin, D.; Owocki, S.; Petit, V.; Rivinus, T.; Townsend, R. H. D.
2015MNRAS.451.1928S Altcode: 2015arXiv150502427S
In this paper, we report 23 magnetic field measurements of the B3IV
star HD 23478: 12 obtained from high-resolution Stokes V spectra using
the ESPaDOnS (Canada-France-Hawaii Telescope) and Narval (Télescope
Bernard Lyot) spectropolarimeters, and 11 from medium-resolution Stokes
V spectra obtained with the DimaPol spectropolarimeter (Dominion
Astronomical Observatory). HD 23478 was one of two rapidly rotating
stars identified as potential `centrifugal magnetosphere' hosts based
on IR observations from the Apache Point Observatory Galactic Evolution
Experiment survey. We derive basic physical properties of this star
including its mass (M=6.1^{+0.8}_{-0.7} M_{⊙), effective temperature
(Teff = 20 ± 2 kK), radius (R=2.7^{+1.6}_{-0.9} R_{⊙}),
and age (τ_age=3^{+37}_{-1} Myr). We repeatedly detect weakly variable
Zeeman signatures in metal, He, and H lines in all our observations
corresponding to a longitudinal magnetic field of &lt;Bz&gt;
≈ -2.0 kG. The rotational period is inferred from Hipparcos photometry
(Prot = 1.0498(4) d). Under the assumption of the Oblique
Rotator Model, our observations yield a surface dipole magnetic field
of strength Bd ≥ 9.5 kG that is approximately aligned
with the stellar rotation axis. We confirm the presence of strong and
broad Hα emission and gauge the volume of this star's centrifugal
magnetosphere to be consistent with those of other Hα emitting
centrifugal magnetosphere stars based on the large inferred Alfvén
to Kepler radius ratio.

---------------------------------------------------------
Title: A Coordinated X-Ray and Optical Campaign of the Nearest Massive
Eclipsing Binary, δ Orionis Aa. I. Overview of the X-Ray Spectrum
Authors: Corcoran, M. F.; Nichols, J. S.; Pablo, H.; Shenar, T.;
Pollock, A. M. T.; Waldron, W. L.; Moffat, A. F. J.; Richardson, N. D.;
Russell, C. M. P.; Hamaguchi, K.; Huenemoerder, D. P.; Oskinova, L.;
Hamann, W. -R.; Nazé, Y.; Ignace, R.; Evans, N. R.; Lomax, J. R.;
Hoffman, J. L.; Gayley, K.; Owocki, S. P.; Leutenegger, M.; Gull,
T. R.; Hole, K. T.; Lauer, J.; Iping, R. C.
2015ApJ...809..132C Altcode: 2015arXiv150705101C
We present an overview of four deep phase-constrained Chandra HETGS
X-ray observations of δ Ori A. Delta Ori A is actually a triple
system that includes the nearest massive eclipsing spectroscopic
binary, δ Ori Aa, the only such object that can be observed with
little phase-smearing with the Chandra gratings. Since the fainter
star, δ Ori Aa2, has a much lower X-ray luminosity than the brighter
primary (δ Ori Aa1), δ Ori Aa provides a unique system with which
to test the spatial distribution of the X-ray emitting gas around δ
Ori Aa1 via occultation by the photosphere of, and wind cavity around,
the X-ray dark secondary. Here we discuss the X-ray spectrum and X-ray
line profiles for the combined observation, having an exposure time of
nearly 500 ks and covering nearly the entire binary orbit. The companion
papers discuss the X-ray variability seen in the Chandra spectra,
present new space-based photometry and ground-based radial velocities
obtained simultaneously with the X-ray data to better constrain the
system parameters, and model the effects of X-rays on the optical and
UV spectra. We find that the X-ray emission is dominated by embedded
wind shock emission from star Aa1, with little contribution from the
tertiary star Ab or the shocked gas produced by the collision of the
wind of Aa1 against the surface of Aa2. We find a similar temperature
distribution to previous X-ray spectrum analyses. We also show that the
line half-widths are about 0.3-0.5 times the terminal velocity of the
wind of star Aa1. We find a strong anti-correlation between line widths
and the line excitation energy, which suggests that longer-wavelength,
lower-temperature lines form farther out in the wind. Our analysis
also indicates that the ratio of the intensities of the strong and
weak lines of Fe xvii and Ne x are inconsistent with model predictions,
which may be an effect of resonance scattering.

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Title: Revisiting the rigidly rotating magnetosphere model for
σ Ori E - II. Magnetic Doppler imaging, arbitrary field RRM, and
light variability
Authors: Oksala, M. E.; Kochukhov, O.; Krtička, J.; Townsend,
R. H. D.; Wade, G. A.; Prvák, M.; Mikulášek, Z.; Silvester, J.;
Owocki, S. P.
2015MNRAS.451.2015O Altcode: 2015arXiv150504839O
The initial success of the Rigidly Rotating Magnetosphere (RRM) model
application to the B2Vp star σ Ori E by Townsend, Owocki &amp; Groote
triggered a renewed era of observational monitoring of this archetypal
object. We utilize high-resolution spectropolarimetry and the magnetic
Doppler imaging (MDI) technique to simultaneously determine the magnetic
configuration, which is predominately dipolar, with a polar strength
Bd = 7.3-7.8 kG and a smaller non-axisymmetric quadrupolar
contribution, as well as the surface distribution of abundance of
He, Fe, C, and Si. We describe a revised RRM model that now accepts
an arbitrary surface magnetic field configuration, with the field
topology from the MDI models used as input. The resulting synthetic
H α emission and broad-band photometric observations generally
agree with observations, however, several features are poorly fit. To
explore the possibility of a photospheric contribution to the observed
photometric variability, the MDI abundance maps were used to compute
a synthetic photospheric light curve to determine the effect of the
surface inhomogeneities. Including the computed photospheric brightness
modulation fails to improve the agreement between the observed and
computed photometry. We conclude that the discrepancies cannot be
explained as an effect of inhomogeneous surface abundance. Analysis
of the UV light variability shows good agreement between observed
variability and computed light curves, supporting the accuracy of
the photospheric light variation calculation. We thus conclude that
significant additional physics is necessary for the RRM model to
acceptably reproduce observations of not only σ Ori E, but also other
similar stars with significant stellar wind-magnetic field interactions.

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Title: The X-ray Lightcurve of the Supermassive star eta Carinae,
1996--2014
Authors: Corcoran, M. F.; Hamaguchi, K.; Liburd, J. K.; Morris, D.;
Gull, T. R.; Madura, T. I.; Teodoro, M.; Moffat, A. F. J.; Richardson,
N. D.; Russell, C. M. P.; Pollock, A. M. T.; Owocki, S. P.
2015arXiv150707961C Altcode:
Eta Carinae is the nearest example of a supermassive, superluminous,
unstable star. Mass loss from the system is critical in shaping its
circumstellar medium and in determining its ultimate fate. Eta Car
currently loses mass via a dense, slow stellar wind and possesses
one of the largest mass loss rates known. It is prone to episodes
of extreme mass ejection via eruptions from some as-yet unspecified
cause; the best examples of this are the large-scale eruptions which
occurred in 19th century. Eta Car is a colliding wind binary in which
strong variations in X-ray emission and in other wavebands are driven
by the violent collision of the wind of eta Car-A and the fast, less
dense wind of an otherwise hidden companion star. X-ray variations
are the simplest diagnostic we have to study the wind-wind collision
and allow us to measure the state of the stellar mass loss from both
stars. We present the X-ray lightcurve over the last 20 years from
ROSAT observations and monitoring with the Rossi X-ray Timing Explorer
and the X-ray Telescope on the Swift satellite. We compare and contrast
the behavior of the X-ray emission from the system over that timespan,
including surprising variations during the 2014 X-ray minimum.

---------------------------------------------------------
Title: Modeling Diffuse X-ray Emission around the Galactic Center
from Colliding Stellar Winds
Authors: Russell, Christopher Michael Post; Cuadra, Jorge; Wang,
Q. Daniel; Owocki, Stanley P.
2015AAS...22520303R Altcode: 2015AAS...22520303P
The Galactic center is a hotbed of astrophysical phenomena. The ~30
evolved massive stars orbiting the SMBH on scales &lt;10" inject a large
fraction of the matter that accretes onto the SMBH, and potentially
creates large swaths of hot, X-ray emitting material around Sgr A*
from their wind-wind collisions. Using the Gadget-2 SPH simulations
of these evolved stars ejecting their winds over the last 1100 years
from Cuadra et al. 2008, we solve the formal solution to the equation
of radiative transfer for a grid of rays through the 6"x6" simulation
volume to calculate the thermal X-ray emission from the diffuse hot
gas. We then fold each of these energy-dependent pixel maps through the
Chandra ACIS-S response function to directly compare with the recent
3Ms X-ray Visionary Program observations of the Galactic center (Wang
et al. 2013). The model X-ray flux, in absolute units, agrees well
with the observations just outside the SMBH (whose emission is not
included in this modeling), indicating that the shocked wind material
from the evolved massive stars is indeed the source of diffuse X-ray
emission at the Galactic center. The emission of the IRS13 cluster,
though, is overestimated by two orders of magnitude, indicating a
potential revision in the cluster stellar parameters. We will conclude
by discussing future work, such as implementing the 'pressure-entropy'
formulation of SPH for this calculation and including O stars and
closely orbiting binaries.

---------------------------------------------------------
Title: V444 Cygni X-ray and polarimetric variability: Radiative and
Coriolis forces shape the wind collision region
Authors: Lomax, J. R.; Nazé, Y.; Hoffman, J. L.; Russell, C. M. P.;
De Becker, M.; Corcoran, M. F.; Davidson, J. W.; Neilson, H. R.;
Owocki, S.; Pittard, J. M.; Pollock, A. M. T.
2015A&A...573A..43L Altcode: 2014arXiv1410.6117L
We present results from a study of the eclipsing, colliding-wind
binary V444 Cyg that uses a combination of X-ray and optical
spectropolarimetric methods to describe the 3D nature of the
shock and wind structure within the system. We have created the
most complete X-ray light curve of V444 Cyg to date using 40 ks
of new data from Swift, and 200 ks of new and archived XMM-Newton
observations. In addition, we have characterized the intrinsic,
polarimetric phase-dependent behavior of the strongest optical emission
lines using data obtained with the University of Wisconsin's Half-Wave
Spectropolarimeter. We have detected evidence of the Coriolis distortion
of the wind-wind collision in the X-ray regime, which manifests itself
through asymmetric behavior around the eclipses in the system's X-ray
light curves. The large opening angle of the X-ray emitting region,
as well as its location (i.e. the WN wind does not collide with the O
star, but rather its wind) are evidence of radiative braking/inhibition
occurring within the system. Additionally, the polarimetric results
show evidence of the cavity the wind-wind collision region carves out
of the Wolf-Rayet star's wind.

---------------------------------------------------------
Title: Instabilities in the Envelopes and Winds of Very Massive Stars
Authors: Owocki, Stanley P.
2015ASSL..412..113O Altcode: 2014arXiv1403.6745O
The high luminosity of Very Massive Stars (VMS) means that radiative
forces play an important, dynamical role both in the structure and
stability of their stellar envelope, and in driving strong stellar-wind
mass loss. Focusing on the interplay of radiative flux and opacity,
with emphasis on key distinctions between continuum vs. line opacity,
this chapter reviews instabilities in the envelopes and winds of
VMS. Specifically, we discuss how: (1) the iron opacity bump can
induce an extensive inflation of the stellar envelope; (2) the density
dependence of mean opacity leads to strange mode instabilities in
the outer envelope; (3) desaturation of line-opacity by acceleration
of near-surface layers initiates and sustains a line-driven stellar
wind outflow; (4) an associated line-deshadowing instability leads to
extensive small-scale structure in the outer regions of such line-driven
winds; (5) a star with super-Eddington luminosity can develop
extensive atmospheric structure from photon bubble instabilities,
or from stagnation of flow that exceeds the "photon tiring" limit;
(6) the associated porosity leads to a reduction in opacity that can
regulate the extreme mass loss of such continuum-driven winds. Two
overall themes are the potential links of such instabilities to Luminous
Blue Variable (LBV) stars, and the potential role of radiation forces
in establishing the upper mass limit of VMS.

---------------------------------------------------------
Title: Hydrodynamic and radiative transfer modeling of X-ray emission
from colliding WR winds: WR 140 and the Galactic center
Authors: Russell, C. M. P.; Corcoran, M. F.; Cuadra, J.; Owocki,
S. P.; Wang, Q. D.; Hamaguchi, K.; Sugawara, Y.; Pollock, A. M. T.;
Kallman, T. R.
2015wrs..conf..309R Altcode:
Colliding Wolf-Rayet (WR) winds produce thermal X-ray emission widely
observed by X-ray telescopes. In wide WR+O binaries, such as WR 140,
the X-ray flux is tied to the orbital phase, and is a direct probe
of the winds’ properties. In the Galactic center, ~30 WRs orbit the
super massive black hole (SMBH) within ~10”, leading to a smorgasbord
of wind-wind collisions. To model the X-ray emission of WR 140 and
the Galactic center, we perform 3D hydrodynamic simulations to trace
the complex gaseous flows, and then carry out 3D radiative transfer
calculations to compute the variable X-ray spectra. The model WR 140
RXTE light curve matches the data well for all phases except the X-ray
minimum associated with periastron, while the model spectra agree
with the RXTE hardness ratio and the shape of the Suzaku observations
throughout the orbit. The Galactic center model of the Chandra flux
and spectral shape match well in the region r ≤ 3”, but the model
flux falls off too rapidly beyond this radius.

---------------------------------------------------------
Title: A Simple Mean-Field Diagnostic from Stokes V Spectra
Authors: Gayley, K. G.; Owocki, S. P.
2015IAUS..307..375G Altcode:
It is shown that the diagnostics from an observed circularly polarized
line in a rapidly rotating star are directly interpretable, not in terms
of the observed Stokes V profiles, but in terms of its antiderivative
with respect to wavelength (in velocity units if preferred). This also
leads to a new mean-field diagnostic that is just as easily obtained as
the standard “center of gravity” approach, and is less susceptible
to cancellation if the line-of-sight field changes sign over the face
of the star.

---------------------------------------------------------
Title: Extremely Hard X-ray Emission from Eta Carinae observed with
XMM-Newton and NuSTAR around Periastron in 2014.5
Authors: Hamaguchi, Kenji; Corcoran, Michael F.; Takahashi, Hiromitsu;
Yuasa, Tadayuki; Groh, Jose H.; Russell, Christopher Michael Post;
Pittard, Julian M.; Madura, Thomas; Owocki, Stanley P.; Grefenstette,
Brian
2015AAS...22534417H Altcode:
The super massive colliding wind binary system, Eta Carinae, experienced
another periastron passage in the summer of 2014. We monitored this
event using the multiple X-ray observatories, Chandra, XMM-Newton,
NuSTAR, Suzaku and Swift. With a high eccentricity of its 5.5 year
orbit, X-ray emission from the wind-wind collision (WWC) increases
strongly toward periastron but then drops sharply by more than two
orders of magnitude in two weeks around periastron due probably to an
eclipse and an intrinsic activity decline of the WWC plasma. In this
observing campaign, XMM-Newton and NuSTAR coordinated two simultaneous
observations around the X-ray flux maximum on June 6 and just before the
flux minimum on July 28. These two observations captured Eta Carinae
with X-ray focusing telescopes in the extreme hard X-ray band above
10 keV for the first time.During the first observation, XMM and NuSTAR
detected stable X-ray emission from the central binary system between
1 - 40 keV. A fit of a 1-temperature bremsstrahlung model to the high
energy slope in the NuSTAR spectrum derives an electron temperature of
~6 keV, which is significantly higher than an ionization temperature
at ~4.5 keV, measured from the Fe K emission lines resolved in the XMM
spectrum.This result suggests the presence of very hot plasma and/or
X-ray reflection at surrounding cold material. During the second
observation, the X-ray flux between 5 - 10 keV declined steadily by
a factor of ~2 in a day, while the other energy bands were rather
stable. This variation may be explained by an increase of the line of
sight absorption to emission from the plasma component that dominates
above 5 keV. NuSTAR did not detect, in either observation, the very
hard non-thermal component that dominated emission above 25 keV seen
in earlier INTEGRAL and Suzaku observations. We discuss the plasma
condition and the wind structure of Eta Carinae around periastron,
and the nature of the non-thermal component.

---------------------------------------------------------
Title: The X-ray Lightcurve of Eta Carinae, 1996-2014
Authors: Corcoran, Michael F.; Hamaguchi, Kenji; Liburd, Jamar; Gull,
Theodore R.; Madura, Thomas; Teodoro, Mairan; Moffat, Anthony F. J.;
Richardson, Noel; Russell, Christopher Michael Post; Pollock, A.;
Owocki, Stanley P.
2015AAS...22534415C Altcode:
Eta Carinae is the nearest example of a supermassive, superluminous,
unstable star. Mass loss from the system is important in shaping
its circumstellar medium and in determining the ultimate fate of the
star. Eta Car loses mass via a dense, slow stellar wind and possesses
one of the largest mass loss rates known. It is prone to episodes of
extreme mass ejection via eruptions from some as-yet unspecified cause;
the best examples of this are the large-scale eruptions which occurred
in the mid-19th century, and then again about 50 years later. Eta
Car is a colliding wind binary in which strong variations in X-ray
emission and in other wavebands are driven by the violent collision
of the wind of Eta Car and the fast, less dense wind of an otherwise
hidden companion star. X-ray variations are the simplest diagnostic
we have to study the wind-wind collision and allow us to measure the
state of the stellar mass loss from both stars. We present the X-ray
lightcurve over the last 20 years from monitoring observations with
the Rossi X-ray Timing Explorer and the X-ray Telescope on the Swift
satellite, and compare and contrast the behavior of the X-ray emission
from the system over that timespan, including surprising variations
during the 2014 X-ray minimum.

---------------------------------------------------------
Title: Measuring the shock-heating rate in the winds of O stars
using X-ray line spectra
Authors: Cohen, David H.; Li, Zequn; Gayley, Kenneth G.; Owocki,
Stanley P.; Sundqvist, Jon O.; Petit, Véronique; Leutenegger,
Maurice A.
2014MNRAS.444.3729C Altcode: 2014arXiv1409.0856C
We present a new method for using measured X-ray emission line fluxes
from O stars to determine the shock-heating rate due to instabilities
in their radiation-driven winds. The high densities of these winds
means that their embedded shocks quickly cool by local radiative
emission, while cooling by expansion should be negligible. Ignoring
for simplicity any non-radiative mixing or conductive cooling, the
method presented here exploits the idea that the cooling post-shock
plasma systematically passes through the temperature characteristic
of distinct emission lines in the X-ray spectrum. In this way, the
observed flux distribution among these X-ray lines can be used to
construct the cumulative probability distribution of shock strengths
that a typical wind parcel encounters as it advects through the
wind. We apply this new method to Chandra grating spectra from five
O stars with X-ray emission indicative of embedded wind shocks in
effectively single massive stars. The results for all the stars are
quite similar: the average wind mass element passes through roughly
one shock that heats it to at least 106 K as it advects
through the wind, and the cumulative distribution of shock strengths
is a strongly decreasing function of temperature, consistent with a
negative power law of index n ≈ 3, implying a marginal distribution
of shock strengths that scales as T-4, and with hints of
an even steeper decline or cut-off above 107 K.

---------------------------------------------------------
Title: 3D Hydrodynamic &amp; Radiative Transfer Models of X-ray
Emission from Colliding Wind Binaries
Authors: Russell, Christopher M. P.; Okazaki, Atsuo T.; Owocki,
Stanley P.; Corcoran, Michael F.; Hamaguchi, Kenji; Sugawara, Yasuharu
2014efxu.conf..208R Altcode: 2014arXiv1405.4803R
Colliding wind binaries (CWBs) are unique laboratories for X-ray
astrophysics. The massive stars in these systems possess powerful
stellar winds with speeds up to $\sim$3000 km s$^{-1}$, and their
collision leads to hot plasma (up to $\sim10^8$K) that emit thermal
X-rays (up to $\sim$10 keV). Many X-ray telescopes have observed
CWBs, including Suzaku, and our work aims to model these X-ray
observations. We use 3D smoothed particle hydrodynamics (SPH) to model
the wind-wind interaction, and then perform 3D radiative transfer
to compute the emergent X-ray flux, which is folded through X-ray
telescopes' response functions to compare directly with observations. In
these proceedings, we present our models of Suzaku observations of
the multi-year-period, highly eccentric systems $\eta$ Carinae and WR
140. The models reproduce the observations well away from periastron
passage, but only $\eta$ Carinae's X-ray spectrum is reproduced at
periastron; the WR 140 model produces too much flux during this more
complicated phase.

---------------------------------------------------------
Title: Theory of Winds from Hot, Luminous Massive Stars
Authors: Owocki, Stanley
2014arXiv1409.2084O Altcode:
The high luminosities of massive stars drive strong stellar winds,
through line scattering of the star's continuum radiation. This paper
reviews the dynamics of such line driving, building first upon the
standard CAK model for steady winds, and deriving the associated
analytic scalings for the mass loss rate and wind velocity law. It
next summarizes the origin and nature of the strong Line Deshadowing
Instability (LDI) intrinsic to such line-driving, including also the
role of a diffuse-line-drag effect that stabilizes the wind base, and
then describes how both instability and drag are incorporated in the
Smooth Source Function (SSF) method for time-dependent simulations of
the nonlinear evolution of the resulting wind structure. The review
concludes with a discussion of the effect of the resulting extensive
structure in temperature, density and velocity for interpreting
observational diagnostics. In addition to the usual clumping effect on
density-squared diagnostics, the spatial porosity of optically thick
clumps can reduce single-density continuum absorption, and a kind of
velocity porosity, or vorocity, can reduce the absorption strength
of spectral lines. An overall goal is to illuminate the rich physics
of radiative driving and the challenges that lie ahead in developing
dynamical models for the often complex structure and variability of
hot-star winds.

---------------------------------------------------------
Title: The magnetic characteristics of Galactic OB stars from the
MiMeS survey of magnetism in massive stars
Authors: Wade, G. A.; Grunhut, J.; Alecian, E.; Neiner, C.; Aurière,
M.; Bohlender, D. A.; David-Uraz, A.; Folsom, C.; Henrichs, H. F.;
Kochukhov, O.; Mathis, S.; Owocki, S.; Petit, V.; Petit
2014IAUS..302..265W Altcode: 2013arXiv1310.3965W
The Magnetism in Massive Stars (MiMeS) project represents the largest
systematic survey of stellar magnetism ever undertaken. Based on a
sample of over 550 Galactic B and O-type stars, the MiMeS project
has derived the basic characteristics of magnetism in hot, massive
stars. Herein we report preliminary results.

---------------------------------------------------------
Title: Mass loss from inhomogeneous hot star winds. III. An
effective-opacity formalism for line radiative transfer in
accelerating, clumped two-component media, and first results on
theory and diagnostics
Authors: Sundqvist, J. O.; Puls, J.; Owocki, S. P.
2014A&A...568A..59S Altcode: 2014arXiv1405.7800S
 Aims: We provide a fast and easy-to-use formalism for treating
the reduction in effective opacity associated with optically thick
clumps in an accelerating two-component medium. Methods: We
develop and benchmark effective-opacity laws for continuum and line
radiative transfer that bridge the limits of optically thin and thick
clumps. We then use this formalism to i) design a simple method for
modeling and analyzing UV wind resonance lines in hot, massive stars,
and ii) derive simple correction factors to the line force driving
the outflows of such stars. Results: Using a vorosity-modified
Sobolev with exact integration (vmSEI) method, we show that, for a given
ionization factor, UV resonance doublets may be used to analytically
predict the upward corrections in empirically inferred mass-loss rates
associated with porosity in velocity space (a.k.a. velocity-porosity, or
vorosity). However, we also show the presence of a solution degeneracy:
in a two-component clumped wind with given inter-clump medium density,
there are always two different solutions producing the same synthetic
doublet profile. We demonstrate this by application to SiIV and PV in B
and O supergiants and derive, for an inter-clump density set to 1% of
the mean density, upward empirical mass-loss corrections of typically
factors of either ~5 or ~50, depending on which of the two solutions
is chosen. Overall, our results indicate that this solution dichotomy
severely limits the use of UV resonance lines as direct mass-loss
indicators in current diagnostic models of clumped hot stellar winds. We
next apply the effective line-opacity formalism to the standard CAK
theory of line-driven winds. A simple vorosity correction factor to
the CAK line force is derived, which for normalized velocity filling
factor fvel simply scales as fvelα,
where α is the slope of the CAK line-strength distribution function. By
analytic and numerical hydrodynamics calculations, we further show that
in cases where vorosity is important at the critical point setting the
mass-loss rate, the reduced line force leads to a lower theoretical
mass loss, by simply a factor fvel. On the other hand, if
vorosity is important only above this critical point, the predicted
mass loss is not affected, but the wind terminal speed is reduced,
by a factor scaling as fvelα/(2-2α). This
shows that porosity in velocity space can have a significant impact
not only on the diagnostics, but also on the dynamics and theory of
radiatively driven winds.

---------------------------------------------------------
Title: X-rays from magnetic massive OB stars
Authors: Petit, V.; Cohen, D. H.; Nazé, Y.; Gagné, M.; Townsend,
R. H. D.; Leutenegger, M. A.; ud-Doula, A.; Owocki, S. P.; Wade, G. A.
2014IAUS..302..330P Altcode:
The magnetic activity of solar-type and low-mass stars is a well known
source of coronal X-ray emission. At the other end of the main sequence,
X-rays emission is instead associated with the powerful, radiatively
driven winds of massive stars. Indeed, the intrinsically unstable
line-driving mechanism of OB star winds gives rise to shock-heated,
soft emission (~0.5 keV) distributed throughout the wind. Recently, the
latest generation of spectropolarimetric instrumentation has uncovered
a population of massive OB-stars hosting strong, organized magnetic
fields. The magnetic characteristics of these stars are similar to
the apparently fossil magnetic fields of the chemically peculiar ApBp
stars. Magnetic channeling of these OB stars' strong winds leads to
the formation of large-scale shock-heated magnetospheres, which can
modify UV resonance lines, create complex distributions of cooled
Halpha emitting material, and radiate hard (~2-5 keV) X-rays. This
presentation summarizes our coordinated observational and modelling
efforts to characterize the manifestation of these magnetospheres in
the X-ray domain, providing an important contrast between the emission
originating in shocks associated with the large-scale fossil fields of
massive stars, and the X-rays associated with the activity of complex,
dynamo-generated fields in lower-mass stars.

---------------------------------------------------------
Title: Wind channeling, magnetospheres, and spindown of magnetic
massive stars
Authors: Owocki, S. P.; ud-Doula, A.; Townsend, R. H. D.; Petit, V.;
Sundqvist, J. O.; Cohen, D. H.
2014IAUS..302..320O Altcode:
A subpopulation (~10%) of hot, luminous, massive stars have been
revealed through spectropolarimetry to harbor strong (hundreds to tens
of thousand Gauss), steady, large-scale (often significantly dipolar)
magnetic fields. This review focuses on the role of such fields
in channeling and trapping the radiatively driven wind of massive
stars, including both in the strongly perturbed outflow from open
field regions, and the wind-fed “magnetospheres” that develop from
closed magnetic loops. For B-type stars with weak winds and moderately
fast rotation, one finds “centrifugal magnetospheres”, in which
rotational support allows magnetically trapped wind to accumulate
to a large density, with quite distinctive observational signatures,
e.g. in Balmer line emission. In contrast, more luminous O-type stars
have generally been spun down by magnetic braking from angular momentum
loss in their much stronger winds. The lack of centrifugal support means
their closed loops form a “dynamical magnetosphere”, with trapped
material falling back to the star on a dynamical timescale; nonetheless,
the much stronger wind feeding leads to a circumstellar density that
is still high enough to give substantial Balmer emission. Overall, this
review describes MHD simulations and semi-analytic dynamical methods for
modeling the magnetospheres, the magnetically channeled wind outflows,
and the associated spin-down of these magnetic massive stars.

---------------------------------------------------------
Title: X-ray emission from magnetic massive stars
Authors: Naze, Y.; Pétit, V.; Rindbrand, M.; Cohen, D.; Owocki, S.;
ud-Doula, A.; Wade, G.; Rauw, G.
2014xru..confE.292N Altcode:
Magnetically confined winds of early-type stars are expected to
be sources of bright and hard X-rays. In an attempt to clarify the
systematics of the observed X-ray properties, we have analyzed a large
series of Chandra and XMM observations, corresponding to over 100
exposures of 60% of the known magnetic massive stars listed recently
by Petit et al. (2013). We notably show that the X-ray luminosity is
strongly correlated with mass-loss rate, in agreement with predictions
of magnetically confined wind models. We also investigated the
behavior of other X-ray properties (plasma temperature, absorption,
variability), yielding additional constraints on models. This work not
only advances our knowledge of the X-ray emission of massive stars,
but also suggests new observational and theoretical avenues to further
explore magnetically confined winds.

---------------------------------------------------------
Title: X-rays from Magnetically Confined Wind Shocks: Effect of
Cooling-Regulated Shock Retreat
Authors: ud-Doula, A.; Owocki, S.; Townsend, R.; Pétit, V.; Cohen, D.
2014xru..confE.345U Altcode:
We use 2D MHD simulations to examine the effects of radiative cooling
and inverse Compton (IC) cooling on X-ray emission from magnetically
confined wind shocks (MCWS) in magnetic massive stars with radiatively
driven stellar winds. For the standard dependence of mass loss rate on
luminosity {dot M} ∼ L^{1.7} , the scaling of IC cooling with L and
radiative cooling with {dot M} means that IC cooling become formally
more important for lower luminosity stars. However, the overall effect
of including IC cooling is quite modest. But for stars with high enough
mass loss to keep the shocks radiative, the MHD simulations indicate a
linear scaling of X-ray luminosity with mass loss rate; but for lower
luminosity stars with weak winds, X-ray emission is reduced and softened
by a shock retreat resulting from the larger post-shock cooling length,
which within the fixed length of a closed magnetic loop forces the
shock back to lower pre-shock wind speeds. A semi-analytic scaling
yields X-ray luminosities that are in close agreement to time-averages
computed from the MHD simulations. The results here provide a good
basis for interpreting available X-ray observations from the growing
list of massive stars with confirmed large-scale magnetic fields.

---------------------------------------------------------
Title: X-rays from magnetically confined wind shocks: effect of
cooling-regulated shock retreat
Authors: ud-Doula, Asif; Owocki, Stanley; Townsend, Richard; Petit,
Veronique; Cohen, David
2014MNRAS.441.3600U Altcode: 2014arXiv1404.5336U
We use 2D magnetohydrodynamic (MHD) simulations to examine the effects
of radiative cooling and inverse Compton (IC) cooling on X-ray emission
from magnetically confined wind shocks (MCWS) in magnetic massive stars
with radiatively driven stellar winds. For the standard dependence of
mass-loss rate on luminosity Ṁ∼ L1.7, the scaling of IC
cooling with L and radiative cooling with Ṁ means that IC cooling
become formally more important for lower luminosity stars. However,
because the sense of the trends is similar, we find the overall
effect of including IC cooling is quite modest. More significantly,
for stars with high enough mass-loss to keep the shocks radiative,
the MHD simulations indicate a linear scaling of X-ray luminosity with
mass-loss rate; but for lower luminosity stars with weak winds, X-ray
emission is reduced and softened by a shock retreat resulting from
the larger post-shock cooling length, which within the fixed length of
a closed magnetic loop forces the shock back to lower pre-shock wind
speeds. A semi-analytic scaling analysis that accounts both for the wind
magnetic confinement and this shock retreat yields X-ray luminosities
that have a similar scaling trend, but a factor few higher values,
compared to time-averages computed from the MHD simulations. The
simulation and scaling results here thus provide a good basis for
interpreting available X-ray observations from the growing list of
massive stars with confirmed large-scale magnetic fields.

---------------------------------------------------------
Title: Measuring mass-loss rates and constraining shock physics
using X-ray line profiles of O stars from the Chandra archive
Authors: Cohen, David H.; Wollman, Emma E.; Leutenegger, Maurice
A.; Sundqvist, Jon O.; Fullerton, Alex W.; Zsargó, Janos; Owocki,
Stanley P.
2014MNRAS.439..908C Altcode: 2014MNRAS.tmp..213C; 2014arXiv1401.7995C
We quantitatively investigate the extent of wind absorption signatures
in the X-ray grating spectra of all non-magnetic, effectively single O
stars in the Chandra archive via line profile fitting. Under the usual
assumption of a spherically symmetric wind with embedded shocks,
we confirm previous claims that some objects show little or no
wind absorption. However, many other objects do show asymmetric and
blueshifted line profiles, indicative of wind absorption. For these
stars, we are able to derive wind mass-loss rates from the ensemble
of line profiles, and find values lower by an average factor of 3 than
those predicted by current theoretical models, and consistent with Hα
if clumping factors of fcl ≈ 20 are assumed. The same
profile fitting indicates an onset radius of X-rays typically at r
≈ 1.5R*, and terminal velocities for the X-ray emitting
wind component that are consistent with that of the bulk wind. We
explore the likelihood that the stars in the sample that do not show
significant wind absorption signatures in their line profiles have
at least some X-ray emission that arises from colliding wind shocks
with a close binary companion. The one clear exception is ζ Oph,
a weak-wind star that appears to simply have a very low mass-loss
rate. We also reanalyse the results from the canonical O supergiant ζ
Pup, using a solar-metallicity wind opacity model and find dot{M}= 1.8
× 10^{-6} {M_{⊙ {yr^{-1}}}}, consistent with recent multiwavelength
determinations.

---------------------------------------------------------
Title: Suppression of X-rays from radiative shocks by their thin-shell
instability
Authors: Kee, Nathaniel Dylan; Owocki, Stanley; ud-Doula, Asif
2014MNRAS.438.3557K Altcode: 2014MNRAS.tmp..136K; 2014arXiv1401.2063K
We examine X-rays from radiatively cooled shocks, focusing on how their
thin-shell instability reduces X-ray emission. For 2D simulations of
collision between equal expanding winds, we carry out a parameter
study of such instability as a function of the ratio of radiative
versus adiabatic-expansion cooling lengths. In the adiabatic regime,
the extended cooling layer suppresses instability, leading to planar
shock compression with X-ray luminosity that follows closely the
expected (L_X ∼ dot{M}^2) quadratic scaling with mass-loss rate
dot{M}. In the strongly radiative limit, the X-ray emission now follows
an expected linear scaling with mass-loss (L_X ∼ dot{M}), but the
instability deforms the shock compression into extended shear layers
with oblique shocks along fingers of cooled, dense material. The spatial
dispersion of shock thermalization limits strong X-ray emission to the
tips and troughs of the fingers, and so reduces the X-ray emission
(here by about a factor 1/50) below what is expected from analytic
radiative-shock models without unstable structure. Between these two
limits, X-ray emission can switch between a high-state associated
with extended shock compression, and a low-state characterized by
extensive shear. Further study is needed to clarify the origin of this
`shear mixing reduction factor' in X-ray emission, and its dependence
on parameters like the shock Mach number.

---------------------------------------------------------
Title: The Delta Ori Very Large Project: X-ray Emission and Stellar
Variability
Authors: Corcoran, Michael F.; Nichols, J. S.; Moffat, A. F.;
Richardson, N.; Pollock, A.; Gull, T. R.; Hamaguchi, K.; Russell,
C. M.; Evans, N. R.; Owocki, S. P.; Waldron, W. L.; Hoffman, J. L.;
Lomax, J. R.; Gayley, K. G.; Oskinova, L.; Hamann, W.; Iping, R.;
Ignace, R.; Naze, Y.; Leutenegger, M. A.; Hole, T.
2014AAS...22315114C Altcode:
Delt Ori is the nearest massive, single-lined eclipsing binary
(O9. II+OB, $P=5^{d}.7324$). High resolution X-ray spectrometry combined
with high-precision photometry from space offers a unique opportunity
to geometrically measure the dynamics of the shocked gas around the
primary star. We summarize our recent campaign of phase-constrained
high-resolution X-ray spectra obtained with the CHANDRA/HETGS plus
high-precision optical photometry with MOST. These observations provide
local measurement of the distribution of the embedded, X-ray emitting
shocks in the wind of an O star via radial velocity variations and
occultation effects, along with standard $f/i$ ratio diagnostics,
and enable us to look for correlations with the broad-band photometric
variability. We discuss how these observations can help determine the
primary star's clumping-corrected mass loss rate, and resolve critical
uncertainties in our understanding of the connection between stellar
and mass loss parameters.

---------------------------------------------------------
Title: Constraints on decreases in Eta Carinae's mass loss from 3D
SPH simulations of its binary colliding winds
Authors: Madura, Thomas; Gull, T. R.; Okazaki, A. T.; Russell, C. M.;
Owocki, S. P.; Groh, J. H.; Corcoran, M. F.; Hamaguchi, K.; Teodoro, M.
2014AAS...22315526M Altcode:
Recent work suggests that the mass-loss rate of the LBV primary star in
the massive, highly eccentric ( 0.9) colliding wind binary Eta Carinae
dropped by a factor of 2-3 between 1999 and 2010. We present results
from large- (±1545 au) and small- (±155 au) domain, 3D smoothed
particle hydrodynamics (SPH) simulations of Eta Car’s colliding winds
for three primary mass-loss rates (2.4, 4.8, and 8.5 × 10^-4 Msun/yr),
investigating the effects on the dynamics of the binary wind-wind
collision (WWC). These simulations include orbital motion, optically
thin radiative cooling, and radiative forces. We find that the primary'
mass-loss rate greatly affects the time-dependent hydrodynamics at
all spatial scales investigated. The simulations also show that the
post-shock wind of the companion star switches from the adiabatic to
the radiative-cooling regime during periastron passage. This switchover
is caused by the encroachment of the wind of the primary into the
acceleration zone of the companion's wind, plus radiative inhibition
of the companion’s wind by the super-luminous primary. The SPH
simulations together with 1D radiative transfer models of the stellar
spectra reveal that a factor of two or more drop in primary mass-loss
rate should lead to substantial changes in numerous multiwavelength
observables. Recent observations are not fully consistent with the
model predictions, indicating that any drop in mass-loss rate was likely
by a factor &lt; 2 and occurred after 2004. We speculate that most of
the recent observed changes in Eta Car are due to a small increase in
the WWC opening angle that produces significant effects because our
line-of-sight to the system lies close to the dense walls of the WWC
zone. A modest decrease in primary mass-loss rate may be responsible,
but changes in the wind/stellar parameters of the companion cannot
yet be fully ruled out. We suggest observations during Eta Car’s
next periastron in 2014 to further test for decreases in mass-loss
rate. If the primary's mass-loss rate is declining and continues to
do so, the 2014 X-ray minimum should be even shorter than that of 2009.

---------------------------------------------------------
Title: Constraints on decreases in η Carinae's mass-loss from 3D
hydrodynamic simulations of its binary colliding winds
Authors: Madura, T. I.; Gull, T. R.; Okazaki, A. T.; Russell, C. M. P.;
Owocki, S. P.; Groh, J. H.; Corcoran, M. F.; Hamaguchi, K.; Teodoro, M.
2013MNRAS.436.3820M Altcode: 2013MNRAS.tmp.2552M; 2013arXiv1310.0487M
Recent work suggests that the mass-loss rate of the primary star
ηA in the massive colliding wind binary η Carinae
dropped by a factor of 2-3 between 1999 and 2010. We present results
from large- (±1545 au) and small- (±155 au) domain, 3D smoothed
particle hydrodynamics (SPH) simulations of η Car's colliding winds
for three ηA mass-loss rates (dot{M}_{η A}=
2.4, 4.8 and 8.5 × 10-4 M⊙ yr-1),
investigating the effects on the dynamics of the binary wind-wind
collision (WWC). These simulations include orbital motion, optically
thin radiative cooling and radiative forces. We find that dot{M}_{η
A} greatly affects the time-dependent hydrodynamics at
all spatial scales investigated. The simulations also show that the
post-shock wind of the companion star ηB switches from the
adiabatic to the radiative-cooling regime during periastron passage
(φ ≈ 0.985-1.02). This switchover starts later and ends earlier
the lower the value of dot{M}_{η A} and is caused by the
encroachment of the wind of ηA into the acceleration zone
of ηB's wind, plus radiative inhibition of ηB's
wind by ηA. The SPH simulations together with 1D radiative
transfer models of ηA's spectra reveal that a factor of
2 or more drop in dot{M}_{η A} should lead to substantial
changes in numerous multiwavelength observables. Recent observations
are not fully consistent with the model predictions, indicating that
any drop in dot{M}_{η A} was likely by a factor of ≲2
and occurred after 2004. We speculate that most of the recent observed
changes in η Car are due to a small increase in the WWC opening
angle that produces significant effects because our line of sight to
the system lies close to the dense walls of the WWC zone. A modest
decrease in dot{M}_{η A} may be responsible, but changes
in the wind/stellar parameters of ηB, while less likely,
cannot yet be fully ruled out. We suggest observations during η Car's
next periastron in 2014 to further test for decreases in dot{M}_{η
A}. If dot{M}_{η A} is declining and continues to
do so, the 2014 X-ray minimum should be even shorter than that of 2009.

---------------------------------------------------------
Title: Erratum: "Constraints on Porosity and Mass Loss in O-star
Winds from Modeling of X-Ray Emission Line Profile Shapes" <A
href="/abs/2013ApJ...770...80L">(2013, ApJ, 770, 80)</A>
Authors: Leutenegger, Maurice A.; Cohen, David H.; Sundqvist, Jon O.;
Owocki, Stanley P.
2013ApJ...777...81L Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On magnetic inhibition of photospheric macroturbulence
generated in the iron-bump opacity zone of O-stars
Authors: Sundqvist, J. O.; Petit, V.; Owocki, S. P.; Wade, G. A.;
Puls, J.; MiMeS Collaboration
2013MNRAS.433.2497S Altcode: 2013arXiv1305.5549S; 2013MNRAS.tmp.1608S
Massive, hot OB-stars show clear evidence of strong macroscopic
broadening (in addition to rotation) in their photospheric spectral
lines. This paper examines the occurrence of such `macroturbulence'
in slowly rotating O-stars with strong, organized surface magnetic
fields. Focusing on the C IV 5811 Å line, we find evidence for
significant macroturbulent broadening in all stars except NGC 1624-2,
which also has (by far) the strongest magnetic field. Instead, the
very sharp C IV lines in NGC 1624-2 are dominated by magnetic Zeeman
broadening, from which we estimate a dipolar field ∼20 kG. By
contrast, magnetic broadening is negligible in the other stars (due
to their weaker field strengths, on the order of 1 kG), and their C
IV profiles are typically very broad and similar to corresponding
lines observed in non-magnetic O-stars. Quantifying this by an
isotropic, Gaussian macroturbulence, we derive vmac =
2.2 ±^{0.9}_{2.2} km s-1 for NGC 1624 and vmac
≈ 20-65 km s-1 for the rest of the magnetic sample. We use these observational results to test the hypothesis that the
field can stabilize the atmosphere and suppress the generation of
macroturbulence down to stellar layers where the magnetic pressure
PB and the gas pressure Pg are comparable. Using
a simple grey atmosphere to estimate the temperature T0
at which PB = Pg, we find that T0
&gt; Teff for all investigated magnetic stars, but that
T0 reaches the ∼ 160 000 K layers associated with the
iron opacity bump in hot stars only for NGC 1624-2. This is consistent
with the view that the responsible physical mechanism for photospheric
O-star macroturbulence may be stellar gravity-mode oscillations excited
by sub-surface convection zones, and it suggests that a sufficiently
strong magnetic field can suppress such iron-bump generated convection
and associated pulsational excitation.

---------------------------------------------------------
Title: The Role of Thin-Shell Mixing in X-Ray Production in the
Winds of Massive Stars
Authors: Kee, Nathaniel; Owocki, Stanley; ud-Doula, Asif
2013msao.confE..76K Altcode:
Strong, X-ray emitting shocks are an inherent feature of the hot,
dense winds of luminous, massive stars. These shocks develop both
due to intrinsic instabilities in the radiative driving and collision
with the winds of a binary companion. In regions of high density where
the shocks are radiative, the resulting narrowness of the the shock
cooling region makes it subject to strong thin-shell instabilities. This
poster presents 2D hydrodynamic modelling of the associated mixing of
hot and cool gas as well as its effect on X-ray emission. An overall
goal is to develop and test simple scaling relations for how thin-shell
mixing reduces both the hardness and luminosity of X-ray emission from
radiative shocks.

---------------------------------------------------------
Title: Constraints on Porosity and Mass Loss in O-star Winds from
the Modeling of X-Ray Emission Line Profile Shapes
Authors: Leutenegger, Maurice A.; Cohen, David H.; Sundqvist, Jon O.;
Owocki, Stanley P.
2013ApJ...770...80L Altcode: 2013arXiv1305.5595L
We fit X-ray emission line profiles in high resolution XMM-Newton and
Chandra grating spectra of the early O supergiant ζ Pup with models
that include the effects of porosity in the stellar wind. We explore
the effects of porosity due to both spherical and flattened clumps. We
find that porosity models with flattened clumps oriented parallel to
the photosphere provide poor fits to observed line shapes. However,
porosity models with isotropic clumps can provide acceptable fits to
observed line shapes, but only if the porosity effect is moderate. We
quantify the degeneracy between porosity effects from isotropic clumps
and the mass-loss rate inferred from the X-ray line shapes, and we show
that only modest increases in the mass-loss rate (lsim 40%) are allowed
if moderate porosity effects (h ∞ &lt;~ R *)
are assumed to be important. Large porosity lengths, and thus strong
porosity effects, are ruled out regardless of assumptions about clump
shape. Thus, X-ray mass-loss rate estimates are relatively insensitive
to both optically thin and optically thick clumping. This supports the
use of X-ray spectroscopy as a mass-loss rate calibration for bright,
nearby O stars.

---------------------------------------------------------
Title: Using 3D Dynamic Models to Reproduce X-ray Properties of
Colliding Wind Binaries
Authors: Russell, C. M. P.; Okazaki, A. T.; Owocki, S. P.; Corcoran,
M. F.; Madura, T. I.; Leyder, J. -C.; Hamaguchi, K.
2013msao.confE.182R Altcode:
Colliding wind binaries (CWBs) are unique laboratories for X-ray
astrophysics. Their wind-wind collisions produce hard X-rays that
have been monitored extensively by several X-ray telescopes, such
as RXTE, XMM, and Chandra. To interpret these X-ray light curves and
spectra, we model the wind-wind interaction using 3D smoothed particle
hydrodynamics (SPH), which incorporates radiative cooling and uses an
anti-gravity approach to accelerate the winds according a β-law, and
then solve the 3D formal solution of radiative transfer to synthesize
the model X-ray properties. The results for the multi-year-period,
highly eccentric CWBs η Carinae and WR140 match well the 2-10 keV
RXTE light curve, hardness ratio, and dynamic spectra. This includes
η Car's ∼3-month-long X-ray minimum associated with the 1998.0 and
2003.5 periastron passages, which we find to occur as the primary
wind encroaches into the secondary wind's acceleration region, and
thus quenches the high temperature gas between the stars. Furthermore,
the η Car modeling suggests the commonly inferred primary mass loss
rate of ∼10^-3 Mo/yr, provides further evidence that the observer
is mainly viewing the system through the secondary's shock cone, and
suggests that periastron occurs ∼1 month after the onset of the X-ray
minimum. For WR140, the decrease in model X-rays around periastron is
less than observed, but there is very good agreement with the observed
XMM spectrum taken on the rise before periastron. We also model the
short-period (2.67 day) CWB HD150136, which harbors the nearest O3
star. The imbalance of the wind strengths suggests a “wind-star”
collision as the primary wind reaches the secondary star's surface, even
when accounting for radiative braking, thus producing high-temperature,
X-ray-emitting gas in a shock cone flowing around the surface of the
secondary star. This model qualitatively reproduces the dip in X-ray
emission associated with superior conjunction observed by Chandra, as
well as an asymmetry around inferior conjunction due to the difference
in occulting the leading and trailing-arms of the wind-star shock. We
also discuss our preliminary results of accelerating the stellar winds
according to CAK theory in the SPH code.

---------------------------------------------------------
Title: The SimpleX Radiative Transfer Algorithm Applied to 3D SPH
Simulations of Eta Car's Colliding Winds
Authors: Clementel, Nicola; Madura, Thomas; Gull, Theodore; Groh,
Jose; Kruip, Chael; Owocki, Stanley; Okazaki, Atsuo; Icke, Vincent
2013msao.confE.144C Altcode:
Eta Carinae is a peculiar object that allow us to study, among other
topics, massive binary interactions and evolution and binary colliding
winds. In order to improve our knowledge of the system, we need to
generate synthetic observations and compare them with the already
available and future HST/STIS data. We present initial results from
full 3D radiative transfer post-processing of 3D SPH hydrodynamical
simulations of the interacting winds of Eta Carinae. We use SimpleX
algorithm to obtain the ionization fractions of hydrogen and helium,
this results in ionization maps of both species that constrain the
regions where these lines can form. These results will allow us to
put constraints on the number of ionizing photons coming from the
companion. This construction of synthetic observations allows us to
obtain insight into the highly complex 3D flows in Eta, from the shape
of the ionized volume and its resulting optical/spectral appearance.

---------------------------------------------------------
Title: Effects of the LBV Primary Star's Mass Loss Rate on Eta
Carinae's Spatially-Extended, Time-Variable Massive Binary Colliding
Winds
Authors: Madura, Thomas; Gull, Theodore; Corcoran, Michael; Owocki,
Stanley; Groh, Jose; Hillier, John; Hamaguchi, Kenji; Okazaki, Atsuo;
Russell, Christopher; Clementel, Nicola
2013msao.confE.164M Altcode:
At the heart of Eta Carinae's spectacular bipolar “Homunculus" nebula
lies an extremely luminous (5 × 10^6 Lo) colliding wind binary with a
highly eccentric (e ~ 0.9), 5.54-year orbit and a total mass ≥ 110
Mo. Our closest (D ~ 2.3 kpc) and best example of a pre-hypernova or
pre-gamma ray burst environment, Eta Carinae is an ideal astrophysical
laboratory for studying massive binary interactions, stellar wind-wind
collisions, and massive star evolution. Over the past fifteen years,
the central source has brightened by one stellar magnitude in the
optical. Changes in P Cygni wind profiles such as Hα have also been
observed. Extended X-ray monitoring and spectroscopy further indicate
that the hydrogen column density in line of sight dropped two-fold
by the 2009 periastron event. A simple interpretation for the cause
of these changes is a factor of two drop in the mass-loss rate of the
LBV primary star over the last decade and a half. However, 1D CMFGEN
radiative transfer models predict that such a large change in the
primary mass-loss rate would lead to huge changes in the observed
stellar spectrum. In an effort to further investigate the effects
that a change in the mass-loss rate of the primary would have on the
system, we performed large- (±1620 au) and small- (±162 au) scale
full 3D smoothed particle hydrodynamic (SPH) simulations of Eta Car's
binary colliding winds assuming three different mass-loss rates for
the LBV primary. Results from these new 3D simulations (which include
optically-thin radiative cooling and radiation-driven stellar winds)
combined with various multi-dimensional radiative transfer codes
indicate that a factor of two change in the mass-loss rate of the
primary should lead to significant changes in various X-ray, UV,
Optical, and near-IR diagnostics. We discuss what changes are expected
and why, and offer some possible explanations for the recent changes
observed in Eta Car. We also discuss the importance of constraining
this likely supernova progenitor's recent mass loss history.

---------------------------------------------------------
Title: Populations of rotating stars. II. Rapid rotators and their
link to Be-type stars
Authors: Granada, A.; Ekström, S.; Georgy, C.; Krtička, J.; Owocki,
S.; Meynet, G.; Maeder, A.
2013A&A...553A..25G Altcode: 2013arXiv1303.2393G
Context. Even though it is broadly accepted that single Be stars are
rapidly rotating stars surrounded by a flat rotating circumstellar
disk, there is still a debate about how fast these stars rotate and
also about the mechanisms involved in the angular-momentum and mass
input in the disk. Aims: We study the properties of stars
that rotate near their critical-rotation rate and investigate the
properties of the disks formed by equatorial mass ejections. 
Methods: We used the most recent Geneva stellar evolutionary tracks
for rapidly rotating stars that reach the critical limit and used a
simple model for the disk structure. Results: We obtain that
for a 9 M⊙ star at solar metallicity, the minimum average
velocity during the main-sequence (MS) phase to reach the critical
velocity is around 330 km s-1, whereas it would be 390
km s-1 at the metallicity of the Small Magellanic Cloud
(SMC). Red giants or supergiants originating from very rapid rotators
rotate six times faster and show N/C ratios three times higher than
those originating from slowly rotating stars. This difference becomes
stronger at lower metallicity. It might therefore be very interesting
to study the red giants in clusters that show a large number of Be
stars on the MS band. On the basis of our single-star models, we show
that the observed Be-star fraction with cluster age is compatible with
the existence of a temperature-dependent lower limit in the velocity
rate required for a star to become a Be star. The mass, extension,
and diffusion time of the disks produced when the star is losing
mass at the critical velocity, obtained from simple parametrized
expressions, are estimated to be between 9.4 × 10-12
and 1.4 × 10-7 M⊙ (3 × 10-6
to 4.7 × 10-2 times the mass of the Earth), 2000 and
6500 R⊙, and 10 and 30 yr. These values are not too
far from those estimated for disks around Be-type stars. At a given
metallicity, the mass and the extension of the disk increase with the
initial mass and with age on the MS phase. Denser disks are expected
in low-metallicity regions.

---------------------------------------------------------
Title: Constraints on Porosity and Mass Loss in O-star Winds from
Modeling of X-ray Emission Line Profile Shapes
Authors: Leutenegger, Maurice A.; Cohen, D. H.; Sundqvist, J.; Owocki,
S. P.
2013HEAD...1312501L Altcode:
Spectrally resolved X-ray line shapes in massive stars provide
important diagnostics of X-ray production mechanisms and they have
also, surprisingly, been used to make some of the most accurate and
model-independent wind mass-loss rate estimates. Measurements of several
nearby O stars using the grating spectrometers onboard {\it Chandra}
and XMM-{\it Newton} have revised downward the mass-loss rates of O
stars, with implications for stellar evolution and the energy budget
in clusters. But if these winds are porous, then the X-ray mass-loss
rates might be subject to systematic underestimates. Here we present a
formalism for modeling the effects of wind porosity on X-ray emission
line profiles, and fit these models to Chandra and XMM observations
of $\zeta$ Pup. We find that strong porosity effects are ruled out,
and for moderate porosity we quantify the degeneracy between assumed
porosity length and derived mass-loss rate. We conclude that mass-loss
rates derived from fitting X-ray line profiles assuming no porosity
effects are overestimated by at most 50\% if moderate porosity effects
are indeed important.

---------------------------------------------------------
Title: Thin-shell mixing in radiative wind-shocks and the
Lx ∼ Lbol scaling of O-star X-rays
Authors: Owocki, S. P.; Sundqvist, J. O.; Cohen, D. H.; Gayley, K. G.
2013MNRAS.429.3379O Altcode: 2013MNRAS.tmp..501O; 2012arXiv1212.4235O
X-ray satellites since Einstein have empirically established that the
X-ray luminosity from single O-stars scales linearly with bolometric
luminosity, Lx ∼ 10-7Lbol. But
straightforward forms of the most favoured model, in which X-rays
arise from instability-generated shocks embedded in the stellar
wind, predict a steeper scaling, either with mass-loss rate L_x ∼
dot{M}∼ L_bol^{1.7} if the shocks are radiative or with L_x ∼
dot{M}2 ∼ L_bol^{3.4} if they are adiabatic. This paper
presents a generalized formalism that bridges these radiative versus
adiabatic limits in terms of the ratio of the shock cooling length to
the local radius. Noting that the thin-shell instability of radiative
shocks should lead to extensive mixing of hot and cool material,
we propose that the associated softening and weakening of the X-ray
emission can be parametrized as scaling with the cooling length ratio
raised to a power m, the `mixing exponent'. For physically reasonable
values m ≈ 0.4, this leads to an X-ray luminosity L_x ∼ dot{M}^{0.6}
∼ L_bol that matches the empirical scaling. To fit observed X-ray
line profiles, we find that such radiative-shock-mixing models require
the number of shocks to drop sharply above the initial shock onset
radius. This in turn implies that the X-ray luminosity should saturate
and even decrease for optically thick winds with very high mass-loss
rates. In the opposite limit of adiabatic shocks in low-density winds
(e.g. from B-stars), the X-ray luminosity should drop steeply with
dot{M}^2. Future numerical simulation studies will be needed to test
the general thin-shell mixing ansatz for X-ray emission.

---------------------------------------------------------
Title: A magnetic confinement versus rotation classification of
massive-star magnetospheres
Authors: Petit, V.; Owocki, S. P.; Wade, G. A.; Cohen, D. H.;
Sundqvist, J. O.; Gagné, M.; Maíz Apellániz, J.; Oksala, M. E.;
Bohlender, D. A.; Rivinius, T.; Henrichs, H. F.; Alecian, E.; Townsend,
R. H. D.; ud-Doula, A.; MiMeS Collaboration
2013MNRAS.429..398P Altcode: 2012arXiv1211.0282P
Building on results from the Magnetism in Massive Stars (MiMeS) project,
this paper shows how a two-parameter classification of massive-star
magnetospheres in terms of the magnetic wind confinement (which sets
the Alfvén radius RA) and stellar rotation (which sets the
Kepler co-rotation radius RK) provides a useful organization
of both observational signatures and theoretical predictions. We
compile the first comprehensive study of inferred and observed values
for relevant stellar and magnetic parameters of 64 confirmed magnetic OB
stars with Teff ≳ 16 kK. Using these parameters, we locate
the stars in the magnetic confinement-rotation diagram, a log-log plot
of RK versus RA. This diagram can be subdivided
into regimes of centrifugal magnetospheres (CM), with RA &gt;
RK, versus dynamical magnetospheres (DM), with RK
&gt; RA. We show how key observational diagnostics, like
the presence and characteristics of Hα emission, depend on a star's
position within the diagram, as well as other parameters, especially
the expected wind mass-loss rates. In particular, we identify two
distinct populations of magnetic stars with Hα emission: namely,
slowly rotating O-type stars with narrow emission consistent with
a DM, and more rapidly rotating B-type stars with broader emission
associated with a CM. For O-type stars, the high mass-loss rates
are sufficient to accumulate enough material for line emission even
within the relatively short free-fall time-scale associated with a DM:
this high mass-loss rate also leads to a rapid magnetic spindown of
the stellar rotation. For the B-type stars, the longer confinement
of a CM is required to accumulate sufficient emitting material from
their relatively weak winds, which also lead to much longer spindown
time-scales. Finally, we discuss how other observational diagnostics,
e.g. variability of UV wind lines or X-ray emission, relate to the
inferred magnetic properties of these stars, and summarize prospects for
future developments in our understanding of massive-star magnetospheres.

---------------------------------------------------------
Title: Clumping in the inner winds of hot, massive stars from
hydrodynamical line-driven instability simulations
Authors: Sundqvist, Jon O.; Owocki, Stanley P.
2013MNRAS.428.1837S Altcode: 2012arXiv1210.1861S; 2012MNRAS.tmp..144S
We investigate the effects of stellar limb darkening and photospheric
perturbations for the onset of wind structure arising from the strong,
intrinsic line-deshadowing instability (LDI) of a line-driven stellar
wind. A linear perturbation analysis shows that including limb darkening
reduces the stabilizing effect of the diffuse radiation, leading
to a net instability growth rate even at the wind base. Numerical
radiation-hydrodynamics simulations of the non-linear evolution of
this instability then show that, in comparison with previous models
assuming a uniformly bright star without base perturbations, wind
structure now develops much closer (r ≲ 1.1R⋆) to the
photosphere. This is in much better agreement with observations of
O-type stars, which typically indicate the presence of strong clumping
quite near the wind base.

---------------------------------------------------------
Title: X-ray Parameters of WR 140 from the RXTE Monitoring Campaign
Authors: Lomax, Jamie R.; Corcoran, M. F.; Pollock, A.; Hoffman, J. L.;
Moffat, A.; Owocki, S. P.; Pittard, J.; Russell, C. M.; Williams, P. M.
2013AAS...22114235L Altcode:
WR 140 is a colliding wind binary star system with an 8-year period and
large eccentricity (0.9) that offers a unique testing ground for the
physics of shocks, in large part due to the strongly variable separation
between the two stars. The system is a made up of a very broad-line,
carbon-rich Wolf-Rayet star and a luminous, hot O-type star, both of
which have terminal velocities of approximately 3000 km/s. Besides
coming from each of the winds, strong X-rays are generated in the
shock where the two winds collide and provide a direct measure of wind
parameters within the system. We present data regularly taken over
11 years with the RXTE satellite. We discuss our spectral fitting and
light curve analysis of these data with emphasis on their implications
for the properties of the winds in WR 140.

---------------------------------------------------------
Title: Discovery of a magnetic field in the rapidly rotating O-type
secondary of the colliding-wind binary HD 47129 (Plaskett's star)
Authors: Grunhut, J. H.; Wade, G. A.; Leutenegger, M.; Petit, V.;
Rauw, G.; Neiner, C.; Martins, F.; Cohen, D. H.; Gagné, M.; Ignace,
R.; Mathis, S.; de Mink, S. E.; Moffat, A. F. J.; Owocki, S.; Shultz,
M.; Sundqvist, J.; MiMeS Collaboration
2013MNRAS.428.1686G Altcode: 2012arXiv1209.6326G
We report the detection of a strong, organized magnetic field in the
secondary component of the massive O8III/I+O7.5V/III double-lined
spectroscopic binary system HD 47129 (Plaskett's star) in the context
of the Magnetism in Massive Stars survey. Eight independent Stokes
V observations were acquired using the Echelle SpectroPolarimetric
Device for the Observations of Stars (ESPaDOnS) spectropolarimeter at
the Canada-France-Hawaii Telescope and the Narval spectropolarimeter at
the Télescope Bernard Lyot. Using least-squares deconvolution we obtain
definite detections of signal in Stokes V in three observations. No
significant signal is detected in the diagnostic null (N) spectra. The
Zeeman signatures are broad and track the radial velocity of the
secondary component; we therefore conclude that the rapidly rotating
secondary component is the magnetized star. Correcting the polarized
spectra for the line and continuum of the (sharp-lined) primary, we
measured the longitudinal magnetic field from each observation. The
longitudinal field of the secondary is variable and exhibits extreme
values of -810 ± 150 and +680 ± 190 G, implying a minimum surface
dipole polar strength of 2850 ± 500 G. In contrast, we derive an upper
limit (3σ) to the primary's surface magnetic field of 230 G. The
combination of a strong magnetic field and rapid rotation leads us
to conclude that the secondary hosts a centrifugal magnetosphere fed
through a magnetically confined wind. We revisit the properties of the
optical line profiles and X-ray emission - previously interpreted as a
consequence of colliding stellar winds - in this context. We conclude
that HD 47129 represents a heretofore unique stellar system - a close,
massive binary with a rapidly rotating, magnetized component - that
will be a rich target for further study.

---------------------------------------------------------
Title: First 3DMHD simulation of a massive-star magnetosphere with
application to Hα emission from θ1 Ori C
Authors: ud-Doula, A.; Sundqvist, J. O.; Owocki, S. P.; Petit, V.;
Townsend, R. H. D.
2013MNRAS.428.2723U Altcode: 2012arXiv1210.5298U; 2012MNRAS.tmp..200U
We present the first fully 3D magnetohydrodynamic (MHD) simulation
for magnetic channelling and confinement of a radiatively driven,
massive-star wind. The specific parameters are chosen to represent
the prototypical slowly rotating magnetic O star θ1
Ori C, for which centrifugal and other dynamical effects of rotation
are negligible. The computed global structure in latitude and radius
resembles that found in previous 2D simulations, with unimpeded outflow
along open field lines near the magnetic poles, and a complex equatorial
belt of inner wind trapping by closed loops near the stellar surface,
giving way to outflow above the Alfvén radius. In contrast to this
previous 2D work, the 3D simulation described here now also shows how
this complex structure fragments in azimuth, forming distinct clumps
of closed loop infall within the Alfvén radius, transitioning in the
outer wind to radial spokes of enhanced density with characteristic
azimuthal separation of 15°-20°. Applying these results in a 3D code
for line radiative transfer, we show that emission from the associated
3D `dynamical magnetosphere' matches well the observed Hα emission
seen from θ1 Ori C, fitting both its dynamic spectrum over
rotational phase and the observed level of cycle-to-cycle stochastic
variation. Comparison with previously developed 2D models for the
Balmer emission from a dynamical magnetosphere generally confirms
that time averaging over 2D snapshots can be a good proxy for the
spatial averaging over 3D azimuthal wind structure. Nevertheless,
fully 3D simulations will still be needed to model the emission from
magnetospheres with non-dipole field components, such as suggested
by asymmetric features seen in the Hα equivalent-width curve of
θ1 Ori C.

---------------------------------------------------------
Title: 3D Radiative Transfer in Eta Carinae: Initial Results from
the SimpleX Algorithm Applied to 3D SPH Simulations of Eta Car’s
Massive Binary Colliding Winds
Authors: Madura, Thomas; Gull, T. R.; Groh, J. H.; Clementel, N.;
Kruip, C.; Owocki, S. P.; Okazaki, A. T.
2013AAS...22114816M Altcode:
An extremely luminous (&gt;5 x 10^6 L_Sun) colliding wind binary with a
highly eccentric (e ~ 0.9), 5.54-year orbit, total mass of at least 110
M_Sun, and distance of ~2.3 kpc, Eta Carinae is an ideal astrophysical
laboratory for studying massive binary interactions, stellar wind-wind
collisions, massive star evolution, and dust formation. Until very
recently, our understanding of the system was limited by the lack of
proper numerical models, which require a full three-dimensional (3D)
treatment since orbital motion, especially during periastron, greatly
affects the shape and dynamics of the wind-wind collision region formed
between the stars. Continuing our theoretical investigations of this
complex system, we present initial results from the application of the
SimpleX algorithm for full 3D radiative transfer on an unstructured
Delaunay grid to new 3D SPH simulations of Eta Car’s massive
binary colliding winds that include radiation-driven stellar winds
and radiative cooling. Depending on the ionizing fluxes assumed for
the stars, different portions of the wind-wind interaction region
and optically-thick wind of the LBV primary star are photoionized and
capable of producing various forms of line emission. Application of the
SimpleX algorithm to the 3D SPH code output further allows synthetic
observations to be generated for comparison to available and future
HST/STIS data. This work will be used to place strong constraints on the
number of ionizing photons from the binary companion star Eta B since
the geometry, spatial extent, and flux of each measured emission line
strongly depends upon the assumed ionizing flux of the companion. By
comparing this ionizing flux to stellar models for a range of O and WR
stars, we hope to obtain a more accurate luminosity and temperature,
and thus evolutionary state, for the as-yet unseen Eta B.

---------------------------------------------------------
Title: Disks Surrounding Be stars: A Stellar Evolution Perspective
Authors: Granada, A.; Ekström, S.; Georgy, C.; Meynet, G.; Krtička,
J.; Owocki, S. P.
2012ASPC..464..117G Altcode:
There is evidence that at least some stars undergoing the Be phenomenon
are single, almost critically rotating stars surrounded by viscous
Keplerian disks. By combining our new stellar evolutionary tracks
for critically rotating B-type stars with different masses and
metallicities, with the parametrized expressions by Krtička et
al. (2011) describing the properties of a stationary viscous Keplerian
disk surrounding a star rotating at its breakup limit, we are able to
explore for the first time how the properties of such a disk changes
along the main sequence evolution of the star. The results we obtain
can be relevant in the understanding of the formation and existence
of Keplerian viscous decretion disks around rapidly rotating single
Be stars.

---------------------------------------------------------
Title: Diagnosing Small- and Large-Scale Structure in the Winds of
Hot, Massive OB-Stars
Authors: Sundqvist, J. O.; Owocki, S. P.
2012ASPC..464..301S Altcode: 2012arXiv1205.3190S
It is observationally as well as theoretically well established that
the winds of hot, massive OB-stars are highly structured on a broad
range of spatial scales. This paper first discusses consequences of
the small-scale structures associated with the strong instability
inherent to the line-driving of these winds. We demonstrate the
importance of a proper treatment of such wind clumping to obtain
reliable estimates of mass-loss rates, and also show that instability
simulations that are perturbed at the lower boundary indeed display
significant clumping quite close to the wind base, in general agreement
with observations. But a growing subset of massive stars has also been
found to possess strong surface magnetic fields, which may channel the
star's outflow and induce also large-scale wind structures and cyclic
behavior of spectral diagnostics. The paper concludes by showing that
multi-dimensional, magneto-hydrodynamical wind simulations, together
with detailed radiative-transfer modeling, can reproduce remarkably
well the periodic Balmer line emission observed in slowly rotating
magnetic O stars like HD 191612.

---------------------------------------------------------
Title: The Dynamical Role of Radiative Driving in the Sources and
Sinks of Circumstellar Matter in Massive Stars
Authors: Owocki, S.
2012ASPC..464..255O Altcode:
The high luminosity of massive stars drives strong stellar wind
outflows. In magnetic massive stars, the channeling and trapping of
wind material can feed a circumstellar magnetosphere, characterized
either by transient suspension and dynamical infall in slow rotators,
or by long-term centrifugal support in moderately fast rotators. In the
non-magnetic but rapidly rotating Be stars, direct centrifugal ejection
of material from the equatorial surface can feed a Keplerian decretion
disk, with radiative forces now playing a potential key role in disk
dissipation, through line-driven ablation from the disk surface. This
contribution reviews these dynamical roles of radiative driving in the
sources and sinks of circumstellar matter, within the context of using
high resolution observations to test and constrain circumstellar models.

---------------------------------------------------------
Title: The Nature and Consequences of Clumping in Hot, Massive
Star Winds
Authors: Sundqvist, J. O.; Owocki, S. P.; Puls, J.
2012ASPC..465..119S Altcode: 2011arXiv1110.0485S
This review describes the evidence for small-scale structure,
‘clumping’, in the radiation line-driven winds of hot, massive
stars. In particular, we focus on examining to what extent simulations
of the strong instability inherent to line-driving can explain the
multitude of observational evidence for wind clumping, as well as
on how to properly account for extensive structures in density and
velocity when interpreting the various wind diagnostics used to derive
mass-loss rates.

---------------------------------------------------------
Title: Massive Stars Near the Eddington Limit: Mass Loss and Envelope
Inflation
Authors: Gräfener, G.; Vink, J. S.; Owocki, S. P.
2012ASPC..465..202G Altcode:
When massive stars approach the Eddington limit, their outer envelopes
and winds become dominated by ‘opacity peaks’, i.e. by material
properties. We discuss the physical consequences, namely the formation
of strong Wolf-Rayet (WR) type winds and a radial extension of the
stellar envelopes. The understanding of the physical processes in this
regime is of basic importance for key phases of stellar evolution,
such as the WR and LBV stage, and thus for questions on how massive
stars evolve and how they end their lives.

---------------------------------------------------------
Title: An X-Ray Survey of Colliding Wind Binaries
Authors: Gagné, M.; Fehon, G.; Savoy, M. R.; Cartagena, C. A.; Cohen,
D. H.; Owocki, S. P.
2012ASPC..465..301G Altcode: 2012arXiv1205.3510G
We have compiled a list of 35 O + O binaries and 86 Wolf-Rayet (WR)
binaries in the Milky Way and Magellanic clouds detected with the
Chandra, XMM-Newton, and ROSAT satellites to probe the connection
between their X-ray properties and their system characteristics. Of the
WR binaries with published model parameters, all have log LX
&gt; 32, kT &gt; 1 keV and log LX/Lbol &gt;
-7. The most X-ray luminous WR binaries are typically very long period
systems. The WR binaries show a nearly four-order of magnitude spread
in X-ray luminosity, even among among systems with very similar WR
primaries. Among the O + O binaries, short-period systems have soft
X-ray spectra and longer period systems show harder X-ray spectra
again with a large spread in LX/Lbol.

---------------------------------------------------------
Title: General Discussion - Session II
Authors: Owocki, S.
2012ASPC..465..111O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Physical Basis of the LX = Lbol
Empirical Law for O-Star X-Rays
Authors: Owocki, S. P.; Sundqvist, J. O.; Cohen, D. H.; Gayley, K. G.
2012ASPC..465..153O Altcode: 2011arXiv1110.0891O
X-ray satellites since Einstein have empirically established that the
X-ray luminosity from single O-stars scales linearly with bolometric
luminosity, LX = 10-7 Lbol. But
straightforward forms of the most favored model, in which X-rays
arise from instability-generated shocks embedded in the stellar wind,
predict a steeper scaling, either with mass-loss rate LX
= M = Lbol1.7 if the shocks are radiative, or
with LX = M2 = Lbol3.4
if they are adiabatic. We present here a generalized formalism that
bridges these radiative vs. adiabatic limits in terms of the ratio
of the shock cooling length to the local radius. Noting that the
thin-shell instability of radiative shocks should lead to extensive
mixing of hot and cool material, we then propose that the associated
softening and weakening of the X-ray emission can be parameterized
by the cooling length ratio raised to a power m, the “mixing
exponent". For physically reasonable values m ≍ 0.4, this leads to
an X-ray luminosity LX = M0.6 = Lbol
that matches the empirical scaling. We conclude by noting that such
thin-shell mixing may also be important for X-rays from colliding wind
binaries, and that future numerical simulation studies will be needed
to test this thin-shell mixing ansatz for X-ray emission.

---------------------------------------------------------
Title: Multi-Wavelength Implications of the Companion Star in
η Carinae
Authors: Madura, T. I.; Gull, T. R.; Groh, J. H.; Owocki, S. P.;
Okazaki, A.; Hillier, D. J.; Russell, C.
2012ASPC..465..313M Altcode: 2011arXiv1111.2280M
η Carinae is considered to be a massive colliding wind binary
system with a highly eccentric (e = 0.9), 5.54-yr orbit. However,
the companion star continues to evade direct detection as the primary
dwarfs its emission at most wavelengths. Using three-dimensional SPH
simulations of η Car's colliding winds and radiative transfer codes,
we are able to compute synthetic observables across multiple wavebands
for comparison to the observations. The models show that the presence
of a companion star has a profound influence on the observed HST/STIS UV
spectrum and Hα line profiles, as well as the ground-based photometric
monitoring. Here, we focus on the bore-hole effect, wherein the fast
wind from the hot secondary star carves a cavity in the dense primary
wind, allowing increased escape of radiation from the hotter/deeper
layers of the primary's extended wind photosphere. The results have
important implications for interpretations of η Car's observables at
multiple wavelengths.

---------------------------------------------------------
Title: Magnetospheres of Massive Stars Across the EM Spectrum
Authors: Petit, V.; Owocki, S. P.; Oksala, M. E.; MiMeS Collaboration
2012ASPC..465...48P Altcode: 2011arXiv1111.1238P
Magnetic massive stars — which are being discovered with increasing
frequency — represent a new category of wind-shaping mechanism for
O and B stars. Magnetic channeling of these stars' radiation-driven
winds, the Magnetically Confined Wind Shock paradigm, leads to
the formation of a shock-heated magnetosphere, which can radiate
X-rays, modify UV resonance lines, and create disks of Hα emitting
material. The dynamical properties of these magnetospheres are well
understood from a theoretical point of view as an interplay between the
magnetic wind confinement and rotation. However, the manifestations
of magnetospheres across the spectrum may be more complex and varied
than first anticipated. On the other hand, recent advances in modeling
these magnetospheres provide a key to better understand massive star
winds in general. We will summarize the coordinated observational,
theoretical, and modeling efforts from the Magnetism in Massive Star
Project, addressing key outstanding questions regarding magnetosphere
manifestations across the spectral domain.

---------------------------------------------------------
Title: HD 96446: a puzzle for current models of magnetospheres?
Authors: Neiner, C.; Landstreet, J. D.; Alecian, E.; Owocki, S.;
Kochukhov, O.; Bohlender, D.; MiMeS Collaboration
2012A&A...546A..44N Altcode:
Context. Oblique magnetic dipole fields have been detected in Bp stars
for several decades, and more recently also in normal massive stars. In
the past decade, it has been established that stellar magnetospheres
form through the channelling and confinement of an outflowing stellar
wind by the stellar magnetic field. This explains specific properties of
magnetic massive stars, such as their rotationally modulated photometric
light curve, Hα emission, UV spectra, and X-ray emission. 
Aims: In the framework of the MiMeS (Magnetism in Massive Stars)
project, four HARPSpol observations of the magnetic Bp star <ASTROBJ>HD
96446</ASTROBJ> have been obtained. <ASTROBJ>HD 96446</ASTROBJ> is
very similar to σ Ori E, the prototype of centrifugally supported
rigidly rotating magnetospheres (CM) and is therefore a perfect target
to study the validity of this model. Methods: We first updated
the basic parameters of <ASTROBJ>HD 96446</ASTROBJ> and studied its
spectral variability. We then analysed the HARPSpol spectropolarimetric
observations using the LSD (Least-Squares Deconvolution) technique to
derive the longitudinal magnetic field and Zeeman signatures in various
types of lines. With LTE spectrum modelling, we derived constraints on
the field modulus, the rotational velocity, and the inclination angle,
and measured non-solar abundances of several elements which we checked
with NLTE modelling. Finally, we calculated the magnetic confinement and
Alfvén and Kepler radii from the stellar magnetic field and rotation
properties, and we examined the various types of magnetospheres that
may be present around <ASTROBJ>HD 96446</ASTROBJ>. Results:
We find radial velocity variations with a period around 2.23 h,
that we attribute to β Cep-type p-mode pulsations. We detect clear
direct magnetic Stokes V signatures with slightly varying values
of the longitudinal magnetic field, typical of an oblique dipole
rotator, and show that these signatures are not much perturbed by
the radial velocity variations. The magnetic confinement parameter
and Alfvén radius in the centrifugally supported, rigidly-rotating
magnetosphere (CM) model points towards the presence of confined
material in the magnetosphere. However, <ASTROBJ>HD 96446</ASTROBJ>
does not present signatures of the presence of such confined material,
such as Hα emission. Conclusions: We conclude that, even
though <ASTROBJ>HD 96446</ASTROBJ> fulfills all criteria to host
a CM with confined material, it does not. The rotation period must
be significantly revised, or another model of magnetosphere with a
leakage mechanism will need to be developed to explain the magnetic
environment of this star. Based on observations obtained with
the HARPSpol spectropolarimeter at ESO, Chile (Program ID 187.D-0917).

---------------------------------------------------------
Title: A dynamical magnetosphere model for periodic Hα emission
from the slowly rotating magnetic O star HD 191612
Authors: Sundqvist, Jon O.; ud-Doula, Asif; Owocki, Stanley P.;
Townsend, Richard H. D.; Howarth, Ian D.; Wade, Gregg A.
2012MNRAS.423L..21S Altcode: 2012MNRAS.tmpL.433S; 2012arXiv1203.1050S
The magnetic O star HD 191612 exhibits strongly variable, cyclic
Balmer line emission on a 538-d period. We show here that its
variable Hα emission can be well reproduced by the rotational phase
variation of synthetic spectra computed directly from full radiation
magnetohydrodynamical simulations of a magnetically confined wind. In
slow rotators such as HD 191612, wind material on closed magnetic
field loops falls back to the star, but the transient suspension
of material within the loops leads to a statistically overdense,
low-velocity region around the magnetic equator, causing the spectral
variations. We contrast such 'dynamical magnetospheres' (DMs) with
the more steady-state 'centrifugal magnetospheres' of stars with
rapid rotation, and discuss the prospects of using this DM paradigm
to explain periodic line emission from also other non-rapidly rotating
magnetic massive stars.

---------------------------------------------------------
Title: Atomic physics of shocked plasma in winds of massive stars
Authors: Leutenegger, Maurice A.; Cohen, David H.; Owocki, Stanley P.
2012AIPC.1438..111L Altcode:
High resolution diffraction grating spectra of X-ray emission from
massive stars obtained with Chandra and XMM-Newton have revolutionized
our understanding of their powerful, radiation-driven winds. Emission
line shapes and line ratios provide diagnostics on a number of key
wind parameters. Modeling of resolved emission line velocity profiles
allows us to derive independent constraints on stellar mass-loss
rates, leading to downward revisions of a factor of a few from previous
measurements. Line ratios in He-like ions strongly constrain the spatial
distribution of Xray emitting plasma, confirming the expectations
of radiation hydrodynamic simulations that X-ray emission begins
moderately close to the stellar surface and extends throughout the
wind. Some outstanding questions remain, including the possibility
of large optical depths in resonance lines, which is hinted at by
differences in line shapes of resonance and intercombination lines
from the same ion. Resonance scattering leads to nontrivial radiative
transfer effects, and modeling it allows us to place constraints on
shock size, density, and velocity structure.

---------------------------------------------------------
Title: Modeling High-energy Light curves of the PSR B1259-63/LS 2883
Binary Based on 3D SPH Simulations
Authors: Takata, J.; Okazaki, A. T.; Nagataki, S.; Naito, T.; Kawachi,
A.; Lee, S. -H.; Mori, M.; Hayasaki, K.; Yamaguchi, M. S.; Owocki,
S. P.
2012ApJ...750...70T Altcode: 2012arXiv1203.2179T
Temporal changes of X-ray to very high energy gamma-ray emissions
from the pulsar-Be-star binary PSR B1259-63/LS 2883 are studied based
on three-dimensional smoothed particle hydrodynamic simulations
of pulsar wind interaction with Be-disk and wind. We focus on the
periastron passage of the binary and calculate the variation of
the synchrotron and inverse-Compton emissions using the simulated
shock geometry and pressure distribution of the pulsar wind. The
characteristic double-peaked X-ray light curve from observations is
reproduced by our simulation under a dense Be-disk condition (base
density ~10-9 g cm-3). We interpret the pre-
and post-periastron peaks as being due to a significant increase
in the conversion efficiency from pulsar spin-down power to the
shock-accelerated particle energy at orbital phases when the pulsar
crosses the disk before periastron passage, and when the pulsar
wind creates a cavity in the disk gas after periastron passage,
respectively. On the contrary, in the model TeV light curve, which
also shows a double-peak feature, the first peak appears around the
periastron phase. The possible effects of cooling processes on the
TeV light curve are briefly discussed.

---------------------------------------------------------
Title: Constraining the absolute orientation of η Carinae's binary
orbit: a 3D dynamical model for the broad [Fe III] emission
Authors: Madura, T. I.; Gull, T. R.; Owocki, S. P.; Groh, J. H.;
Okazaki, A. T.; Russell, C. M. P.
2012MNRAS.420.2064M Altcode: 2011arXiv1111.2226M
We present a three-dimensional (3D) dynamical model for the broad
[Fe III] emission observed in η Carinae using the Hubble Space
Telescope/Space Telescope Imaging Spectrograph (STIS). This model is
based on full 3D smoothed particle hydrodynamics simulations of η Car's
binary colliding winds. Radiative transfer codes are used to generate
synthetic spectroimages of [Fe III] emission-line structures at various
observed orbital phases and STIS slit position angles (PAs). Through
a parameter study that varies the orbital inclination i, the PA θ
that the orbital plane projection of the line of sight makes with
the apastron side of the semimajor axis and the PA on the sky of the
orbital axis, we are able, for the first time, to tightly constrain
the absolute 3D orientation of the binary orbit. To simultaneously
reproduce the blueshifted emission arcs observed at orbital phase 0.976,
STIS slit PA =+38° and the temporal variations in emission seen at
negative slit PAs, the binary needs to have an i≈ 130° to 145°,
θ≈-15° to +30° and an orbital axis projected on the sky at a PA
≈ 302° to 327° east of north. This represents a system with an
orbital axis that is closely aligned with the inferred polar axis of
the Homunculus nebula, in 3D. The companion star, ηB, thus
orbits clockwise on the sky and is on the observer's side of the system
at apastron. This orientation has important implications for theories
for the formation of the Homunculus and helps lay the groundwork
for orbital modelling to determine the stellar masses. &lt;title
type="main"&gt;Footnotes&lt;label&gt;1&lt;/label&gt;Low- and
high-ionization refer here to atomic species with ionizations
potentials (IPs) below and above the IP of hydrogen, 13.6
eV.&lt;label&gt;2&lt;/label&gt;Measured in degrees from north to
east.&lt;label&gt;3&lt;/label&gt;θ is the same as the angle φ
defined in fig. 3 of O08.&lt;label&gt;4&lt;/label&gt;The outer edge
looks circular only because this marks the edge of the spherical
computational domain of the SPH simulation.

---------------------------------------------------------
Title: A generalized porosity formalism for isotropic and anisotropic
effective opacity and its effects on X-ray line attenuation in
clumped O star winds
Authors: Sundqvist, Jon O.; Owocki, Stanley P.; Cohen, David H.;
Leutenegger, Maurice A.; Townsend, Richard H. D.
2012MNRAS.420.1553S Altcode: 2011MNRAS.tmp.2109S; 2011arXiv1111.1762S
We present a generalized formalism for treating the porosity-associated
reduction in continuum opacity that occurs when individual clumps
in a stochastic medium become optically thick. As in previous work,
we concentrate on developing bridging laws between the limits of
optically thin and thick clumps. We consider geometries resulting in
either isotropic or anisotropic effective opacity, and, in addition to
an idealized model in which all clumps have the same local overdensity
and scale, we also treat an ensemble of clumps with optical depths set
by Markovian statistics. This formalism is then applied to the specific
case of bound-free absorption of X-rays in hot star winds, a process
not directly affected by clumping in the optically thin limit. We find
that the Markov model gives surprisingly similar results to those found
previously for the single-clump model, suggesting that porous opacity
is not very sensitive to details of the assumed clump distribution
function. Further, an anisotropic effective opacity favours escape
of X-rays emitted in the tangential direction (the 'venetian blind'
effect), resulting in a 'bump' of higher flux close to line centre
as compared to profiles computed from isotropic porosity models. We
demonstrate how this characteristic line shape may be used to diagnose
the clump geometry, and we confirm previous results that for optically
thick clumping to significantly influence X-ray line profiles, very
large porosity lengths, defined as the mean free path between clumps,
are required. Moreover, we present the first X-ray line profiles
computed directly from line-driven instability simulations using a 3D
patch method, and find that porosity effects from such models also are
very small. This further supports the view that porosity has, at most,
a marginal effect on X-ray line diagnostics in O stars, and therefore
that these diagnostics do indeed provide a good 'clumping insensitive'
method for deriving O star mass-loss rates.

---------------------------------------------------------
Title: Stellar envelope inflation near the Eddington
limit. Implications for the radii of Wolf-Rayet stars and luminous
blue variables
Authors: Gräfener, G.; Owocki, S. P.; Vink, J. S.
2012A&A...538A..40G Altcode: 2011arXiv1112.1910G
Context. It has been proposed that the envelopes of luminous stars may
be subject to substantial radius inflation. The peculiar structure of
such inflated envelopes, with an almost void, radiatively dominated
region beneath a thin, dense shell could mean that many in reality
compact stars are hidden below inflated envelopes, displaying much
lower effective temperatures. The inflation effect has been discussed
in relation to the radius problem of Wolf-Rayet (WR) stars, but has yet
failed to explain the large observed radii of Galactic WR stars. Aims: We wish to obtain a physical perspective of the inflation
effect, and study the consequences for the radii of WR stars, and
luminous blue variables (LBVs). For WR stars the observed radii are
up to an order of magnitude larger than predicted by theory, whilst S
Doradus-type LBVs are subject to humongous radius variations, which
remain as yet ill-explained. Methods: We use a dual approach
to investigate the envelope inflation, based on numerical models
for stars near the Eddington limit, and a new analytic formalism
to describe the effect. An additional new aspect is that we take
the effect of density inhomogeneities (clumping) within the outer
stellar envelopes into account. Results: Due to the effect of
clumping we are able to bring the observed WR radii in agreement with
theory. Based on our new formalism, we find that the radial inflation
is a function of a dimensionless parameter W, which largely depends on
the topology of the Fe-opacity peak, i.e., on material properties. For
W &gt; 1, we discover an instability limit, for which the stellar
envelope becomes gravitationally unbound, i.e. there no longer exists
a static solution. Within this framework we are also able to explain
the S Doradus-type instabilities for LBVs like AG Car, with a possible
triggering due to changes in stellar rotation. Conclusions: The
stellar effective temperatures in the upper Hertzsprung-Russell (HR)
diagram are potentially strongly affected by the inflation effect. This
may have particularly strong effects on the evolved massive LBV and
WR stars just prior to their final collapse, as the progenitors of
supernovae (SNe) Ibc, SNe II, and long-duration gamma-ray bursts
(long GRBs).

---------------------------------------------------------
Title: Revisiting the Rigidly Rotating Magnetosphere model for σ
Ori E - I. Observations and data analysis
Authors: Oksala, M. E.; Wade, G. A.; Townsend, R. H. D.; Owocki,
S. P.; Kochukhov, O.; Neiner, C.; Alecian, E.; Grunhut, J.
2012MNRAS.419..959O Altcode: 2011MNRAS.tmp.1620O; 2011arXiv1109.0328O
We have obtained 18 new high-resolution spectropolarimetric
observations of the B2Vp star σ Ori E with both the Narval and ESPaDOnS
spectropolarimeters. The aim of these observations is to test, with
modern data, the assumptions of the Rigidly Rotating Magnetosphere (RRM)
model of Townsend &amp; Owocki, applied to the specific case of σ Ori
E by Townsend, Owocki &amp; Groote. This model includes a substantially
offset dipole magnetic field configuration, and approximately reproduces
previous observational variations in longitudinal field strength,
photometric brightness and Hα emission. We analyse new spectroscopy,
including H I, He I, C II, Si III and Fe III lines, confirming the
diversity of variability in photospheric lines, as well as the double
S-wave variation of circumstellar hydrogen. Using the multiline
analysis method of least-squares deconvolution (LSD), new, more
precise longitudinal magnetic field measurements reveal a substantial
variance between the shapes of the observed and RRM model time-varying
field. The phase-resolved Stokes V profiles of He I 5876 and 6678
Å lines are fitted poorly by synthetic profiles computed from the
magnetic topology assumed by Townsend et al.. These results challenge
the offset dipole field configuration assumed in the application of the
RRM model to σ Ori E, and indicate that future models of its magnetic
field should also include complex, higher order components. &lt;title
type="main"&gt;Footnotes&lt;label&gt;1&lt;/label&gt;

---------------------------------------------------------
Title: The Many Facets of Massive Star Mass Loss
Authors: Owocki, S.
2012iac..talk..363O Altcode: 2012iac..talk..284O
No abstract at ADS

---------------------------------------------------------
Title: Instability &amp; Mass Loss near the Eddington Limit
Authors: Owocki, S. P.; Shaviv, N. J.
2012ASSL..384..275O Altcode:
We review the physics of continuum-driven mass loss and its likely
role in η Carinae and LBVs. Unlike a line-driven wind, which is
inherently limited by self-shadowing, continuum driving can in
principle lead to mass-loss rates up to the "photon-tiring" limit,
for which the entire luminosity is expended in lifting the outflow. We
discuss how instabilities near the Eddington limit give rise to a
clumped atmosphere, and how the associated "porosity" can regulate
a continuum-driven flow. We also summarize recent time-dependent
simulations in which a mass flow stagnates because it exceeds the
tiring limit, leading to complex time-dependent inflow and outflow
regions. Porosity-regulated continuum driving in super-Eddington
epochs can probably explain the large, near tiring-limit mass loss
inferred for LBV giant eruptions. However, while these extreme flows
can persist over dynamically long periods, they cannot be sustained
for an evolutionary timescale; so ultimately it is stellar structure
and evolution that sets the overall mass loss.

---------------------------------------------------------
Title: X-ray Spectral Variations of the Extremely Massive Colliding
Wind Binaries Eta Carinae and WR 140
Authors: Corcoran, Michael F.; Hamaguchi, K.; Pollock, A. M. T.;
Russell, C. M. P.; Moffat, A. F. J.; Owocki, S.; Ishibashi, B.;
Davidson, K.; Pittard, J. M.; Parkin, R.
2012AAS...21924901C Altcode:
The Rossi X-ray Timing Explorer has, for the first time, provided
detailed measures of the X-ray spectral variations in the two most
important, high mass, evolved, highly eccentric colliding wind binaries,
Eta Carinae and WR 140 though multiple orbital cycles. We report on the
breakthroughs RXTE has achieved for these two binaries in observations
spanning 15 years.

---------------------------------------------------------
Title: X-ray Modeling of \eta\ Carinae and WR140 from SPH Simulations
Authors: Russell, Christopher M. P.; Owocki, Stanley P.; Corcoran,
Michael F.; Okazaki, Atsuo T.; Madura, Thomas I.
2011arXiv1111.0100R Altcode:
The colliding wind binary (CWB) systems \eta\ Carinae and WR140 provide
unique laboratories for X-ray astrophysics. Their wind-wind collisions
produce hard X-rays that have been monitored extensively by several
X-ray telescopes, including RXTE. To interpret these X-ray light curves
and spectra, we apply 3D hydrodynamic simulations of the wind-wind
collision using smoothed particle hydrodynamics (SPH), with the recent
improvements of radiative cooling and the acceleration of the stellar
winds according to a \beta\ law. For both systems, the 2-10 keV RXTE
light curves are well-reproduced in absolute units for most phases,
but the light curve dips associated with the periastron passages are
not well matched. In WR140, the dip is too weak, and in \eta\ Carinae,
the large difference in wind speeds of the two stars leads to a hot,
post-periastron bubble that produces excess emission toward the end
of the X-ray minimum.

---------------------------------------------------------
Title: Hydrodynamic Interaction between the Be Star and the Pulsar
in the TeV Binary PSR B1259-63/LS 2883
Authors: Okazaki, Atsuo T.; Nagataki, Shigehiro; Naito, Tsuguya;
Kawachi, Akiko; Hayasaki, Kimitake; Owocki, Stanley P.; Takata, Jumpei
2011PASJ...63..893O Altcode: 2011arXiv1105.1481O
We have been studying the interaction between the Be star and
the pulsar in the TeV binary PSR B1259-63/LS 2883, using 3-D SPH
simulations of the tidal and wind interactions in this Be-pulsar
system. We first ran a simulation without pulsar wind nor Be wind,
while taking into account only the gravitational effect of the pulsar
on the Be disk. In this simulation, the gas particles are ejected at
a constant rate from the equatorial surface of the Be star, which is
tilted in a direction consistent with multi-waveband observations. We
ran the simulation until the Be disk was fully developed and started to
repeat a regular tidal interaction with the pulsar. Then, we turned on
the pulsar wind and the Be wind. We ran two simulations with different
wind mass-loss rates for the Be star, one for a B2 V type and the other
for a significantly earlier spectral type. Although the global shape of
the interaction surface between the pulsar wind and the Be wind agrees
with the analytical solution, the effect of the pulsar wind on the Be
disk is profound. The pulsar wind strips off an outer part of the Be
disk, truncating the disk at a radius significantly smaller than the
pulsar orbit. Our results, therefore, rule out the idea that the pulsar
passes through the Be disk around periastron, which has been assumed in
previous studies. It also turns out that the location of the contact
discontinuity can be significantly different between phases when the
pulsar wind directly hits the Be disk and those when the pulsar wind
collides with the Be wind. It is thus important to adequately take
into account the circumstellar environment of the Be star, in order
to construct a satisfactory model for this prototypical TeV binary.

---------------------------------------------------------
Title: Chandra X-ray spectroscopy of the very early O supergiant HD
93129A: constraints on wind shocks and the mass-loss rate
Authors: Cohen, David H.; Gagné, Marc; Leutenegger, Maurice A.;
MacArthur, James P.; Wollman, Emma E.; Sundqvist, Jon O.; Fullerton,
Alex W.; Owocki, Stanley P.
2011MNRAS.415.3354C Altcode: 2011MNRAS.tmp..890C; 2011arXiv1104.4786C
We present an analysis of both the resolved X-ray emission-line
profiles and the broad-band X-ray spectrum of the O2 If* star HD 93129A,
measured with the Chandra High Energy Transmission Grating Spectrometer
(HETGS). This star is among the earliest and most massive stars in
the Galaxy, and provides a test of the embedded wind-shock scenario in
a very dense and powerful wind. A major new result is that continuum
absorption by the dense wind is the primary cause of the hardness of
the observed X-ray spectrum, while intrinsically hard emission from
colliding wind shocks contributes less than 10 per cent of the X-ray
flux. We find results consistent with the predictions of numerical
simulations of the line-driving instability, including line broadening
indicating an onset radius of X-ray emission of several tenths of
R*. Helium-like forbidden-to-intercombination line ratios
are consistent with this onset radius, and inconsistent with being
formed in a wind-collision interface with the star's closest visual
companion at a distance of 100 au. The broad-band X-ray spectrum is
fitted with a dominant emission temperature of just kT= 0.6 keV along
with significant wind absorption. The broad-band wind absorption and
the line profiles provide two independent measurements of the wind
mass-loss rate: 5.2+1.8-1.5 × 10-6
M⊙ yr-1 and 6.8+2.8-2.2×
10-6 M⊙ yr-1, respectively. This
is the first consistent modelling of the X-ray line-profile shapes
and broad-band X-ray spectral energy distribution in a massive star,
and represents a reduction of a factor of 3-4 compared to the standard
Hα mass-loss rate that assumes a smooth wind.

---------------------------------------------------------
Title: Interaction between the Be star and the compact companion in
TeV γ-ray binaries
Authors: Okazaki, Atsuo T.; Nagataki, Shigehiro; Naito, Tsuguya;
Kawachi, Akiko; Hayasaki, Kimitake; Owocki, Stanley P.; Takata, Jumpei
2011IAUS..272..628O Altcode:
We report on the results from 3-D SPH simulations of TeV binaries
with Be stars. Since there is only one TeV binary (B 1259-63) where
the nature of the compact companion has been established, we mainly
focus on this Be-pulsar system. From simulations of B 1259-63 around
periastron, we find that the pulsar wind dominates the Be-star wind
and strips off an outer part of the Be-star disk, causing a strongly
asymmetric, phase-dependent structure of the circumstellar material
around the Be star. Such a large modulation may be detected by optical,
IR, and/or UV observations at phases near periastron. We also discuss
the results from simulations of another TeV binary LS I+61 303, for
which the nature of the compact object is not yet known.

---------------------------------------------------------
Title: X-Ray modeling of η Carinae &amp; WR 140 from SPH simulations
Authors: Russell, Christopher M. P.; Corcoran, Michael F.; Okazaki,
Atsuo T.; Madura, Thomas I.; Owocki, Stanley P.
2011IAUS..272..630R Altcode:
The colliding wind binary (CWB) systems η Carinae and WR140 provide
unique laboratories for X-ray astrophysics. Their wind-wind collisions
produce hard X-rays that have been monitored extensively by several
X-ray telescopes, including RXTE. To interpret these RXTE X-ray
light curves, we apply 3D hydrodynamic simulations of the wind-wind
collision using smoothed particle hydrodynamics (SPH). We find
adiabatic simulations that account for the absorption of X-rays
from an assumed point source of X-ray emission at the apex of the
wind-collision shock cone can closely match the RXTE light curves of
both η Car and WR140. This point-source model can also explain the
early recovery of η Car's X-ray light curve from the 2009.0 minimum
by a factor of 2-4 reduction in the mass loss rate of η Car. Our more
recent models account for the extended emission and absorption along the
full wind-wind interaction shock front. For WR140, the computed X-ray
light curves again match the RXTE observations quite well. But for η
Car, a hot, post-periastron bubble leads to an emission level that
does not match the extended X-ray minimum observed by RXTE. Initial
results from incorporating radiative cooling and radiative forces via
an anti-gravity approach into the SPH code are also discussed.

---------------------------------------------------------
Title: An Introduction to the Chandra Carina Complex Project
Authors: Townsley, Leisa K.; Broos, Patrick S.; Corcoran, Michael
F.; Feigelson, Eric D.; Gagné, Marc; Montmerle, Thierry; Oey,
M. S.; Smith, Nathan; Garmire, Gordon P.; Getman, Konstantin V.;
Povich, Matthew S.; Remage Evans, Nancy; Nazé, Yaël; Parkin, E. R.;
Preibisch, Thomas; Wang, Junfeng; Wolk, Scott J.; Chu, You-Hua; Cohen,
David H.; Gruendl, Robert A.; Hamaguchi, Kenji; King, Robert R.;
Mac Low, Mordecai-Mark; McCaughrean, Mark J.; Moffat, Anthony F. J.;
Oskinova, L. M.; Pittard, Julian M.; Stassun, Keivan G.; ud-Doula,
Asif; Walborn, Nolan R.; Waldron, Wayne L.; Churchwell, Ed; Nichols,
J. S.; Owocki, Stanley P.; Schulz, N. S.
2011ApJS..194....1T Altcode: 2011arXiv1102.4779T
The Great Nebula in Carina provides an exceptional view into the
violent massive star formation and feedback that typifies giant H II
regions and starburst galaxies. We have mapped the Carina star-forming
complex in X-rays, using archival Chandra data and a mosaic of 20
new 60 ks pointings using the Chandra X-ray Observatory's Advanced
CCD Imaging Spectrometer, as a testbed for understanding recent and
ongoing star formation and to probe Carina's regions of bright diffuse
X-ray emission. This study has yielded a catalog of properties of
&gt;14,000 X-ray point sources; &gt;9800 of them have multiwavelength
counterparts. Using Chandra's unsurpassed X-ray spatial resolution, we
have separated these point sources from the extensive, spatially-complex
diffuse emission that pervades the region; X-ray properties of this
diffuse emission suggest that it traces feedback from Carina's massive
stars. In this introductory paper, we motivate the survey design,
describe the Chandra observations, and present some simple results,
providing a foundation for the 15 papers that follow in this special
issue and that present detailed catalogs, methods, and science results.

---------------------------------------------------------
Title: Mass loss from inhomogeneous hot star winds. II. Constraints
from a combined optical/UV study
Authors: Sundqvist, J. O.; Puls, J.; Feldmeier, A.; Owocki, S. P.
2011A&A...528A..64S Altcode: 2011arXiv1101.5293S
Context. Mass loss is essential for massive star evolution, thus
also for the variety of astrophysical applications relying on its
predictions. However, mass-loss rates currently in use for hot,
massive stars have recently been seriously questioned, mainly because
of the effects of wind clumping. Aims: We investigate the
impact of clumping on diagnostic ultraviolet resonance and optical
recombination lines often used to derive empirical mass-loss rates
of hot stars. Optically thick clumps, a non-void interclump medium,
and a non-monotonic velocity field are all accounted for in a single
model. The line formation is first theoretically studied, after which an
exemplary multi-diagnostic study of an O-supergiant is performed. Methods: We used 2D and 3D stochastic and radiation-hydrodynamic
wind models, constructed by assembling 1D snapshots in radially
independent slices. To compute synthetic spectra, we developed
and used detailed radiative transfer codes for both recombination
lines (solving the "formal integral") and resonance lines (using a
Monte-Carlo approach). In addition, we propose an analytic method to
model these lines in clumpy winds, which does not rely on optically
thin clumping. Results: The importance of the "vorosity"
effect for line formation in clumpy winds is emphasized. Resonance
lines are generally more affected by optically thick clumping than
recombination lines. Synthetic spectra calculated directly from current
radiation-hydrodynamic wind models of the line-driven instability are
unable to in parallel reproduce strategic optical and ultraviolet lines
for the Galactic O-supergiant λ Cep. Using our stochastic wind models,
we obtain consistent fits essentially by increasing the clumping in
the inner wind. A mass-loss rate is derived that is approximately two
times lower than what is predicted by the line-driven wind theory,
but much higher than the corresponding rate derived when assuming
optically thin clumps. Our analytic formulation for line formation
is used to demonstrate the potential importance of optically thick
clumping in diagnostic lines in so-called weak-winded stars and to
confirm recent results that resonance doublets may be used as tracers
of wind structure and optically thick clumping. Conclusions:
We confirm earlier results that a re-investigation of the structures
in the inner wind predicted by line-driven instability simulations
is needed. Our derived mass-loss rate for λ Cep suggests that only
moderate reductions of current mass-loss predictions for OB-stars are
necessary, but this nevertheless prompts investigations on feedback
effects from optically thick clumping on the steady-state, NLTE wind
models used for quantitative spectroscopy.

---------------------------------------------------------
Title: Mass and angular momentum loss via decretion disks
Authors: Krtička, J.; Owocki, S. P.; Meynet, G.
2011A&A...527A..84K Altcode: 2011arXiv1101.1732K
We examine the nature and role of mass loss via an equatorial decretion
disk in massive stars with near-critical rotation induced by evolution
of the stellar interior. In contrast to the usual stellar wind mass loss
set by exterior driving from the stellar luminosity, such decretion-disk
mass loss stems from the angular momentum loss needed to keep the star
near and below critical rotation, given the interior evolution and
decline in the star's moment of inertia. Because the specific angular
momentum in a Keplerian disk increases with the square root of the
radius, the decretion mass loss associated with a required level of
angular momentum loss depends crucially on the outer radius for viscous
coupling of the disk, and can be significantly less than the spherical,
wind-like mass loss commonly assumed in evolutionary calculations. We
discuss the physical processes that affect the outer disk radius,
including thermal disk outflow, and ablation of the disk material
via a line-driven wind induced by the star's radiation. We present
parameterized scaling laws for taking account of decretion-disk mass
loss in stellar evolution codes, including how these are affected
by metallicity, or by presence within a close binary and/or a dense
cluster. Effects similar to those discussed here should also be present
in accretion disks during star formation, and may play an important
role in shaping the distribution of rotation speeds on the ZAMS.

---------------------------------------------------------
Title: Constraining the Properties of the Eta Carinae System via 3-D
SPH Models of Space-Based Observations: The Absolute Orientation of
the Binary Orbit
Authors: Madura, Thomas I.; Gull, Theodore R.; Owocki, Stanley P.;
Okazaki, Atsuo T.; Russell, Christopher M. P.
2011BSRSL..80..694M Altcode:
The extremely massive (&gt; 90 M_⊙) and luminous ( = 5 × 10^{6}
L_⊙) star Eta Carinae, with its spectacular bipolar “Homunculus”
nebula, comprises one of the most remarkable and intensely observed
stellar systems in the Galaxy. However, many of its underlying
physical parameters remain unknown. Multiwavelength variations
observed to occur every 5.54 years are interpreted as being due
to the collision of a massive wind from the primary star with the
fast, less dense wind of a hot companion star in a highly elliptical
(e ∼ 0.9) orbit. Using three-dimensional (3-D) Smoothed Particle
Hydrodynamics (SPH) simulations of the binary wind-wind collision,
together with radiative transfer codes, we compute synthetic spectral
images of [Fe III] emission line structures and compare them to existing
Hubble Space Telescope/Space Telescope Imaging Spectrograph (HST/STIS)
observations. We are thus able, for the first time, to tightly constrain
the absolute orientation of the binary orbit on the sky. An orbit with
an inclination of i ∼ 40°, an argument of periapsis ω ∼ 255°,
and a projected orbital axis with a position angle of ∼ 312° east
of north provides the best fit to the observations, implying that
the orbital axis is closely aligned in 3-D space with the Homunculus
symmetry axis, and that the companion star orbits clockwise on the
sky relative to the primary.

---------------------------------------------------------
Title: X-ray Modeling of η Carinae &amp; WR140 from SPH Simulations
Authors: Russell, Christopher M. P.; Corcoran, Michael F.; Okazaki,
Atsuo T.; Madura, Thomas I.; Owocki, Stanley P.
2011BSRSL..80..719R Altcode: 2011arXiv1110.1692R
The colliding wind binary (CWB) systems η Carinae and WR140 provide
unique laboratories for X-ray astrophysics. Their wind-wind collisions
produce hard X-rays that have been monitored extensively by several
X-ray telescopes, including RXTE. To interpret these RXTE X-ray light
curves, we model the wind-wind collision using 3D smoothed particle
hydrodynamics (SPH) simulations. Adiabatic simulations that account
for the emission and absorption of X-rays from an assumed point source
at the apex of the wind-collision shock cone by the distorted winds
can closely match the observed 2-10keV RXTE light curves of both η
Car and WR140. This point-source model can also explain the early
recovery of η Car's X-ray light curve from the 2009.0 minimum by
a factor of 2-4 reduction in the mass loss rate of η Car. Our more
recent models relax the point-source approximation and account for
the spatially extended emission along the wind-wind interaction shock
front. For WR140, the computed X-ray light curve again matches the
RXTE observations quite well. But for η Car, a hot, post-periastron
bubble leads to an emission level that does not match the extended
X-ray minimum observed by RXTE. Initial results from incorporating
radiative cooling and radiatively-driven wind acceleration via a new
anti-gravity approach into the SPH code are also discussed.

---------------------------------------------------------
Title: A proper description of clumping in hot star winds: the key
to obtaining reliable mass-loss rates?
Authors: Sundqvist, Jon O.; Puls, Joachim; Feldmeier, Achim; Owocki,
Stanley P.
2011BSRSL..80...48S Altcode: 2010arXiv1010.3987S
Small-scale inhomogeneities, or `clumping', in the winds of hot,
massive stars are conventionally included in spectral analyses
by assuming optically thin clumps. To reconcile investigations of
different diagnostics using this microclumping technique, very low
mass-loss rates must be invoked for O stars. Recently it has been
suggested that by using the microclumping approximation one may actually
drastically underestimate the mass-loss rates. Here we demonstrate
this, present a new, improved description of clumpy winds, and show
how corresponding models, in a combined UV and optical analysis, can
alleviate discrepancies between previously derived rates and those
predicted by the line-driven wind theory. Furthermore, we show that
the structures obtained in time-dependent, radiation-hydrodynamic
simulations of the intrinsic line-driven instability of such winds,
which are the basis to our current understanding of clumping, in
their present-day form seem unable to provide a fully self-consistent,
simultaneous fit to both UV and optical lines. The reasons for this
are discussed.

---------------------------------------------------------
Title: Testing 3D SPH Models Of Eta Carina's Winds By HST, RXTE,
VLT And VLTI Observations
Authors: Gull, Theodore R.; Madura, T.; Groh, J.; Weigelt, G.;
Corcoran, M.; Owocki, S.; Russell, C.; Okazaki, A.
2011AAS...21733817G Altcode: 2011BAAS...4333817G
Observations of Eta Carina have been combined with three-dimensional
smoothed-particle hydrodynamic (3DSPH) simulations providing
considerable insight on this &gt;100 Mo binary that may become
near-term supernovae, a GRB, or a staid WR binary. Understanding how
this system loses 1e-3 Mo/yr, 500 km/s will provide new understanding
of massive stellar evolution, including the first progenitors of GRBs,
supernovae and pseudo-supernovae. The 3DSPH models extend to 100
semi-major axes ( 2000 AU, &lt;2” at 2300 pc). At these scales,
HST/STIS resolves [Fe III] and [Fe II] spatial-velocity structures
that change with orbital phase and position angle. Radiative transfer
models combining temperature and density with EtaCar B's FUV lead to
synthetic spectroimages of extended wind-wind interfaces. Model X-ray
light curves provide orbital inclination and location of periastron
but cannot determine sky PA. Synthetic spectro-images generated for
a range of possible binary orientations lead to best-fit when the
orbital axis is closely aligned with the Homunculus axis of symmetry,
and periastron with EtaCar B on the far side of EtaCar A. VLTI/AMBER
measures of the continuum, extended hydrogen and helium structures
of EtaCar A demonstrate that, across periastron, EtaCar B penetrates
the primary extended atmosphere. Spectroimagery observations of He
10830 by VLT/CRIRES show blue-shifted emission extending to -1500
km/s, consistent with wind-wind structures driven by the companion's
fast wind. The 2009.0 RXTE X-ray recovery and return of the
spectroscopic high state was much sooner than the 1998.0 and 2003.5
recoveries. What has changed? Suggestions range from a drop in the
primary wind, changes in the secondary wind or line-of-sight shifting
of the wind-wind boundary. We will discuss potential observational
tests based upon predictions by 3DSPH models.

---------------------------------------------------------
Title: Recent X-ray Variability of η Carinae: The Quick Road
to Recovery
Authors: Corcoran, M. F.; Hamaguchi, K.; Pittard, J. M.; Russell,
C. M. P.; Owocki, S. P.; Parkin, E. R.; Okazaki, A.
2010ApJ...725.1528C Altcode:
We report continued monitoring of the superluminous binary system η Car
by the Proportional Counter Array on the Rossi X-ray Timing Observatory
(RXTE) through the 2009 X-ray minimum. The RXTE campaign shows that
the minimum began on 2009 January 16, consistent with the phasings
of the two previous minima, and overall, the temporal behavior of the
X-ray emission was similar to that observed by RXTE in the previous two
cycles. However, important differences did occur. The 2-10 keV X-ray
flux and X-ray hardness decreased in the 2.5 year interval leading up
to the 2009 minimum compared to the previous cycle. Most intriguingly,
the 2009 X-ray minimum was about 1 month shorter than either of the
previous two minima. During the egress from the 2009 minimum the
X-ray hardness increased markedly as it had during egress from the
previous two minima, although the maximum X-ray hardness achieved was
less than the maximum observed after the two previous recoveries. We
suggest that the cycle-to-cycle variations, especially the unexpectedly
early recovery from the 2009 X-ray minimum, might have been the result
of a decline in η Car's wind momentum flux produced by a drop in η
Car's mass loss rate or wind terminal velocity (or some combination),
though if so the change in wind momentum flux required to match the
X-ray variation is surprisingly large.

---------------------------------------------------------
Title: Numerical models of collisions between core-collapse supernovae
and circumstellar shells
Authors: van Marle, Allard Jan; Smith, Nathan; Owocki, Stanley P.;
van Veelen, Bob
2010MNRAS.407.2305V Altcode: 2010arXiv1004.2791V; 2010MNRAS.tmp.1295V
Recent observations of luminous Type IIn supernovae (SNe) provide
compelling evidence that massive circumstellar shells surround their
progenitors. In this paper we investigate how the properties of such
shells influence the SN light curve by conducting numerical simulations
of the interaction between an expanding SN and a circumstellar shell
ejected a few years prior to core collapse. Our parameter study explores
how the emergent luminosity depends on a range of circumstellar
shell masses, velocities, geometries and wind mass-loss rates, as
well as variations in the SN mass and energy. We find that the shell
mass is the most important parameter, in the sense that higher shell
masses (or higher ratios of Mshell/MSN) lead to
higher peak luminosities and higher efficiencies in converting shock
energy into visual light. Lower mass shells can also cause high peak
luminosities if the shell is slow or if the SN ejecta are very fast,
but only for a short time. Sustaining a high luminosity for durations
of more than 100 d requires massive circumstellar shells of the order
of 10 Msolar or more. This reaffirms previous comparisons
between pre-SN shells and shells produced by giant eruptions of
luminous blue variables (LBVs), although the physical mechanism
responsible for these outbursts remains uncertain. The light-curve
shape and observed shell velocity can help diagnose the approximate
size and density of the circumstellar shell, and it may be possible to
distinguish between spherical and bipolar shells with multi-wavelength
light curves. These models are merely illustrative. One can, of course,
achieve even higher luminosities and longer duration light curves from
interaction by increasing the explosion energy and shell mass beyond
values adopted here.

---------------------------------------------------------
Title: Modeling Broadband X-ray Absorption of Massive Star Winds
Authors: Leutenegger, Maurice A.; Cohen, David H.; Zsargó, Janos;
Martell, Erin M.; MacArthur, James P.; Owocki, Stanley P.; Gagné,
Marc; Hillier, D. John
2010ApJ...719.1767L Altcode: 2010arXiv1007.0783L
We present a method for computing the net transmission of X-rays emitted
by shock-heated plasma distributed throughout a partially optically
thick stellar wind from a massive star. We find the transmission
by an exact integration of the formal solution, assuming that the
emitting plasma and absorbing plasma are mixed at a constant mass
ratio above some minimum radius, below which there is assumed to be no
emission. This model is more realistic than either the slab absorption
associated with a corona at the base of the wind or the exospheric
approximation that assumes that all observed X-rays are emitted without
attenuation from above the radius of optical depth unity. Our model is
implemented in XSPEC as a pre-calculated table that can be coupled to a
user-defined table of the wavelength-dependent wind opacity. We provide
a default wind opacity model that is more representative of real wind
opacities than the commonly used neutral interstellar medium (ISM)
tabulation. Preliminary modeling of Chandra grating data indicates
that the X-ray hardness trend of OB stars with spectral subtype can
largely be understood as a wind absorption effect.

---------------------------------------------------------
Title: Detection of high-velocity material from the wind-wind
collision zone of Eta Carinae across the 2009.0 periastron passage
Authors: Groh, J. H.; Nielsen, K. E.; Damineli, A.; Gull, T. R.;
Madura, T. I.; Hillier, D. J.; Teodoro, M.; Driebe, T.; Weigelt, G.;
Hartman, H.; Kerber, F.; Okazaki, A. T.; Owocki, S. P.; Millour, F.;
Murakawa, K.; Kraus, S.; Hofmann, K. -H.; Schertl, D.
2010A&A...517A...9G Altcode: 2010arXiv1003.4527G
We report near-infrared spectroscopic observations of the Eta Carinae
massive binary system during 2008-2009 using the CRIRES spectrograph
mounted on the 8 m UT 1 Very Large Telescope (VLT Antu). We detect a
strong, broad absorption wing in He i λ10833 extending up to -1900
km s-1 across the 2009.0 spectroscopic event. Analysis of
archival Hubble Space Telescope/Space Telescope Imaging Spectrograph
ultraviolet and optical data identifies a similar high-velocity
absorption (up to -2100 km s-1) in the ultraviolet resonance
lines of Si iv λλ1394, 1403 across the 2003.5 event. Ultraviolet
resonance lines from low-ionization species, such as Si ii λλ1527,
1533 and C ii λλ1334, 1335, show absorption only up to -1200
km s-1, indicating that the absorption with velocities
-1200 to -2100 km s-1 originates in a region markedly
more rapidly moving and more ionized than the nominal wind of the
primary star. Seeing-limited observations obtained at the 1.6 m
OPD/LNA telescope during the last four spectroscopic cycles of
Eta Carinae (1989-2009) also show high-velocity absorption in He i
λ10833 during periastron. Based on the large OPD/LNA dataset, we
determine that material with velocities more negative than -900 km
s-1 is present in the phase range 0.976 ≤ ϕ ≤ 1.023 of
the spectroscopic cycle, but absent in spectra taken at ϕ ≤ 0.94 and
ϕ ≥ 1.049. Therefore, we constrain the duration of the high-velocity
absorption to be 95 to 206 days (or 0.047 to 0.102 in phase). We propose
that the high-velocity absorption component originates in shocked gas
in the wind-wind collision zone, at distances of 15 to 45 AU in the
line-of-sight to the primary star. With the aid of three-dimensional
hydrodynamical simulations of the wind-wind collision zone, we find that
the dense high-velocity gas is along the line-of-sight to the primary
star only if the binary system is oriented in the sky such that the
companion is behind the primary star during periastron, corresponding
to a longitude of periastron of ω ~ 240°-270°. We study a possible
tilt of the orbital plane relative to the Homunculus equatorial
plane and conclude that our data are broadly consistent with orbital
inclinations in the range i = 40°-60°. Based on observations
made with ESO Telescopes at the La Silla Paranal Observatory under
programme IDs 381.D-0262, 282.D-5043, and 383.D-0240; with the Hubble
Space Telescope Imaging Spectrograph (HST/STIS) under programs 9420
and 9973; and with the 1.6 m telescope of the OPD/LNA (Brazil).

---------------------------------------------------------
Title: A mass-loss rate determination for ζ Puppis from the
quantitative analysis of X-ray emission-line profiles
Authors: Cohen, David H.; Leutenegger, Maurice A.; Wollman, Emma E.;
Zsargó, Janos; Hillier, D. John; Townsend, Richard H. D.; Owocki,
Stanley P.
2010MNRAS.405.2391C Altcode: 2010arXiv1003.0892C; 2010MNRAS.tmp..603C
We fit every emission line in the high-resolution Chandra grating
spectrum of ζ Pup with an empirical line profile model that accounts
for the effects of Doppler broadening and attenuation by the bulk
wind. For each of 16 lines or line complexes that can be reliably
measured, we determine a best-fitting fiducial optical depth, , and
place confidence limits on this parameter. These 16 lines include
seven that have not previously been reported on in the literature. The
extended wavelength range of these lines allows us to infer, for the
first time, a clear increase in τ* with line wavelength,
as expected from the wavelength increase of bound-free absorption
opacity. The small overall values of τ*, reflected in the
rather modest asymmetry in the line profiles, can moreover all be
fitted simultaneously by simply assuming a moderate mass-loss rate
of 3.5 +/- 0.3 × 10-6Msolaryr-1,
without any need to invoke porosity effects in the wind. The quoted
uncertainty is statistical, but the largest source of uncertainty in
the derived mass-loss rate is due to the uncertainty in the elemental
abundances of ζ Pup, which affects the continuum opacity of the wind,
and which we estimate to be a factor of 2. Even so, the mass-loss
rate we find is significantly below the most recent smooth-wind Hα
mass-loss rate determinations for ζ Pup, but is in line with newer
determinations that account for small-scale wind clumping. If ζ Pup
is representative of other massive stars, these results will have
important implications for stellar and Galactic evolution.

---------------------------------------------------------
Title: Is Eta Carinae a Fast Rotator, and How Much Does the Companion
Influence the Inner Wind Structure?
Authors: Groh, J. H.; Madura, T. I.; Owocki, S. P.; Hillier, D. J.;
Weigelt, G.
2010ApJ...716L.223G Altcode: 2010arXiv1006.4816G
We analyze interferometric measurements of the luminous blue variable
Eta Carinae with the goal of constraining the rotational velocity of
the primary star and probing the influence of the companion. Using
two-dimensional radiative transfer models of latitude-dependent
stellar winds, we find that prolate-wind models with a ratio of the
rotational velocity (v rot) to the critical velocity (v
crit) of W = 0.77-0.92, inclination angle of i = 60°-90°,
and position angle (P.A.) =108°-142° reproduce simultaneously K-band
continuum visibilities from VLTI/VINCI and closure phase measurements
from VLTI/AMBER. Interestingly, oblate models with W = 0.73-0.90 and
i = 80°-90° produce similar fits to the interferometric data, but
require P.A. =210°-230°. Therefore, both prolate and oblate models
suggest that the rotation axis of the primary star is not aligned
with the Homunculus polar axis. We also compute radiative transfer
models of the primary star allowing for the presence of a cavity and
dense wind-wind interaction region created by the companion star. We
find that the wind-wind interaction has a significant effect on the
K-band image mainly via free-free emission from the compressed walls
and, for reasonable model parameters, can reproduce the VLTI/VINCI
visibilities taken at phivb03 = 0.92-0.93. We conclude that
the density structure of the primary wind can be sufficiently disturbed
by the companion, thus mimicking the effects of fast rotation in the
interferometric observables. Therefore, fast rotation may not be the
only explanation for the interferometric observations. Intense temporal
monitoring and three-dimensional modeling are needed to resolve these
issues. Based on observations made with VLTI/AMBER and VLTI/VINCI.

---------------------------------------------------------
Title: Hot-Star Mass-Loss Mechanisms: Winds and Outbursts
Authors: Owocki, S.
2010ASPC..425..199O Altcode:
Mass loss from hot, massive stars can occur both through steady
winds of OB and WR phases, and through relatively brief eruptions
during their Luminous Blue Variable (LBV) phase. This talk reviews
how radiative momentum associated with the extreme luminosity of such
stars is tapped to drive their mass loss. For OB stars, the steady
outflows seem well described by the classical theory of Castor, Abbott,
and Klein (CAK) for scattering of continuum radiation from a stellar
core by line-transitions of metal ions in an otherwise optically
thin wind. For WR stars, the winds themselves become optically thick,
leading to ionization shifts and mass loss that can exceed the single
scattering limit, with driving now arising from a complex combination
of continuum and line opacity. In LBVs, the mass loss can be even more
extreme, with the mechanical energy even becoming comparable to the
radiative luminosity, for example in giant eruptions when the star's
luminosity can actually exceed the classical Eddington limit. A key
theme of this review is to compare and contrast the nature of radiative
driving in these various stages of massive-star evolution.

---------------------------------------------------------
Title: Gamma-Ray Variability from Stellar Wind Porosity in Microquasar
Systems
Authors: Owocki, S. P.; Romero, G. E.; Townsend, R. H. D.; Araudo,
A. T.
2010ASPC..422...49O Altcode:
In the subclass of high-mass X-ray binaries known as “microquasars,”
relativistic hadrons in the jets launched by the compact object can
interact with cold protons from the star's radiatively driven wind,
producing pions that then quickly decay into gamma rays. Since the
resulting gamma-ray emissivity depends on the target density, the
detection of rapid variability in microquasars with GLAST and the
new generation of Cherenkov imaging arrays could be used to probe the
clumped structure of the stellar wind. This paper summarizes recent
analyses of how the “porosity length” of the stellar wind structure
can set the level of fluctuation in gamma rays. A key result is that,
for a porosity length defined by h ≡ L/f, i.e. as the ratio of the
characteristic size L of clumps to their volume filling factor f, the
relative fluctuation in gamma-ray emission in a binary with orbital
separation a scales as (h/πa)1/2 in the “thin-jet”
limit, and is reduced by a factor ( 1 + φ a/2L )-1/2 for a
jet with a finite opening angle φ. For a thin jet and quite moderate
porosity length h ≍ 0.03a, this implies a ca. 10% variation in the
gamma-ray emission.

---------------------------------------------------------
Title: Discovery of Rotational Braking in the Magnetic Helium-strong
Star Sigma Orionis E
Authors: Townsend, R. H. D.; Oksala, M. E.; Cohen, D. H.; Owocki,
S. P.; ud-Doula, A.
2010ApJ...714L.318T Altcode: 2010arXiv1004.2038T
We present new U-band photometry of the magnetic helium-strong star
σ Ori E, obtained over 2004-2009 using the SMARTS 0.9 m telescope at
Cerro Tololo Inter-American Observatory. When combined with historical
measurements, these data constrain the evolution of the star's 1.19 day
rotation period over the past three decades. We are able to rule out
a constant period at the p null = 0.05% level, and instead
find that the data are well described (p null = 99.3%)
by a period increasing linearly at a rate of 77 ms per year. This
corresponds to a characteristic spin-down time of 1.34 Myr, in good
agreement with theoretical predictions based on magnetohydrodynamical
simulations of angular momentum loss from magnetic massive stars. We
therefore conclude that the observations are consistent with σ Ori
E undergoing rotational braking due to its magnetized line-driven wind.

---------------------------------------------------------
Title: Mass and angular momentum loss of first stars via decretion
disks
Authors: Krtička, Jiří; Owocki, Stanley P.; Meynet, Georges
2010IAUS..265...69K Altcode:
Although the first stars were likely very hot and luminous, their low
or zero metallicity implies that any mass loss through winds driven
by line-scattering of radiation in metal ions was likely small or
non-existent. Here we examine the potential role of another possible
mechanism for mass loss in these first stars, namely via decretion
disks associated with near-critical rotation induced from evolution of
the stellar interior. In this case the mass loss is set by the angular
momentum needed to keep the stellar rotation at or below the critical
rate. In present evolutionary models, that mass loss is estimated by
assuming effective release from a spherical shell at the surface. Here
we examine the potentially important role of viscous coupling of the
decretion disk in outward angular momentum transport, emphasizing
that the specific angular momentum at the outer edge of the disk can
be much larger than at the stellar surface. The net result is that,
for a given stellar interior angular momentum excess, the mass loss
required from a decretion disk can be significantly less than invoked
in previous models assuming a direct, near-surface release.

---------------------------------------------------------
Title: Signatures of the 3-D Wind-Wind Collision Cavity in η Car
Authors: Madura, T. I.; Owocki, S. P.
2010RMxAC..38...52M Altcode:
We discuss recent efforts to apply 3-D Smoothed Particle Hydrodynamics
(SPH) simulations to model the binary wind collision in η Carinae,
focusing on the Bore Hole effect, wherein the fast wind from the hot
secondary star carves a cavity in the dense primary wind, allowing
increased escape of radiation from the hotter/deeper layers of
the primary's extended photosphere. This model may provide clues on
how/where UV light is escaping the system, the illumination of distant
material in various directions, and the parameters/orientation of the
binary orbit. The role of interferometric observations in testing the
models is also discussed.

---------------------------------------------------------
Title: High Velocity Absorption during Eta Car B's periastron passage
Authors: Nielsen, Krister E.; Groh, J. H.; Hillier, J.; Gull, T. R.;
Madura, T. I.; Owocki, S. P.; Okazaki, A. T.; Damineli, A.; Teodoro,
M.; Weigelt, G.; Hartman, H.
2010AAS...21542605N Altcode: 2010BAAS...42..341N
Eta Car is one of the most luminous massive stars in the Galaxy,
with repeated eruptions with a 5.5 year periodicity. These events are
caused by the periastron passage of a massive companion in an eccentric
orbit. We report the VLT/CRIRES detection of a strong high-velocity
(&lt; 1900 km/s), broad absorption wing in He I at 10833 A during the
2009.0 periastron passage. Previous observations during the 2003.5
event have shown evidence of such high-velocity absorption in the He
I 10833 transition, allowing us to conclude that the high-velocity
gas is crossing the line-of-sight toward Eta Car over a time period
of approximately 2 months. Our analysis of HST/STIS archival data
with observations of high velocity absorption in the ultraviolet Si
IV and C IV resonance lines, confirm the presence of a high-velocity
material during the spectroscopic low state. The observations provide
direct detection of high-velocity material flowing from the wind-wind
collision zone around the binary system, and we discuss the implications
of the presence of high-velocity gas in Eta Car during periastron.

---------------------------------------------------------
Title: X-ray Modeling of η Carinae and WR140 From Hydrodynamic
Simulations
Authors: Russell, Christopher M. P.; Corcoran, M. F.; Okazaki, A. T.;
Madura, T. I.; Owocki, S. P.
2010AAS...21542602R Altcode: 2010BAAS...42..341R
The colliding wind binary (CWB) systems Eta Carinae and WR140 provide
unique laboratories for X-ray astrophysics. Their wind-wind collisions
produce hard X-rays, which have been monitored extensively by several
X-ray telescopes, such as RXTE and Chandra. To interpret these X-ray
light curves and spectra, we apply 3D hydrodynamic simulations of the
wind-wind collision using both smoothed particle hydrodynamics (SPH)
and finite difference methods. We find isothermal simulations that
account for the absorption of X-rays from an assumed point source of
X-ray emission at the apex of the wind-collision shock cone can closely
match the RXTE light curves of both Eta Carinae and WR140. We are now
applying simulations with self-consistent energy balance and extended
X-ray emission to model the observed X-ray spectra. We present these
results and discuss efforts to understand the earlier recovery of Eta
Carinae's RXTE light curve from the 2009 minimum.

---------------------------------------------------------
Title: 3D Modeling of Forbidden Line Emission in the Binary Wind
Interaction Region of Eta Carinae
Authors: Madura, Thomas; Gull, T. R.; Owocki, S.; Okazaki, A. T.;
Russell, C. M. P.
2010AAS...21542606M Altcode: 2010BAAS...42..341M
We present recent work using three-dimensional (3D) Smoothed Particle
Hydrodynamics (SPH) simulations to model the high ([Fe III], [Ar III],
[Ne III] and [S III]) and low ([Fe II], [Ni II]) ionization forbidden
emission lines observed in Eta Carinae using the HST/STIS. These
structures are interpreted as the time-averaged, outer extensions
of the primary wind and the wind-wind interaction region directly
excited by the FUV of the hot companion star of this massive binary
system. We discuss how analyzing the results of the 3D SPH simulations
and synthetic slit spectra and comparing them to the spectra obtained
with the HST/STIS helps us determine the absolute orientation of
the binary orbit and helps remove the degeneracy inherent to models
based solely on the observed RXTE X-ray light curve. A key point of
this work is that spatially resolved observations like those with
HST/STIS and comparison to 3D models are necessary to determine the
alignment or misalignment of the orbital angular momentum axis with
the Homunculus, or correspondingly, the alignment of the orbital plane
with the Homunculus skirt.

---------------------------------------------------------
Title: The X-ray Variability Of Eta Car, 1996-2010
Authors: Corcoran, Michael F.; Hamaguchi, K.; Gull, T.; Owocki, S.;
Pittard, J.
2010AAS...21542601C Altcode: 2010BAAS...42..340C
X-ray photometry in the 2--10 keV band of the the supermassive binary
star Eta Car has been measured with the Rossi X-ray Timing Explorer
from 1996--2010. The ingress to X-ray minimum is consistent with
a period of 2024 days. The 2009 X-ray minimum began on January 16
2009 and showed an unexpectedly abrupt recovery starting after 12 Feb
2009. The X-ray colors become harder about half-way through all three
minima and continue until flux recovery. The behavior of the fluxes
and X-ray colors for the most recent X-ray minimum, along with Chandra
high resolution grating spectra at key phases suggests a significant
change in the inner wind of Eta Car, a possible indicator that the
star is entering a new unstable phase of mass loss.

---------------------------------------------------------
Title: Origin of the Central Constant Emission Component of Eta
Carinae
Authors: Hamaguchi, Kenji; Corcoran, M. F.; Gull, T.; Ishibashi, K.;
Pittard, J. M.; Hillier, D. J.; Damineli, A.; Davidson, K.; Nielsen,
K. E.; Owocki, S.; Henley, D.; Pollock, A.; Okazaki, A.
2010AAS...21542603H Altcode: 2010BAAS...42..341H
The X-ray campaign observation of the wind-wind colliding (WWC) binary
system, Eta Carinae, targeted at its periastron passage in 2003,
presented a detailed view of the flux and spectral variations of the
X-ray minimum phase. One of the discoveries in this campaign was a
central constant emission (CCE) component very near the central WWC
source (Hamaguchi et al. 2007, ApJ, 663, 522). The CCE component was
noticed between 1-3 keV during the X-ray minima and showed no variation
on either short timescales within any observation or long timescales
of up to 10 years. Hamaguchi et al. (2007) discussed possible origins
as collisionally heated shocks from the fast polar winds from Eta
Car or the fast moving outflow from the WWC with the ambient gas,
or shocked gas that is intrinsic to the wind of Eta Car. During
the 2009 periastron passage, we launched another focussed observing
campaign of Eta Carinae with the Chandra, XMM-Newton and Suzaku
observatories, concentrating on the X-ray faintest phase named the
deep X-ray minimum. Thanks to multiple observations during the deep
X-ray minimum, we found that the CCE spectrum extended up to 10 keV,
indicating presence of hot plasma of kT 4-6 keV. This result excludes
two possible origins that assume relatively slow winds (v 1000 km
s-1) and only leaves the possibility that the CCE plasma
is wind blown bubble at the WWC downstream. The CCE spectrum in 2009
showed a factor of 2 higher soft band flux as the CCE spectrum in
2003, while the hard band flux was almost unchanged. This variation
suggests decrease in absorption column along the line of sight. We
compare this result with recent increase in V-band magnitude of Eta
Carinae and discuss location of the CCE plasma.

---------------------------------------------------------
Title: The Spatially-resolved Interacting Winds of Eta Carinae:
Implications on the Orbit Orientation
Authors: Gull, Theodore R.; Nielsen, K. E.; Corcoran, M.; Hamaguchi,
K.; Madura, T.; Russell, C.; Hillier, D. J.; Owocki, S.; Okazaki, A. T.
2010AAS...21542604G Altcode: 2010BAAS...42..341G
Medium-dispersion long slit spectra, recorded by HST/STIS (R=8000,
Theta=0.1"), resolve the extended wind-wind interaction region of the
massive binary, Eta Carinae. During the high state, extending for about
five years of the 5.54-year binary period, lines of [N II], [Fe III],
[S III], [Ar III] and [Ne III] extend outwards to 0.4" with a velocity
range of -500 to +200 km/s. By comparison, lines of [Fe II] and [Ni II]
extend to 0.7" with a velocity range of -500 to +500 km/s. During the
high state, driven by the lesser wind of Eta Car B and photo-ionized by
the FUV of Eta Car B, the high excitation lines originate in or near
the outer ballistic portions of the wind-wind interaction region. The
lower excitation lines ([Fe II] and [Ni II]) originate from the
boundary regions of the dominating wind of Eta Car A. As the binary
system has an eccentricity exceeding 0.9, the two stars approach quite
close across the periastron, estimated to be within 1 to 2 AU. As a
result, Eta Car B moves into the primary wind structure, cutting off
the FUV supporting the ionization of the high state lines. Forbidden
emission lines of the doubly-ionized species disappear, He II 4686
drops along with the collapse of the X-ray flux. This behavior is
understood through the 3-D models of A. Okazaki and of E. R. Parkin
and Pittard. Discussion will address the orbit orientation relative
to the geometry of the Homunculus, ejected by Eta Carinae in the 1840s.

---------------------------------------------------------
Title: The extended interacting wind structure of Eta Carinae
Authors: Gull, T. R.; Nielsen, K. E.; Corcoran, M. F.; Madura, T. I.;
Owocki, S. P.; Russell, C. M. P.; Hillier, D. J.; Hamaguchi, K.;
Kober, G. V.; Weis, K.; Stahl, O.; Okazaki, A. T.
2009MNRAS.396.1308G Altcode: 2009MNRAS.tmp..693G
The highly eccentric binary system, η Car, provides clues to the
transition of massive stars from hydrogen-burning via the CNO cycle to
a helium-burning evolutionary state. The fast-moving wind of η Car B
creates a cavity in η Car A's slower, but more massive, stellar wind,
providing an in situ probe. The Hubble Space Telescope/Space Telescope
Imaging Spectrograph (HST/STIS), with its high spatial and spectral
resolutions, is well matched to follow temporal spatial and velocity
variations of multiple wind features. We use observations obtained
across 1998-2004 to produce a rudimentary three-dimensional model of the
wind interaction in the η Car system. Broad (+/-500 km s-1)
[FeII] emission line structures extend 0.7arcsec (~1600 au) from the
stellar core. In contrast, [FeIII], [ArIII], [NeIII] and [SIII] lines
extend only 0.3arcsec (700 au) from NE to SW and are blue shifted
from -500 to +200 km s-1. All observed spectral features
vary with the 5.54-year orbital period. The highly ionized, forbidden
emission disappears during the low state, associated with periastron
passage. The high-ionization emission originates in the outer wind
interaction region that is directly excited by the far-ultraviolet
radiation from η Car B. The HST/STIS spectra reveal a time-varying,
distorted paraboloidal structure, caused by the interaction of the
massive stellar winds. The model and observations are consistent with
the orbital plane aligned with the skirt of the Homunculus. However,
the axis of the distorted paraboloid, relative to the major axis of the
binary orbit, is shifted in a prograde rotation along the plane, which
projected on the sky is from NE to NW. Based on observations made
with the National Aeronautics and Space Agency/European Space Agency
(NASA/ESA) HST. Support for Programme numbers 7302, 8036, 8483, 8619,
9083, 9337, 9420, 9973, 10957 and 11273 was provided by NASA directly
to the Space Telescope Imaging Spectrograph Science Team and through
grants from the Space Telescope Science Institute (STScI), which is
operated by the Association of Universities for Research in Astronomy,
Incorporated, under NASA contract NAS5-26555. Based on observations made
with European Southern Observatory telescopes at the La Silla or Paranal
Observatories under programme IDs 070.D-0607, 071.D-0168, 074.D-0141,
077.D-0618 and 380.D-0036. E-mail: theodore.r.gull@nasa.gov

---------------------------------------------------------
Title: Gamma-Ray Variability from Wind Clumping in High-Mass X-Ray
Binaries with Jets
Authors: Owocki, S. P.; Romero, G. E.; Townsend, R. H. D.; Araudo,
A. T.
2009ApJ...696..690O Altcode: 2009arXiv0902.2278O
In the subclass of high-mass X-ray binaries known as "microquasars,"
relativistic hadrons in the jets launched by the compact object can
interact with cold protons from the star's radiatively driven wind,
producing pions that then quickly decay into gamma rays. Since the
resulting gamma-ray emissivity depends on the target density, the
detection of rapid variability in microquasars with Gamma-Ray Large
Area Space Telescope and the new generation of Cherenkov imaging arrays
could be used to probe the clumped structure of the stellar wind. We
show here that the fluctuation in gamma rays can be modeled using a
"porosity length" formalism, usually applied to characterize clumping
effects. In particular, for a porosity length defined by h ≡ ell/f,
i.e., as the ratio of the characteristic size ell of clumps to their
volume filling factor f, we find that the relative fluctuation in
gamma-ray emission in a binary with orbital separation a scales as
√{h/π a} in the "thin-jet" limit, and is reduced by a factor 1/√{1
+ φ a/2 ℓ} for a jet with a finite opening angle phi. For a thin jet
and quite moderate porosity length h ≈ 0.03a, this implies a ca. 10%
variation in the gamma-ray emission. Moreover, the illumination of
individual large clumps might result in isolated flares, as has been
recently observed in some massive gamma-ray binaries.

---------------------------------------------------------
Title: On the behaviour of stellar winds that exceed the photon-tiring
limit
Authors: van Marle, Allard Jan; Owocki, Stanley P.; Shaviv, Nir J.
2009MNRAS.394..595V Altcode: 2009MNRAS.tmp..152V; 2008arXiv0812.0242V
Stars can produce steady-state winds through radiative driving as long
as the mechanical luminosity of the wind does not exceed the radiative
luminosity at its base. This upper bound on the mass-loss rate is
known as the photon-tiring limit. Once above this limit, the radiation
field is unable to lift all the material out of the gravitational
potential of the star, such that only part of it can escape and reach
infinity. The rest stalls and falls back towards the stellar surface,
making a steady-state wind impossible. Photon-tiring is not an issue
for line-driven winds since they cannot achieve sufficiently high
mass-loss rates. It can, however, become important if the star exceeds
the Eddington limit and continuum interaction becomes the dominant
driving mechanism. This paper investigates the time-dependent
behaviour of stellar winds that exceed the photon-tiring limit,
using one-dimensional numerical simulations of a porosity-moderated,
continuum-driven stellar wind. We find that the regions close to the
star show a hierarchical pattern of high-density shells moving back
and forth, unable to escape the gravitational potential of the star. At
larger distances, the flow eventually becomes uniformly outward, though
still quite variable. Typically, these winds have a very high density
but a terminal flow speed well below the escape speed at the stellar
surface. Since most of the radiative luminosity of the star is used
to drive the stellar wind, such stars would appear much dimmer than
expected from the super-Eddington energy generation at their core. The
visible luminosity typically constitutes less than half of the total
energy flow and can become as low as 10 per cent or less for those
stars that exceed the photon-tiring limit by a large margin.

---------------------------------------------------------
Title: Angular momentum loss and stellar spin-down in magnetic
massive stars
Authors: ud-Doula, Asif; Owocki, Stanley P.; Townsend, Richard H. D.
2009IAUS..259..423U Altcode: 2008arXiv0812.2836U
We examine the angular momentum loss and associated rotational
spin-down for magnetic hot stars with a line-driven stellar wind
and a rotation-aligned dipole magnetic field. Our analysis here
is based on our previous 2-D numerical MHD simulation study that
examines the interplay among wind, field, and rotation as a function
of two dimensionless parameters, W(=Vrot/Vorb) and ‘wind magnetic
confinement’, η∗ defined below. We compare and contrast the 2-D,
time variable angular momentum loss of this dipole model of a hot-star
wind with the classical 1-D steady-state analysis by Weber and Davis
(WD), who used an idealized monopole field to model the angular momentum
loss in the solar wind. Despite the differences, we find that the total
angular momentum loss averaged over both solid angle and time follows
closely the general WD scaling ~ ṀΩR2A. The
key distinction is that for a dipole field Alfvèn radius RA
is significantly smaller than for the monopole field WD used in their
analyses. This leads to a slower stellar spin-down for the dipole
field with typical spin-down times of order 1 Myr for several known
magnetic massive stars.

---------------------------------------------------------
Title: Dynamical simulations of magnetically channelled line-driven
stellar winds - III. Angular momentum loss and rotational spin-down
Authors: Ud-Doula, Asif; Owocki, Stanley P.; Townsend, Richard H. D.
2009MNRAS.392.1022U Altcode: 2008arXiv0810.4247U
We examine the angular momentum loss and associated rotational
spin-down for magnetic hot stars with a line-driven stellar wind
and a rotation-aligned dipole magnetic field. Our analysis here is
based on our previous two-dimensional numerical magnetohydrodynamics
simulation study that examines the interplay among wind, field
and rotation as a function of two dimensionless parameters: one
characterizing the wind magnetic confinement () and the other the
ratio (W ≡ Vrot/Vorb) of stellar rotation to
critical (orbital) speed. We compare and contrast the two-dimensional,
time-variable angular momentum loss of this dipole model of a hot-star
wind with the classical one-dimensional steady-state analysis by
Weber and Davis (WD), who used an idealized monopole field to model
the angular momentum loss in the solar wind. Despite the differences,
we find that the total angular momentum loss averaged over both solid
angle and time closely follows the general WD scaling , where is the
mass-loss rate, Ω is the stellar angular velocity and RA
is a characteristic Alfvén radius. However, a key distinction here
is that for a dipole field, this Alfvén radius has a strong-field
scaling RA/R* ~ η1/4*,
instead of the scaling for a monopole field. This leads to a slower
stellar spin-down time that in the dipole case scales as , where is
the characteristic mass loss time and k is the dimensionless factor
for stellar moment of inertia. The full numerical scaling relation
that we cite gives typical spin-down times of the order of 1 Myr for
several known magnetic massive stars.

---------------------------------------------------------
Title: Inter-Division IV-V / Working Group Active B-Type Stars
Authors: Fabregat, Juan; Peters, Geraldine J.; Owocki, Stanley P.;
Bjorkman, Karen S.; Gies, Douglas R.; Henrichs, Hubertus F.; McDavid,
David A.; Neiner, Coralie; Stee, Philippe
2009IAUTA..27..242F Altcode:
The Working Group on Active B-type Stars (formerly known as the
Working Group on Be Stars) was re-established under IAU Commission
29 at the IAU General Assembly in Montreal, Quebec (Canada) in 1979,
and has been continuously active to the present. Its main goal is to
promote and stimulate research and international collaboration on the
field of the active early-type (OB) stars.

---------------------------------------------------------
Title: Division IV / Working Group Massive Stars
Authors: Owocki, Stanley P.; Crowther, Paul A.; Fullerton, Alexander
W.; Koenigsberger, Gloria; Langer, Norbert; Leitherer, Claus; Massey,
Philip L.; Meynet, Georges; Puls, Joachim; St-Louis, Nicole
2009IAUTA..27..236O Altcode:
Our Working Group studies massive, luminous stars, with historical
focus on early-type (OB) stars, but extending in recent years to
include massive red supergiants that evolve from hot stars. There
is also emphasis on the role of massive stars in other branches of
astrophysics, particularly regarding starburst galaxies, the first
stars, core-collapse gamma-ray bursts, and formation of massive stars.

---------------------------------------------------------
Title: Modeling The RXTE Light Curve of Eta Carinae and WR140
Authors: Russell, Christopher M. P.; Corcoran, M. F.; Owocki, S. P.;
Okazaki, A. T.; Madura, T. I.
2009AAS...21340801R Altcode: 2009BAAS...41..205R
Colliding Wind Binaries (CWBs) are a key mechanism for X-ray production
in young clusters and star-forming regions, such as the Orion and
Carina Nebulae. Their extremely fast winds slam together to produce
hard X-rays, allowing a direct diagnostic of the CWB's wind parameters
and hence the evolutionary state of the stars. The Rossi X-ray Timing
Explorer (RXTE) has been regularly monitoring the 2-10keV X-rays of
Eta Carinae and WR140, two massive-star CWBs with highly eccentric
orbits. Using 3D Smoothed Particle Hydrodynamics (SPH) simulations to
model the interaction of these winds, along with a simple model for
the X-ray emission and absorption, we have been able to reproduce the
RXTE light curves of both systems very well.

---------------------------------------------------------
Title: IXO Spectroscopy Of High-mass Stars: Wind Shocks, Magnetic
Confinement, And Colliding-wind Binaries
Authors: Gagné, Marc; Huenemoerder, D.; Osten, R.; Cohen, D.;
Townsend, R.; Leutenegger, M.; Owocki, S.
2009AAS...21345414G Altcode: 2009BAAS...41..352G
High-resolution x-ray spectra of single and binary high-mass stars
observed with the Chandra and XMM gratings in the 6-30 Å region show
resolved, sometimes asymmetric line profiles of highly ionized N, O, Ne,
Mg, Si, S and Fe. The line profiles are being used to probe mass-loss
in the winds of some of the most luminous single stars like ζ Puppis
(O4 If). Meanwhile, the forbidden-to-intercombination line ratio of
the He-like ions are being used to localize the X-ray emitting regions
around many hot, massive stars. The f/i ratios have been used to show,
for example, that the x-rays from magnetic stars like θ1
Orionis C (O6 V) arise close to the stellar photosphere, suggesting
magnetic confinement, while the x-rays from Wolf-Rayet binaries
are produced further away, in a wind interaction zone between the
stars. The superior effective area of the CAT grating spectrometer
on IXO will allow similar studies to be undertaken for a larger, more
distant sample of stars spanning a range of masses, mass-loss rates,
ages, and binary separations and with higher time cadence to look for
dynamic phenomena. The high efficiency of the XMS microcalorimeter
will allow us to detect rapid changes in temperature, column density
and emission measure. The unsurpassed spectral resolution of the XMS
at high energies will probe the very hottest, time variable lines of
Ca XIX, Fe XXV, Fe XXVI and Fe Kα.

---------------------------------------------------------
Title: Stellar Magnetospheres
Authors: Owocki, S.
2009EAS....39..223O Altcode:
The term “magnetosphere” originated historically from early
spacecraft measurements of plasma trapped by the magnetic field of
earth and other planets. But over the years this concept has also
been applied to the magnetically channeled wind outflows from magnetic
stars. The review here describes the basic magnetohydrodynamics (MHD)
approach used to model such stellar magnetospheres, with emphasis on
the central competition between confinement by the magnetic field
vs. expansion of the stellar wind outflow. A key result is that,
for a star with a dipole surface field Bast, surface
radius {Rast}, and asymptotic wind momentum {dot M}
v∞, this competition can be well characterized by a
single “wind magnetic confinement parameter”, ηast
equiv Bast2 Rast2/{dot M}
v∞. For large ηast, closed magnetic looops
can confine parts of the wind up to an Alfvén radius RA
≈ η1/4 Rast, leading to “magnetically
confined wind shocks” that might produce the relatively hard X-ray
emission seen in some magnetic stars. In rotating stars, RA
also roughly characterizes the radius up to which material co-rotates
with the underlying star. For the outflowing wind, the associated
loss of angular momentum, can lead to spindown in the stellar rotation
over a time much shorter than the star's evolutionary timescale. For
confined material within RA but beyond the star's Keplerian
corotation radius RK, the net centrifugal support against
gravity can lead to a “rigidly rotating magnetosphere” composed
of accumulating trapped wind. This can provide a natural explanation
for the rotationally modulated Balmer line emission observed from
magnetic Bp stars. Moreover, magnetic reconnection heating from
episodic centrifugal breakout events might explain the occasional very
hard X-ray flares seen from such stars. Overall, it seems clear that
magnetic fields can play a strong role in confining and channeling such
stellar wind outflows, providing a natural explanation for various
observational signatures structure and variability in the winds and
circumstellar envelopes of massive stars.

---------------------------------------------------------
Title: 3D Modeling of the Massive Binary Wind Interaction Region in
Eta Carinae
Authors: Madura, Thomas; Gull, T.; Owocki, S.; Okazaki, A.; Russell, C.
2009AAS...21340802M Altcode: 2009BAAS...41..205M
We present recent work on the theoretical modeling of low excitation
([Fe II]) and high excitation ([Fe III]) wind lines observed in Eta
Carinae using the HST/STIS. The spatially resolved structures seen in
these lines are interpreted as the time-averaged, outer extensions of
the wind from the primary star and the wind-wind interaction region
of the massive binary system. For most of the orbit, the wind-wind
interface can be approximated as a cone with a half-opening angle of
65° whose axis of rotation is aligned with the major axis of the binary
orbit and appears to lie in the plane of the Homunculus disk. However,
because the orbit is highly elliptical, this approximation breaks down
at periastron and so full 3D Smoothed Particle Hydrodynamics (SPH)
simulations become necessary. By analyzing the results of these 3D
SPH simulations of the binary interactions and comparing them to the
spectra obtained with the HST/STIS we place further constraints on
the orientation of the binary orbit, and hope to eventually determine
how/where UV light is escaping in the system, to search for any direct
signatures of the companion star, and to ultimately establish a mass
ratio for the system.

---------------------------------------------------------
Title: Resolving the Massive Binary Wind Interaction Of Eta Carinae
with HST/STIS
Authors: Gull, Theodore R.; Nielsen, K.; Corcoran, M.; Hillier,
J.; Madura, T.; Hamaguchi, K.; Kober, G.; Owocki, S.; Russell, C.;
Okazaki, A.; Weis, K.; Stahl, O.
2009AAS...21340803G Altcode: 2009BAAS...41..205G
We have resolved the outer structures of the massive binary interacting
wind of Eta Carinae using the HST/STIS. They extend as much as 0.7"
(1600AU} and are highly distorted due to the very elliptical orbit
of the binary system. Observations conducted from 1998.0 to 2004.3
show spatial and temporal variations consistent with a massive,
low excitation wind, seen by spatially resolved, velocity-broadened
[Fe II], and a high excitation extended wind interaction region, seen
by[Fe III], in the shape of a distorted paraboloid. The highly excited
[Fe III] structure is visible for 90% of the 5.5-year period, but
disappears as periastron occurs along with the drop of X-Rays as seen by
RXTE. Some components appear in [Fe II] emission across the months long
minimum. We will discuss the apparent differences between the bowshock
orientation derived from the RXTE light curve and these structures
seen by HST/STIS. Monitoring the temporal variations with phase using
high spatial resolution with appropriate spectral dispersions proves
to be a valuable tool for understanding massive wind interactions.

---------------------------------------------------------
Title: Numerical simulations of continuum-driven winds of
super-Eddington stars
Authors: van Marle, A. J.; Owocki, S. P.; Shaviv, N. J.
2008MNRAS.389.1353V Altcode: 2008MNRAS.tmp..977V; 2008arXiv0806.4536V
We present the results of numerical simulations of continuum-driven
winds of stars that exceed the Eddington limit and compare these
against predictions from earlier analytical solutions. Our models
are based on the assumption that the stellar atmosphere consists of
clumped matter, where the individual clumps have a much larger optical
thickness than the matter between the clumps. This `porosity' of the
stellar atmosphere reduces the coupling between radiation and matter,
since photons tend to escape through the more tenuous gas between the
clumps. This allows a star that formally exceeds the Eddington limit
to remain stable, yet produce a steady outflow from the region where
the clumps become optically thin. We have made a parameter study of
wind models for a variety of input conditions in order to explore
the properties of continuum-driven winds. The results show that
the numerical simulations reproduce quite closely the analytical
scalings. The mass-loss rates produced in our models are much larger
than can be achieved by line driving. This makes continuum driving a
good mechanism to explain the large mass-loss and flow speeds of giant
outbursts, as observed in η Carinae and other luminous blue variable
stars. Continuum driving may also be important in population III stars,
since line driving becomes ineffective at low metallicities. We also
explore the effect of photon tiring and the limits it places on the
wind parameters.

---------------------------------------------------------
Title: Magnetic Models of Circumstellar Clouds around Massive Stars
Authors: Owocki, S.; Townsend, R.; Ud-Doula, A.
2008RMxAC..33...80O Altcode:
This talk reviewed recent efforts to develop dynamical models
for the effects of a surface dipole field on radiatively driven
wind outflows. One particular project applies magnetohydrodynamic
(MHD) simulations of a Magnetically Confined Wind Shock (MCWS) model
(originally developed by Babel &amp; Montmerle 1997) to explain X-ray
emission observed by Rosat (Gagné et al. 1997) from the magnetic O7V
star θ^{1 Ori C.

---------------------------------------------------------
Title: High-Resolution Chandra X-Ray Imaging and Spectroscopy of
the σ Orionis Cluster
Authors: Skinner, Stephen L.; Sokal, Kimberly R.; Cohen, David H.;
Gagné, Marc; Owocki, Stanley P.; Townsend, Richard D.
2008ApJ...683..796S Altcode: 2008arXiv0805.0714S
We present results of a 90 ks Chandra X-ray observation of the young
σ Orionis cluster (age ~ 3 Myr ) obtained with the HETGS. We use
the high-resolution grating spectrum and moderate-resolution CCD
spectrum of the massive central star σ Ori AB (O 9.5 V + B 0.5 V
) to test wind shock theories of X-ray emission and also analyze
the high spatial resolution zero-order ACIS-S image of the central
cluster region. Chandra detected 42 X-ray sources on the primary CCD
(ACIS-S3). All but five have near-IR or optical counterparts and about
one-fourth are variable. Notable high-mass stellar detections are σ Ori
AB, the magnetic B star σ Ori E, and the B5 V binary HD 37525. Most
of the other detections have properties consistent with lower mass K-
or M-type stars. We present the first X-ray spectrum of the unusual
infrared source IRS 1, located ≈3” north of σ Ori
AB. Its X-ray properties and elongated mid-IR morphology suggest that
it is an embedded low-mass T Tauri star whose disk/envelope is being
photoevaporated by σ Ori AB. We focus on the radiative wind shock
interpretation of the soft luminous X-ray emission from σ Ori AB, but
also consider possible alternatives including magnetically confined wind
shocks and colliding wind shocks. Its emission lines show no significant
asymmetries or centroid shifts and are moderately broadened to HWHM ≈
264 km s-1, or one-fourth the terminal wind speed. Forbidden
lines in He-like ions are formally undetected, implying strong UV
suppression. The Mg XI triplet forms in the wind acceleration zone
within one stellar radius above the surface. These X-ray properties
are consistent in several respects with the predictions of radiative
wind shock theory for an optically thin wind, but explaining the narrow
line widths presents a challenge to the theory.

---------------------------------------------------------
Title: Modelling the RXTE light curve of η Carinae from a 3D SPH
simulation of its binary wind collision
Authors: Okazaki, A. T.; Owocki, S. P.; Russell, C. M. P.; Corcoran,
M. F.
2008MNRAS.388L..39O Altcode: 2008arXiv0805.1794O
The very massive star system η Carinae exhibits regular 5.54 yr
(2024 d) period disruptive events in wavebands ranging from the radio
to X-ray. There is a growing consensus that these events likely stem
from periastron passage of an (as yet) unseen companion in a highly
eccentric (e ~ 0.9) orbit. This Letter presents 3D smoothed particle
hydrodynamics (SPH) simulations of the orbital variation of the binary
wind-wind collision, and applies these to modelling the X-ray light
curve observed by the Rossi X-ray Timing Explorer (RXTE). By providing a
global 3D model of the phase variation of the density of the interacting
winds, the simulations allow computation of the associated variation in
X-ray absorption, presumed here to originate from near the apex of the
wind-wind interaction cone. We find that the observed RXTE light curve
can be readily fitted if the observer's line of sight is within this
cone along the general direction of apastron. Specifically, the data
are well fitted by an assumed inclination i = 45° for the orbit's
polar axis, which is thus consistent with orbital angular momentum
being along the inferred polar axis of the Homunculus nebula. The fits
also constrain the position angle φ that an orbital-plane projection
makes with the apastron side of the semimajor axis, strongly excluding
positions φ &lt; 9° along or to the retrograde side of the axis,
with the best-fitting position given by φ = 27°. Overall the results
demonstrate the utility of a fully 3D dynamical model for constraining
the geometric and physical properties of this complex colliding wind
binary system.

---------------------------------------------------------
Title: Rotation and Mass Loss
Authors: Owocki, S.
2008ASPC..388...57O Altcode:
Stellar rotation can play an important role in structuring and enhancing
the mass loss from massive stars. Initial 1D models focussed on the
expected centrifugal enhancement of the line-driven mass flux from the
equator of a rotating star, but the review here emphasizes that the
loss of centrifugal support away from the stellar surface actually
limits the steady mass flux to just the point-star CAK value, with
models near critical rotation characterized by a slow, subcritical
acceleration. Recent suggestions that such slow outflows might have
high enough density to explain disks in Be or B[e] stars are examined
in the context of 2D simulations of the “Wind Compressed Disk” (WCD)
paradigm, together with a review of the tendency for poleward components
of the line-driving force to inhibit WCD formation. When one accounts
for equatorial gravity darkening, the net tendency is in fact for the
relatively bright regions at higher latitude to drive a faster, denser
“bipolar” outflow. I discuss the potential relevance for the bipolar
form of nebulae from LBV stars like η Carinae, but emphasize that,
since the large mass loss associated with the eruption of eta Carinae's
Homunculus would heavily saturate line-driving, explaining its bipolar
form requires development of analogous models for continuum-driven mass
loss. I conclude with a discussion of how radiation seems inherently
ill-suited to supporting or driving a geometrically thin, but optically
thick disk or disk outflow. The disks inferred in Be and B[e] stars may
instead be centrifugally ejected, with radiation inducing an ablation
flow from the disk surface, and thus perhaps playing a greater role
in destroying (rather than creating) an orbiting, circumstellar disk.

---------------------------------------------------------
Title: 3-D SPH Simulations of Colliding Winds in η Carinae
Authors: Okazaki, Atsuo T.; Owocki, Stanley P.; Russell, Christopher
M. P.; Corcoran, Michael F.
2008IAUS..250..133O Altcode: 2008arXiv0803.3977O
We study colliding winds in the superluminous binary η Carinae by
performing three-dimensional, Smoothed Particle Hydrodynamics (SPH)
simulations. For simplicity, we assume both winds to be isothermal. We
also assume that wind particles coast without any net external
forces. We find that the lower density, faster wind from the secondary
carves out a spiral cavity in the higher density, slower wind from the
primary. Because of the phase-dependent orbital motion, the cavity is
very thin on the periastron side, whereas it occupies a large volume
on the apastron side. The model X-ray light curve using the simulated
density structure fits very well with the observed light curve for a
viewing angle of i = 54° and φ = 36°, where i is the inclination
angle and φ is the azimuth from apastron.

---------------------------------------------------------
Title: The Propagation of Kinks in Line-driven Winds
Authors: Feldmeier, A.; Rätzel, D.; Owocki, S. P.
2008ApJ...679..704F Altcode:
We show that discontinuities in spatial derivatives of the velocity and
density law, so-called kinks, can propagate upstream at Mach numbers
&gt; 1 with respect to radiative-acoustic waves in stellar winds
driven by radiation scattering in spectral lines. This fast upstream
propagation of kinks can, for example, explain the slow evolution of
discrete absorption components found in P Cygni line profiles from
O stars.

---------------------------------------------------------
Title: Dynamical simulation of the “velocity-porosity” reduction
in observed strength of stellar wind lines
Authors: Owocki, S. P.
2008cihw.conf..121O Altcode:
I use dynamical simulations of the line-driven instability to examine
the potential role of the resulting flow structure in reducing the
observed strength of wind absorption lines. Instead of the porosity
length formalism used to model effects on continuum absorption, I
suggest reductions in line strength can be better characterized in
terms of a velocity clumping factor that is insensitive to spatial
scales. Examples of dynamic spectra computed directly from instability
simulations do exhibit a net reduction in absorption, but only at a
modest 10-20% level that is well short of the ca. factor 10 required
by recent analyses of PV lines.

---------------------------------------------------------
Title: Using gamma-rays to probe the clumped structure of stellar
winds
Authors: Romero, G. E.; Owocki, S. P.; Araudo, A. T.; Townsend,
R. H. D.; Benaglia, P.
2008cihw.conf..191R Altcode: 2007arXiv0708.1525R
Gamma-rays can be produced by the interaction of a relativistic jet
and the matter of the stellar wind in the subclass of massive X-ray
binaries known as “microquasars”. The relativistic jet is ejected
from the surroundings of the compact object and interacts with cold
protons from the stellar wind, producing pions that then quickly decay
into gamma-rays. Since the resulting gamma-ray emissivity depends on
the target density, the detection of rapid variability in microquasars
with GLAST and the new generation of Cherenkov imaging arrays could be
used to probe the clumped structure of the stellar wind. In particular,
we show here that the relative fluctuation in gamma rays may scale with
the square root of the ratio of porosity length to binary separation,
√{h/a}, implying for example a ca. 10% variation in gamma ray emission
for a quite moderate porosity, h/a ∼ 0.01.

---------------------------------------------------------
Title: Resonance scattering in the X-ray emission lines profiles of
ζ Puppis
Authors: Leutenegger, M. A.; Cohen, D. H.; Kahn, S. M.; Owocki, S. P.;
Paerels, F. B. S.
2008cihw.conf..213L Altcode: 2007arXiv0708.1011L
We present XMM-Newton Reflection Grating Spectrometer observations
of pairs of X-ray emission line profiles from the O star ζ Pup that
originate from the same He-like ion. The two profiles in each pair
have different shapes and cannot both be consistently fit by models
assuming the same wind parameters. We show that the differences in
profile shape can be accounted for in a model including the effects of
resonance scattering, which affects the resonance line in the pair but
not the intercombination line. This implies that resonance scattering
is also important in single resonance lines, where its effect is
difficult to distinguish from a low effective continuum optical depth
in the wind. Thus, resonance scattering may help reconcile X-ray line
profile shapes with literature mass-loss rates.

---------------------------------------------------------
Title: General Discussion
Authors: Moffat, Anthony F. J.; Hillier, D. J.; Hamann, Wolf-Rainer;
Owocki, S. P.
2008cihw.conf..233M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamical simulations of magnetically channelled line-driven
stellar winds - II. The effects of field-aligned rotation
Authors: Ud-Doula, Asif; Owocki, Stanley P.; Townsend, Richard H. D.
2008MNRAS.385...97U Altcode: 2007arXiv0712.2780U; 2008MNRAS.tmp..176U
Building upon our previous magnetohydrodynamics (MHD) simulation study
of magnetic channelling in radiatively driven stellar winds, we examine
here the additional dynamical effects of stellar rotation in the (still)
2D axisymmetric case of an aligned dipole surface field. In addition to
the magnetic confinement parameter η* introduced in Paper
I, we characterize the stellar rotation in terms of a parameter W ≡
Vrot/Vorb (the ratio of the equatorial surface
rotation speed to orbital speed), examining specifically models with
moderately strong rotation W = 0.25 and 0.5, and comparing these to
analogous non-rotating cases. Defining the associated Alfvén radius
RA ~ η1/4* R* and Kepler
corotation radius RK ~ W-2/3 R*,
we find rotation effects are weak for models with RA &lt;
RK, but can be substantial and even dominant for models with
RA &gt;~ RK. In particular, by extending our
simulations to magnetic confinement parameters (up to η*
= 1000) that are well above those (η* = 10) considered
in Paper I, we are able to study cases with RA &gt;&gt;
RK; we find that these do indeed show clear formation of the
rigid body disc predicted in previous analytic models, with however
a rather complex, dynamic behaviour characterized by both episodes
of downward infall and outward breakout that limit the build-up of
disc mass. Overall, the results provide an intriguing glimpse into the
complex interplay between rotation and magnetic confinement, and form
the basis for a full MHD description of the rigid body discs expected
in strongly magnetic Bp stars like σ Ori E.

---------------------------------------------------------
Title: Continuum-Driven Winds from Super-Eddington Stars: A Tale of
Two Limits
Authors: van Marle, A. J.; Owocki, S. P.; Shaviv, N. J.
2008AIPC..990..250V Altcode: 2007arXiv0708.4207V
Continuum driving is an effective method to drive a strong stellar
wind. It is governed by two limits: the Eddington limit and the
photon-tiring limit. A star must exceed the effective Eddington limit
for continuum driving to overcome the stellar gravity. The photon-tiring
limit places an upper limit on the mass loss rate that can be driven
to infinity, given the energy available in the radiation field of the
star. Since continuum driving does not require the presence of metals
in the stellar atmosphere it is particularly suited to removing mass
from low- and zero-metallicity stars and can play a crucial part
in their evolution. Using a porosity length formalism we compute
numerical simulations of super-Eddington, continuum driven winds to
explore their behavior for stars both below and above the photon-tiring
limit. We find that below the photon tiring limit, continuum driving
can produce a large, steady mass loss rate at velocities on the order
of the escape velocity. If the star exceeds the photon-tiring limit,
a steady solution is no longer possible. While the effective mass loss
rate is still very large, the wind velocity is much smaller.

---------------------------------------------------------
Title: Ls i +61 303:. Microquasar or not Microquasar?
Authors: Romero, G. E.; Orellana, M.; Okazaki, A. T.; Owocki, S. P.
2008IJMPD..17.1875R Altcode: 2007arXiv0712.1832R
LS I +61 303 is a puzzling object detected from radio up to very
high-energy gamma-rays. Variability has recently been observed in its
high-energy emission. The object is a binary system, with a compact
object and a Be star as primary. The nature of the secondary and
the origin of the gamma-ray emission are not clearly established at
present. Recent VLBA radio data have been used to claim that the system
is a Be/neutron star colliding wind binary, instead of a microquasar. We
review the main views on the nature of LS I +61 303 and present results
of 3D SPH simulations that can shed some light on the nature of the
system. Our results support an accretion powered source, compatible
with a microquasar interpretation.

---------------------------------------------------------
Title: The accretion regime of LS 5039: 3-D SPH simulations
Authors: Okazaki, A. T.; Romero, G. E.; Owocki, S. P.
2008int..workE..74O Altcode: 2008PoS....67E..74O; 2008arXiv0811.1958O
LS 5039 is a TeV gamma-ray binary with extended radio emission. It
consists of a compact object in the mildly eccentric (e=0.35), 3.9-day
orbit around a massive O star. The nature of the compact object is not
yet established. In this paper, assuming that the compact object is a
black hole, we study the accretion of O-star wind by the black hole,
by performing three-dimensional Smoothed Particle Hydrodynamics (SPH)
simulations. In order to roughly emulate the effect of the stellar
radiation effectively canceling the stellar gravity, we assume that
the O star's gravity does not exert on the wind. The wind particles
are ejected with half the observed terminal velocity in a narrow
range of azimuthal and vertical angles toward the black hole, in
order to emulate the wind significantly slower than the terminal
speed, and optimize the resolution and computational efficiency of
simulations. We find that the mass-accretion rate closely follows the
classical Bondi-Hoyle-Littleton accretion rate, which is of the order
of 10^{16}g/s around periastron. The accretion rate at this level
would provide jets enough power to produce the gamma-rays detected
by HESS. Since the accretion peak occurs near the periastron passage,
we need a strong gamma-ray absorption around periastron in order for
the microquasar scenario to be consistent with the observed orbital
modulation of the TeV gamma-ray flux.

---------------------------------------------------------
Title: Division Iv-V / Wg: Active B-Type Stars
Authors: Owocki, Stanley P.; Štefl, Stanislav; Aerts, Conny; Barbuy,
Beatriz; Fabregat, Juan; Gies, Douglas R.; Guinan, Edward F.; Henrichs,
Hubertus F.; Peters, Geraldine J.; Porter, John M.; Rivinius, Thomas;
Štefl, Stanislav
2007IAUTB..26..151O Altcode:
The meeting of the Working Group on Active B-type Stars consisted of a
business meeting followed by a scientific meeting containing invited
and contributed talks. The titles of the talks and their presenters
are listed below. We plan to publish a series of articles containing
summaries of these talks in Issue No. 39 of the Be Star Newsletter.

---------------------------------------------------------
Title: Eta Carinae: Preparing for the Next Spectroscopic Event and
What We May Learn
Authors: Gull, Theodore R.; Corcoran, M.; Damineli, A.; Hillier, D.;
Owocki, S.; Madura, T.; Weigelt, G.; Groh, J.; Nielsen, K.; Vieira
Kober, G.; Weis, K.; Hartman, H.; Smith, N.; Hamaguchi, K.; Bautista,
M.; Carinae Bunch, Eta
2007AAS...211.5103G Altcode: 2007BAAS...39..815G
Eta Carinae, with its historical ejection events of the 19th century
and propinquity, provides an excellent test bed for understanding
how the most massive stars transition from hydrogen burning to the
presupernova stage. The next X-ray and visible/UV spectroscopic
event, associated with the binary periastron, is predicted to be January
11, 2009 +/- 2 days. Observations are being prepared and proposed
to test models of the binary system and response by the ejecta to
changes in the photo-excitation. This poster will describe current
and developing models and observations that are planned to test and
further enhance these models. We solicit additional observational
and modeling efforts. We acknowledge the resources of the many
observatories supporting this project.

---------------------------------------------------------
Title: A Rigid-Field Hydrodynamics approach to modelling the
magnetospheres of massive stars
Authors: Townsend, R. H. D.; Owocki, S. P.; Ud-Doula, A.
2007MNRAS.382..139T Altcode: 2007MNRAS.tmp..978T; 2007arXiv0709.0694T
We introduce a new Rigid-Field Hydrodynamics approach to modelling the
magnetospheres of massive stars in the limit of very strong magnetic
fields. Treating the field lines as effectively rigid, we develop
hydrodynamical equations describing the one-dimensional flow along each,
subject to pressure, radiative, gravitational and centrifugal forces. We
solve these equations numerically for a large ensemble of field lines
to build up a three-dimensional time-dependent simulation of a model
star with parameters similar to the archetypal Bp star σOriE. Since
the flow along each field line can be solved independently of other
field lines, the computational cost of this approach is a fraction of
an equivalent magnetohydrodynamical treatment. The simulations
confirm many of the predictions of previous analytical and numerical
studies. Collisions between wind streams from opposing magnetic
hemispheres lead to strong shock heating. The post-shock plasma cools
initially via X-ray emission, and eventually accumulates into a warped,
rigidly rotating disc defined by the locus of minima of the effective
(gravitational plus centrifugal) potential. However, a number of novel
results also emerge. For field lines extending far from the star,
the rapid area divergence enhances the radiative acceleration of the
wind, resulting in high shock velocities (up to ~3000kms-1)
and hard X-rays. Moreover, the release of centrifugal potential energy
continues to heat the wind plasma after the shocks, up to temperatures
around twice those achieved at the shocks themselves. Finally, in some
circumstances the cool plasma in the accumulating disc can oscillate
about its equilibrium position, possibly due to radiative cooling
instabilities in the adjacent post-shock regions.

---------------------------------------------------------
Title: Accretion vs. colliding wind models for the gamma-ray binary
LS I +61 303: an assessment
Authors: Romero, G. E.; Okazaki, A. T.; Orellana, M.; Owocki, S. P.
2007A&A...474...15R Altcode: 2007arXiv0706.1320R
Context: LS I +61 303 is a puzzling Be/X-ray binary with variable
gamma-ray emission up to TeV energies. The nature of the compact
object and the origin of the high-energy emission are unclear. One
family of models invokes particle acceleration in shocks from the
collision between the B-star wind and a relativistic pulsar wind,
whereas another centers on a relativistic jet powered by accretion from
the Be star decretion disc onto a black hole. Recent high-resolution
radio observations showing a putative “cometary tail” pointing away
from the Be star near periastron have been cited as support for the
pulsar-wind model. Aims: We wish to carry out a quantitative
assessment of these competing models. Methods: We apply a
“Smoothed Particle Hydrodynamics” (SPH) code in 3D dynamical
simulations for both the pulsar-wind-interaction and accretion-jet
models. The former yields a dynamical description of the shape of
the wind-wind interaction surface. The latter provides a dynamical
estimation of the accretion rate under a variety of conditions, and
how this varies with orbital phase. Results: The results allow
critical evaluation of how the two distinct models confront the data
in various wavebands. When one accounts for the 3D dynamical wind
interaction under realistic constraints for the relative strength of
the B-star and pulsar winds, the resulting form of the interaction
front does not match the putative “cometary tail” claimed from
radio observations. On the other hand, dynamical simulations of
the accretion-jet model indicate that the orbital phase variation
of accretion power includes a secondary broad peak well away from
periastron, thus providing a plausible way to explain the observed
TeV gamma ray emission toward apastron. Conclusions: Contrary to
previous claims, the colliding-wind model is not clearly established
for LS I +61 303, whereas the accretion-jet model can reproduce many
key characteristics, such as required energy budget, lightcurve,
and spectrum of the observed TeV gamma-ray emission.

---------------------------------------------------------
Title: Continuum-driven versus line-driven mass loss and the
Eddington limit
Authors: Owocki, Stanley P.
2007HiA....14..202O Altcode:
Basic stellar structure dictates that stars of ∼ 100 M or
more will be close to the Eddington limit, with luminosities in excess
of 106 L, and radiation pressure contributing
prominently to the support against gravity. Although it is formally
possible to generate static structure models of even more massive stars,
recent studies of dense clusters show there is a sharp cutoff at masses
above ∼ 150 M. This talk examines the role of extreme mass
loss is limiting the masses of stars, emphasizing in particular that
continuum driving, possibly associated with structural instabilities of
radiation dominated envelope, can lead to much stronger mass loss than
is possible by the usual line-scattering mechanism of steady stellar
winds. However, population studies of very young, dense stellar
clusters now suggest quite strongly that there is a sharp cutoff at
masses above ca. 150 M (see, e.g., the talk by Sally Oey,
in this JD 05, p. 206). This is sometimes attributed to a mass limit
on star formation by accretion processes, though there are competing
formation scenarios by binary or cluster merging that would seem likely
to lead to formation of even higher mass stars (see talks in JD14 and
S237). So given the above rough coincidence of the observational
upper mass limit with the Eddington-limit domain of radiation-pressure
dominance, it seems associated instabilities in stellar structure
might actually be a more important factor in this upper mass limit,
leading to extreme mass loss in LBV and/or giant eruption events,
much as inferred from circumstellar nebulae observed around high mass
stars like eta Carinae and the Pistol star.

---------------------------------------------------------
Title: A Nozzle Analysis of Slow-Acceleration Solutions in
One-dimensional Models of Rotating Hot-Star Winds
Authors: Madura, Thomas I.; Owocki, Stanley P.; Feldmeier, Achim
2007ApJ...660..687M Altcode: 2007astro.ph..2007M
One-dimensional (1D) stellar wind models for hot stars rotating at
&gt;~75% of the critical rate show a sudden shift to a slow-acceleration
mode, implying a slower, denser equatorial outflow that might
be associated with the dense equatorial regions inferred for B[e]
supergiants. Here we analyze the steady 1D flow equations for a rotating
stellar wind based on a “nozzle” analogy for terms that constrain
the local mass flux. For low rotation, we find the nozzle minimum
occurs near the stellar surface, allowing a transition to a standard,
CAK-type steep-acceleration solution; but for rotations &gt;~75% of the
critical rate, this inner nozzle minimum exceeds the global minimum,
implying near-surface supercritical solutions would have an overloaded
mass-loss rate. In steady, analytic models in which the acceleration is
assumed to be monotonically positive, this leads the solution to switch
to a slow-acceleration mode. However, time-dependent simulations using
a numerical hydrodynamics code show that, for rotation rates 75%-85%
of critical, the flow can develop abrupt “kink” transitions from a
steep acceleration to a decelerating solution. For rotations above 85%
of critical, the hydrodynamic simulations confirm the slow acceleration,
with the lower flow speed implying densities 5-30 times higher than
the polar (or a nonrotating) wind. Still, when gravity darkening and
2D flow effects are accounted for, it seems unlikely that rotationally
modified equatorial wind outflows could account for the very large
densities inferred for the equatorial regions around B[e] supergiants.

---------------------------------------------------------
Title: Evidence for the Importance of Resonance Scattering in X-Ray
Emission Line Profiles of the O Star ζ Puppis
Authors: Leutenegger, Maurice A.; Owocki, Stanley P.; Kahn, Steven M.;
Paerels, Frits B. S.
2007ApJ...659..642L Altcode: 2006astro.ph.10181L
We fit the Doppler profiles of the He-like triplet complexes of O VII
and N VI in the X-ray spectrum of the O star ζ Pup, using XMM-Newton
RGS data collected over ~400 ks of exposure. We find that they cannot
be well fit if the resonance and intercombination lines are constrained
to have the same profile shape. However, a significantly better fit is
achieved with a model incorporating the effects of resonance scattering,
which causes the resonance line to become more symmetric than the
intercombination line for a given characteristic continuum optical
depth τ*. We discuss the plausibility of this hypothesis,
as well as its significance for our understanding of Doppler profiles
of X-ray emission lines in O stars.

---------------------------------------------------------
Title: Overview of Active OB Stars
Authors: Owocki, S. P.
2007ASPC..361....3O Altcode:
In contrast to their classical ideal as constant, spherical, radiative
envelopes, the hot, luminous, OB-type stars often exhibit signatures
of activity, with associated variability and structure on a range of
temporal and spatial scales. For example, spectral monitoring shows that
even “normal" OB stars commonly exhibit variable Discrete Absorption
Components (DACs) in UV lines formed in their stellar wind. And certain
special classes -- e.g. Be, B[e], LBV, Bp stars -- are in effect
defined by their particular signatures of activity. A common element is
often the emission and/or absorption by circumstellar material. Thus
a general theme here is to consider the physical mechanisms that can
drive material off the nominally tightly bound hydrostatic stellar
surface. Specifically I discuss the dynamical roles played by radiation,
rotation, pulsation, and magnetic fields, and how these, individually
or in combination, can eject the circumstellar clouds, disks, and/or
mass outflows that seem to be at the root of many observed signatures of
hot-star activity. In emphasizing a framework of physical commonalities,
an overall goal is to foster exchanges among researchers specializing
in the various classes of active OB stars.

---------------------------------------------------------
Title: Discussion Session 4a: Structure and Dynamics of Circumstellar
Disks
Authors: Owocki, S. P.
2007ASPC..361..256O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Active OB-Stars: Laboratories for Stellare and Circumstellar
Physics
Authors: Okazaki, A. T.; Owocki, S. P.; Stefl, S.
2007ASPC..361.....O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Fields and UV-line Variability in β Cephei
Authors: Schnerr, R. S.; Henrichs, H. F.; Owocki, S. P.; Ud-Doula,
A.; Townsend, R. H. D.
2007ASPC..361..488S Altcode: 2006astro.ph..3418S
We present results of numerical simulations of wind variability in the
magnetic B1 IVe star β Cephei. 2D-MHD simulations are used to determine
the structure of the wind. From these wind models we calculate line
profiles for different aspect angles to simulate rotation. The results
are compared with the observed UV wind line profiles.

---------------------------------------------------------
Title: Surface Flows and the Struve-Sahade Effect
Authors: Gayley, K. G.; Townsend, R.; Parsons, J.; Owocki, S.
2007ASPC..367..393G Altcode:
The Struve-Sahade effect is the tendency for double-line spectroscopic
hot-star binaries to show anomalously deeper absorption in the
blueshifted profile, particularly for the secondary. This effect breaks
the expected profile reflection symmetry at mirror opposite phases,
which is not easily done. It was suggested at the meeting in Les
Isles-de-la-Madeleine that this effect might be explained by surface
flows induced by irradiation from the companion, but no effort was made
to constrain the observational characteristics of such a flow. Here
we apply a binary synthesis code to the generation of observable
line profiles to characterize the required flow speed and geometry,
and argue that the generation of such flows is a plausible, though
not proven, explanation for the source of the Struve- Sahade effect.

---------------------------------------------------------
Title: The accretion mass regimes in the gamma-ray binary LS I
+61° 303
Authors: Orellana, M.; Romero, G. E.; Okazaki, A. T.; Owocki, S. P.
2007BAAA...50..311O Altcode:
LS I +61° 303 is a puzzling Be/X-ray binary with variable emission up
to TeV energies. The nature of the compact object and the origin of the
high-energy emission are unclear. The usual assumption of microquasar
models for the source is that the accretion rate is coupled to the jet
power hence an energy budget constrain is imposed by the former. Here
we compare the results of 3D-SPH simulations of the mass transference
in LS I +61° 303 with analytically obtained values for the spherical
accretion. In terms of the position of the local maxima, both kind
of estimates show a similar evolution for the accretion rate along
the orbit, but the accretion rates and the underlying physics are
remarkably different.

---------------------------------------------------------
Title: Gamma-ray emission from jet-clump interaction
Authors: Romero, G. E.; Owocki, S. P.; Araudo, A. T.; Townsend, R.
2007BAAA...50..319R Altcode:
Microquasars can produce gamma-rays through the interaction of a
relativistic jet and matter of the wind from the stellar companion. The
jet is ejected from the surroundings of the compact object and
interacts with cold protons from the wind, producing pions that then
quickly decay into gamma-rays. In the present contribution we
calculate the spectral energy distribution and light curve produced
by the interaction of a clumpy wind with the relativistic jet.

---------------------------------------------------------
Title: Radiative Forces in Interacting Binaries
Authors: Owocki, S.
2007ASPC..367..233O Altcode:
I review the role of radative forces in massive interacting binary
systems, with particular emphasis on the distinction between forces
arising from an internal vs. external radiation source. For an internal
source, the multiple scattering of radiation makes possible a multiple
momentum deposition that can keep the radiative flux strong even through
a dense medium. In contrast, the force from an external illumination
of an optically thick surface is inherently limited by the single
scattering value of radiative momentum. As such, radiative forces from
even a close companion star should have little effect in modifying the
overall tidal distortion of stellar surfaces that define conditions for
Roche lobe overflow, or in inhibiting an associated dense accretion
stream. They can, however, play an important role in inhibiting or
braking of a companion star's wind outflow, thereby reducing the
strength and X-ray emission of wind-wind collision shocks. In very
close systems, they can also induce parallel surface flows that might
help explain certain peculiar spectroscopic signatures of such systems.

---------------------------------------------------------
Title: A Simple Nozzle Analysis of Slow-Acceleration Solutions in
1-D Models of Rotating Line-Driven Stellar Winds
Authors: Owocki, S.
2006ASPC..355...65O Altcode:
For a star rotating at more than about 75% of the critical rate,
one-dimensional (1-D) models for the equatorial regions of a
line-driven stellar wind show a sudden shift to a slow-acceleration
solution, implying a slower, denser equatorial outflow that might be
associated with the dense disks inferred for sgB[e] stars. To clarify
the nature of this solution shift, I present here a simple analysis
of the 1-D flow equations based on a nozzle analogy for the terms
that constrain the local mass flux. At low rotation rates the nozzle
minimum (or “throat”) occurs near the stellar surface, allowing
a near-surface transition to a steeply accelerating, supercritical
flow solution. But for rotations above about 75% of the critical rate,
this local, inner nozzle minimum exceeds the global minimum approached
asymptotically at large radii, implying that near-surface supercritical
solutions would now have an overloaded mass loss rate. Maintaining a
monotonically positive acceleration is then only possible if the flow
is kept subcritical out to large radii, where the nozzle function
approaches its absolute minimum. For fixed line-driving parameters,
the associated enhancements in equatorial density are typically a
factor 5-30 relative to the polar (or nonrotating) wind. However, when
gravity darkening and 2-D flow effects are accounted for, it still
seems unlikely that rotationally modified equatorial wind outflows
could account for the very large densities inferred for the disks
around supergiant B[e] stars.

---------------------------------------------------------
Title: Quantitative Analysis of Resolved X-ray Emission Line Profiles
of O Stars
Authors: Cohen, David H.; Leutenegger, M. A.; ud-Doula, A.; Owocki,
S. P.
2006AAS...20913307C Altcode: 2006BAAS...38.1081C
The resolved x-ray emission line profiles of O stars carry a
significant amount of information about the kinematics of the hot
plasma in their massive stellar winds. They provide important clues
to the x-ray production mechanism, and via the effects of continuum
absorption, also place key constraints on conditions in the bulk,
cool wind. Initial interpretations of the relatively symmetric form
of observed profiles suggested significant mass-loss rate reductions,
but recent studies argue that the profile shapes can be affected by
wind clumping and the associated porosity, as well as by resonance
scattering. We report here on the results of quantitative model fitting
to several relatively high-resolution, high signal-to-noise Chandra and
XMM-Newton spectra of O stars. We interpret the results of this analysis
in light of recent adjustments to O star mass-loss rates based on the
analysis of UV data and also in light of state-of-the-art 2D numerical
simulations of the line-force instability. We show that reduced optical
depth is statistically favored over porosity when high-quality data
are quantitatively analyzed and that, in any case, porosity can only
significantly affect line profile shapes when unrealistically large
porosity scales are assumed. Further, we show that resonance scattering
seems to be important in understanding the longer-wavelength oxygen and
nitrogen lines in O stars with very dense winds, but is probably not
significant for most other lines in the x-ray spectra of O stars or in
hot stars with lower mass-loss rates. We acknowledge support from
grant AR5-6003X to Swarthmore College from the Chandra X-ray Center
at the Smithsonian Astrophysical Observatory.

---------------------------------------------------------
Title: Synthesis of Observables from Numerical Simulations of
Magnetized Hot-Star Winds
Authors: St. Vincent, Stephen; Cohen, D. H.; ud-Doula, A.; Townsend,
R. H.; Owocki, S. P.
2006AAS...20910103S Altcode: 2006BAAS...38.1045S
With the recent detection of large scale magnetic fields on several O
and B stars, the anomalous X-ray emission detected from a subset of hot
stars, and advances in numerical modeling, there is increasing interest
in the role magnetic fields play in hot stars. We present synthesized
diagnostics of magnetic hot stars, derived from state-of-the-art
numerical simulations of both slowly rotating O stars with strong winds
(e.g. theta1 Ori C) and rapidly rotating B stars with weaker winds and
stronger fields (e.g. sigma Ori E). The numerical simulations include
2-D ZEUS MHD simulations, as well as rigidly-rotating magnetosphere
(RRM) simulations of the more magnetically dominated winds of early B
stars. We present results on rotationally modulated H-alpha emission,
as well as x-ray emission arising from magnetically channeled wind
shocks and centrifugally driven magnetospheric breakout with the
associated magnetic reconnection.

---------------------------------------------------------
Title: Formation and Evolution of Disks around Classical Be Stars
Authors: Owocki, S.
2006ASPC..355..219O Altcode:
I review models for formation of Be star disks, with particular emphasis
on the role of rapid stellar rotation in providing basis for launching
stellar material into circumstellar orbit. Two key questions are: 1)
How close must the rotation be to the critical rate at which material
at the equtorial surface would effectively be in orbit? and 2) What
mechanism(s) (e.g. radiative driving, magnetic torquing, pulsational
overshoot) might provide the final boost of material from the surface
into an orbiting disk? I also discuss the subsequent evolution of disk
material, with emphasis on outward diffusion vs. radiative and/or
wind ablation as possible destruction mechanisms that might explain
the episodic disappearance of the signature disk emission lines in
some Be stars.

---------------------------------------------------------
Title: Modeling the Magnetospheres of Luminous Stars: Interactions
Between Supersonic Radiation-Driven Winds and Stellar Magnetic Fields
Authors: Owocki, Stanley
2006APS..DPPNI2005O Altcode:
Hot, luminous stars (spectral types O and B) lack the hydrogen
recombination convection zones that drive magnetic dynamo generation
in the sun and other cool stars. Nonetheless observed rotational
modulation of spectral lines formed in the strong, radiatively
driven winds of hot-stars suggest magnetic perturbations analogous
to those that induce “corotating interaction regions” in the solar
wind. Indeed, recent advances in spectropolarimetric techniques have now
led to direct detection of moderate to strong (100-10,000 G), tilted
dipole magnetic fields in several hot stars. Using a combination of
analytic and numerical MHD models, this talk will focus on the role
of such magnetic fields in channelling, and sometimes confining,
the radiatively driven mass outflows from such stars. In particular,
I discuss how the resulting “magnetically confined wind shocks”
can explain the moderately hard X-ray emission seen from the O7V
star Theta-1 Ori C, and how the trapping of material in a “rigidly
rotating magnetosphere” can explain the periodically modulated Balmer
line emission seen from the magnetic B2pV star sigma Ori E. I also
discuss how magnetic reconnection heating from episodic centrifugal
breakout events might explain the occasional very hard X-ray flares
seen from the latter star. I conclude with a brief discussion on the
generation of hot-star fields and the broader relationship to other
types of magnetospheres.

---------------------------------------------------------
Title: The Effect of Porosity on X-Ray Emission-Line Profiles from
Hot-Star Winds
Authors: Owocki, Stanley P.; Cohen, David H.
2006ApJ...648..565O Altcode: 2006astro.ph..2054O
We investigate the degree to which the nearly symmetric form of X-ray
emission lines seen in Chandra spectra of early-type supergiant stars
could be explained by the possibly porous nature of their spatially
structured stellar winds. Such porosity could effectively reduce the
bound-free absorption of X-rays emitted by embedded wind shocks, and
thus allow a more similar transmission of redshifted and blueshifted
emission from the back and front hemispheres, respectively. To obtain
the localized self-shielding that is central to this porosity effect,
it is necessary that the individual clumps be optically thick. In a
medium consisting of clumps of size l and volume filling factor f,
we argue that the general modification in effective opacity should
scale approximately as κeff~κ/(1+τc), where,
for a given atomic opacity κ and mean density ρ, the clump optical
thickness scales as τc=κρl/f. For a simple wind structure
parameterization in which the “porosity length” h≡l/f increases with
local radius r as h=h'r, we find that a substantial reduction
in wind absorption requires a quite large porosity scale factor,
h'&gt;~1, implying large porosity lengths h&gt;~r. The
associated wind structure must thus have either a relatively large
scale l&lt;~r, or a small volume filling factor f~l/r&lt;&lt;1, or
some combination of these. We argue that the relatively small-scale,
moderate compressions generated by intrinsic instabilities in line
driving are unlikely to give such large porosity lengths. This raises
questions about whether porosity effects could play a significant
role in explaining nearly symmetric X-ray line profiles, leaving the
prospect of instead having to invoke a substantial (approximately a
factor of 5) downward revision in the assumed mass-loss rates.

---------------------------------------------------------
Title: Dynamical Simulation of Magnetic Channeling and Reconnection
for Hot-Star X-ray Emission
Authors: Owocki, Stanley
2006cxo..prop.4071O Altcode:
We propose to extend our development of dynamical models of magnetic
mechanisms for hot-star X-ray emission, focusing on two parallel
initiatives. The first will be to develop general 3D simulations of the
Magnetically Confined Wind Shock (MCWS) model with application to stars
with tilted-dipole or multipole fields. The second will be to analyze
detailed plasma processes for the flare heating and X-ray emission
from a newly discovered Centrifugually Driven Magnetic Reconnection
(CDMR) mechanism. Both developments aim to enhance our capacity to
interpret planned and archival Chandra observations of a growing list
of magnetic hot-stars.

---------------------------------------------------------
Title: Centrifugal Breakout of Magnetically Confined Line-Driven
Stellar Winds
Authors: Ud-Doula, A.; Townsend, R. H. D.; Owocki, S. P.
2006IAUJD...1E..29U Altcode:
We present 2D MHD simulations of the radiatively driven outflow from
a rotating hot star with a dipole magnetic field aligned with the
star's rotation axis. We focus primarily on a model with moderately
rapid rotation (half the critical value), and also a large magnetic
confinement parameter, η_* ≡ {B_star^2 R_star^2}/{Mν_∞}=600 . The
magnetic field channels and torques the wind outflow into an equatorial,
rigidly rotating disk extending from near the Kepler corotation radius
outwards. Even with fine-tuning at lower magnetic confinement, none
of the MHD models produce a stable Keplerian disk. Instead, material
below the Kepler radius falls back on to the stellar surface, while
the strong centrifugal force on material beyond the corotation escape
radius stretches the magnetic loops outwards, leading to episodic
breakout of mass when the field reconnects. The associated dissipation
of magnetic energy heats material to temperatures of nearly 10^8 K,
high enough to emit hard (several keV) X-rays. Such centrifugal mass
ejection represents a novel mechanism for driving magnetic reconnection,
and seems a very promising basis for modeling X-ray flares recently
observed in rotating magnetic Bp stars like Sigma Ori E.

---------------------------------------------------------
Title: Continuum vs. Line Driven Mass Loss and the Eddington Limit
Authors: Owocki, S.
2006IAUJD...5E...3O Altcode:
Basic stellar structure dictates that stars of ca. 100 Msun or more will
be close to the Eddington limit, with luminosities in excess of 10^6
Lsun, and radiation pressure contributing prominently to the support
against gravity. Although it is formally possible to generate static
structure models of even more massive stars, there is little or no
evidence such stars actually exist. This talk will focus on the role
of extreme mass loss is limiting the masses of stars, emphasizing in
particular that continuum driving, possibly associated with structural
instabilities of radiation dominated envelope, can lead to much stronger
mass loss than is possible by the usual line-scattering mechanism of
steady stellar winds. I will briefly discuss the implications for the
evolution of the most massive stars, including both the first stars
and GRB progenitors.

---------------------------------------------------------
Title: On the Role of Continuum-driven Eruptions in the Evolution
of Very Massive Stars and Population III Stars
Authors: Smith, Nathan; Owocki, Stanley P.
2006ApJ...645L..45S Altcode: 2006astro.ph..6174S
We suggest that the mass lost during the evolution of very massive
stars may be dominated by optically thick, continuum-driven outbursts
or explosions, instead of by steady line-driven winds. In order for
a massive star to become a Wolf-Rayet star, it must shed its hydrogen
envelope, but new estimates of the effects of clumping in winds from
O-type stars indicate that line driving is vastly insufficient. We
discuss massive stars above roughly 40-50 Msolar, which do
not become red supergiants and for which the best alternative is mass
loss during brief eruptions of luminous blue variables (LBVs). Our
clearest example of this phenomenon is the 19th century outburst
of η Carinae, when the star shed 12-20 Msolar or more
in less than a decade. Other examples are circumstellar nebulae of
LBVs and LBV candidates, extragalactic η Car analogs (the so-called
supernova impostors), and massive shells around supernovae and gamma-ray
bursters. We do not yet fully understand what triggers LBV outbursts or
what supplies their energy, but they occur nonetheless, and they present
a fundamental mystery in stellar astrophysics. Since line opacity
from metals becomes too saturated, the extreme mass loss probably
arises from a continuum-driven wind or a hydrodynamic explosion,
both of which are insensitive to metallicity. As such, eruptive mass
loss could have played a pivotal role in the evolution and ultimate
fate of massive metal-poor stars in the early universe. If they occur
in these Population III stars, such eruptions would also profoundly
affect the chemical yield and types of remnants from early supernovae
and hypernovae thought to be the origin of long gamma-ray bursts.

---------------------------------------------------------
Title: FUSE Observations of Variability in Hot-Star Winds
Authors: Fullerton, A. W.; Massa, D. L.; Prinja, R. K.; Howarth,
I. D.; Willis, A. J.; Owocki, S. P.
2006ASPC..348..130F Altcode:
New constraints on the large-scale wind structures responsible for
discrete absorption components are obtained from far-ultraviolet
time-series observations of O supergiants in the Large Magellanic Cloud.

---------------------------------------------------------
Title: Wind signatures in the X-ray emission-line profiles of the
late-O supergiant ζ Orionis
Authors: Cohen, David H.; Leutenegger, Maurice A.; Grizzard, Kevin T.;
Reed, Catherine L.; Kramer, Roban H.; Owocki, Stanley P.
2006MNRAS.368.1905C Altcode: 2006astro.ph..2599C; 2006MNRAS.tmp..377C
X-ray line-profile analysis has proved to be the most direct
diagnostic of the kinematics and spatial distribution of the very hot
plasma around O stars. The Doppler-broadened line profiles provide
information about the velocity distribution of the hot plasma, while
the wavelength-dependent attenuation across a line profile provides
information about the absorption to the hot plasma, thus providing a
strong constraint on its physical location. In this paper, we apply
several analysis techniques to the emission lines in the Chandra
High Energy Transmission Grating Spectrometer (HETGS) spectrum of
the late-O supergiant ζ Ori (O9.7 Ib), including the fitting of a
simple line-profile model. We show that there is distinct evidence for
blueshifts and profile asymmetry, as well as broadening in the X-ray
emission lines of ζ Ori. These are the observational hallmarks of a
wind-shock X-ray source, and the results for ζ Ori are very similar
to those for the earlier O star, ζ Pup, which we have previously
shown to be well fit by the same wind-shock line-profile model. The
more subtle effects on the line-profile morphologies in ζ Ori, as
compared to ζ Pup, are consistent with the somewhat lower density wind
in this later O supergiant. In both stars, the wind optical depths
required to explain the mildly asymmetric X-ray line profiles imply
reductions in the effective opacity of nearly an order of magnitude,
which may be explained by some combination of mass-loss rate reduction
and large-scale clumping, with its associated porosity-based effects
on radiation transfer. In the context of the recent reanalysis of the
helium-like line intensity ratios in both ζ Ori and ζ Pup, and also
in light of recent work questioning the published mass-loss rates in
OB stars, these new results indicate that the X-ray emission from ζ
Ori can be understood within the framework of the standard wind-shock
scenario for hot stars.

---------------------------------------------------------
Title: Centrifugal Breakout of Magnetically Confined Line-driven
Stellar Winds
Authors: ud-Doula, Asif; Townsend, Richard H. D.; Owocki, Stanley P.
2006ApJ...640L.191U Altcode: 2006astro.ph..1193U
We present two-dimensional MHD simulations of the radiatively
driven outflow from a rotating hot star with a dipole
magnetic field aligned with the star's rotation axis. We
focus primarily on a model with moderately rapid rotation (half
the critical value) and also a large magnetic confinement parameter,
η*≡B2*R2*/M˙V∞=600.
The magnetic field channels and torques the wind outflow into an
equatorial, rigidly rotating disk extending from near the Kepler
corotation radius outward. Even with fine-tuning at lower magnetic
confinement, none of the MHD models produce a stable Keplerian
disk. Instead, material below the Kepler radius falls back onto the
stellar surface, while the strong centrifugal force on material beyond
the corotation escape radius stretches the magnetic loops outward,
leading to the episodic breakout of mass when the field reconnects. The
associated dissipation of magnetic energy heats material to temperatures
of nearly 108 K, high enough to emit hard (several keV)
X-rays. Such centrifugal mass ejection represents a novel mechanism
for driving magnetic reconnection and seems a very promising basis
for modeling X-ray flares recently observed in rotating magnetic Bp
stars like σ Ori E.

---------------------------------------------------------
Title: Magnetic Coupling between Hot Stars and their Radiatively
Driven Winds and Decretion Disks
Authors: Owocki, S.; Townsend, R.; Ud-Doula, A.
2006cosp...36.3548O Altcode: 2006cosp.meet.3548O
Spectropolarimetric techniques have led to detection of strong dipole
magnetic fields in several hot early-type stars Using a combination
of analytic and numerical MHD models this talk will focus on the
role of such magnetic fields in channelling and sometimes confining
the radiatively driven mass outflows from such stars In particular
I discuss how the resulting magnetically confined wind shocks MCWS
can explain the moderately hard X-ray emission seen from the O7V star
Theta-1 Ori C and how the trapping of material in a rigidly rotating
magnetosphere RRM can explain the periodically modulated Balmer line
emission seen from the magnetic B2pV star sigma Ori E I also discuss
how magnetic reconnection heating from episodic centrifugal breakout
events might also explain the occasional very hard X-ray flares seen
from the latter star

---------------------------------------------------------
Title: Wind Signatures in the X-ray Emission Line Profiles of the
O Supergiant zeta Orionis
Authors: Grizzard, K.; Cohen, D. H.; Leutenegger, M. A.; Reed, C.;
Kramer, R. H.; Owocki, S. P.
2005AAS...20718222G Altcode: 2005BAAS...37.1467G
The profiles of X-ray emission lines from several hot stars are resolved
by the Chandra grating spectrometers. These profile shapes provide
information about the kinematics of the hot plasma around these stars
and, via signatures of continuum attenuation, also provide information
about the overlying cold stellar wind. We present a quantitative
analysis of roughly one dozen emission lines in the Chandra HETGS
spectrum of zeta Ori (O 9.7 Ib), and analyze the results in the context
of the line-force instability shock model. The distribution in velocity
space of the emission, along with the separate helium-like line ratio
results, are consistent with the wind-shock scenario. But, although
there is statistically significant evidence for some wind attenuation,
the degree of wind attenuation is a factor of five to ten less than what
would be expected in the context of a smooth, spherically symmetric wind
and the UV-derived wind mass-loss rate. These results are consistent
with those derived from the X-ray emission line profiles of the hotter
O star, zeta Pup, and also with recent, independent evidence from UV
spectroscopy, that O star mass-loss rates have been overestimated. We acknowledge support from NASA contract NAS8-03060 and from the
National Science Foundation, through their REU program at the Keck
Northeast Astronomy Consortium, via contract AST-0353997.

---------------------------------------------------------
Title: The Young Magnetic O Star θ1 Ori C: Multi-phase
Chandra Grating Spectra
Authors: Oksala, M. E.; Gagné, M.; Cohen, D. H.; Tonnesen, S. K.;
Ud-Doula, A.; Owocki, S. P.; Townsend, R. H. D.; Macfarlane, J. J.
2005ASPC..337..289O Altcode:
Chandra high-energy grating spectra of the oblique magnetic rotator
θ1 Ori C (O6 V) obtained at four rotational phases,
corresponding to four different viewing angles with respect to the
magnetic axis, are used to constrain the temperature, spatial location,
and kinematics of the hot plasma on this very young hot star with a
strong (1100 G) dipole field. The plasma is moving but only at speeds
of a few 100 km s-1, much slower than the terminal wind
velocity. It is close to the star (r ≤ 1.8 R*) and hot
(peak temperature ∼30 MK). We analyze these diagnostics in conjuction
with new 2D MHD simulations of the magnetically channeled wind shock
mechanism on θ1 Ori C. This model fits the data surprisingly
well, reproducing the very high temperatures, relatively narrow lines,
overall emission measure, and location of the X-ray emitting plasma.

---------------------------------------------------------
Title: Erratum: “Chandra HETGS Multiphase Spectroscopy of
the Young Magnetic O Star θ1 Orionis C” (<A
href="/abs/2005ApJ...628..986G">ApJ, 628, 986 [2005]</A>)
Authors: Gagné, Marc; Oksala, Mary E.; Cohen, David H.; Tonnesen,
Stephanie K.; ud-Doula, Asif; Owocki, Stanley P.; Townsend, Richard
H. D.; MacFarlane, Joseph J.
2005ApJ...634..712G Altcode:
Figure 13 of our paper shows the dependence of the
forbidden-to-intercombination line ratio on formation radius
and electron density for two He-like ions, Mg XI and S XV,
assuming a 45,000 K photosphere. In calculating the PRISMSPECT
line ratios, we neglected to include the flux from the upper
3P2--&gt;1S0 transition. For
lower-Z ions such as Mg XI, the intercombination doublet is dominated by
flux from the lower transition. But for higher-Z elements, this omission
led us to underestimate the intercombination line strength from our
model. In addition, comparing the spectral type and luminosity
of θ1 Ori C with the recent calibrations of P. Massey et
al. (<A href="/abs/2005ApJ...628..986G">ApJ, 628, 986 [2005]</A>)
and F. Martins et al. (<A href="/abs/2005ApJ...628..986G">ApJ,
628, 986 [2005]</A>) suggests that θ1 Ori C is an
O5.5 V star with radius R~10.6 Rsolar and effective
temperature Teff~40,000 K. This new calibration makes
θ1 Ori C larger and cooler than both the “hot”
and “cool” models in Table 3 of our original paper. In the
revised Figure 13 below, we show the corrected PRISMSPECT line ratios
for Mg XI, Si XIII, and S XV as a function of u=R*/R
for Teff=40,000 K. Also shown are the predictions
from the analytic parametrization of G. R. Blumenthal et al. (<A
href="/abs/2005ApJ...628..986G">ApJ, 628, 986 [2005]</A>). For Mg XI
and Si XIII, we further show the predictions based on the R0
values of D. Porquet et al. (<A href="/abs/2005ApJ...628..986G">ApJ,
628, 986 [2005]</A>). The hatched regions in Figure 13 represent
the 1 σ upper and lower bounds on f/i from the HETG data and
u=R*/R from the PRISMSPECT model. Although the three f/i
ratios yield different bounds on the formation radius, there is a
range of radii, 1.7&lt;R/R*&lt;2.1, that is consistent
with all three measured f/i ratios. Using G. R. Blumenthal et al. (<A
href="/abs/2005ApJ...628..986G">ApJ, 628, 986 [2005]</A>) (Fig. 13,
solid lines), the range of radii is 1.6&lt;R/R*&lt;2.0. The
net effect of the corrected model and lower effective temperature
is to place the X-ray-emitting plasma at a slightly larger radius
than reported originally. Figure 5c added here is a gray-scale
image of emission measure per unit volume from the 375 ks snapshot
of the MHD simulation shown in the upper and lower panels of
Figure 5 of the original paper. Overlaid on the emission-measure
map is a T=106 K contour. The new Figure 5c shows
that the X-rays in the magnetically channeled wind shock (MCWS)
model are formed over a range of radii, with an effective emission
radius R~2R*, consistent with the revised f/i ratio
calculations. Thus, the only remaining discrepancy between
the HETG data and the MCWS model is the overall X-ray variability,
1-LminX/LmaxX~0.33. If the
X-rays are produced, on average, at R~2R*, then occultation
of an X-ray torus by the photosphere would produce a ~17% dip in visible
X-ray luminosity at phase 0.5, only about half the observed drop. We
suggest that the remaining absorption must occur in the dense gas in the
magnetic equatorial plane. To test this idea, future absorption models
will need to properly account for the ionization of the outflowing
circumstellar plasma. M. R. G. and D. H. C. would like to thank
Swarthmore College students Micah Walter-Range for uncovering the
PRISMSPECT modeling error and Steve St. Vincent for producing Figure 5c.

---------------------------------------------------------
Title: A Rigidly Rotating Magnetosphere Model for the Circumstellar
Environments of Magnetic OB Stars
Authors: Townsend, R.; Owocki, S.; Groote, D.
2005ASPC..337..314T Altcode:
We report on a new model for the circumstellar environments of rotating,
magnetic hot stars. This model predicts the channeling of wind plasma
into a corotating magnetosphere, where -- supported against gravity
by centrifugal forces -- it can steadily accumulate over time. We
apply the model to the B2p star σ Ori E, demonstrating that it can
simultaneously reproduce the spectroscopic, photometric and magnetic
variations exhibited by the star.

---------------------------------------------------------
Title: Centrifugal Breakout of Magnetically Confined Line-Driven
Stellar Winds
Authors: Ud-Doula, A.; Townsend, R.; Owocki, S.
2005ASPC..337..319U Altcode:
We present 2D MHD simulations of radiatively driven winds from a hot
star having a dipole magnetic field aligned with the star's rotation
axis. We focus in particular on a model with a moderately rapid
rotation (about half the critical value), and also a strong magnetic
confinement parameter, η* ≡ Beq2
R*2/{dot M} v∞ = 600. The magnetic
field channels and torques the wind outflow into an equatorial,
rigidly rotating disk extending from near the Keplerian co-rotation
radius outwards. The strong centrifugal force on material in the
outer edge of this disk stretches the magnetic loops, leading to
episodic breakout of mass when the field reconnects. The associated
dissipation of magnetic energy heats material to temperatures of nearly
108 K, high enough to emit hard (several keV) X-rays. Such
centrifugal mass ejection represents a novel mechanism for explaining
X-ray flares recently observed in the magnetic Bp star σ Ori E.

---------------------------------------------------------
Title: Dynamical Processes in the Formation of Hot-Star Disks
Authors: Owocki, S.
2005ASPC..337..101O Altcode:
The characteristic signature of Be stars is the Balmer line emission
understood to arise in a circumstellar disk. Unlike the accretion
disks of protostars or mass-exchange binary systems, the evolved and
generally single or wide-binary status of Be stars seems to require
that its disk must form from mass ejection (a.k.a. decretion) from the
star itself. In this review, I use analogies with launching orbital
satellites to discuss candidate processes (radiation, pulsation,
magnetic) for driving such orbital mass ejection, with particular
emphasis on the role of the rapid, possibly near-critical, rotation
of Be stars in facilitating the formation of their signature disks.

---------------------------------------------------------
Title: Radiatively Driven Stellar Winds and Aspherical Mass Loss
Authors: Owocki, S.
2005ASPC..332..169O Altcode: 2005ASPC..332..171O
The high luminosity of massive stars can drive substantial mass loss,
with significant consequences for their evolution and ultimate fate. A
key general issue regards the relative importance of the cumulative
loss in the comparitively moderate, line-driven stellar winds that
persist through much of the star's evolution, versus briefer episodes
of much stronger, continuum-driven mass loss associated with Luminous
Blue Variable (LBV) phases, when the star may approach or exceed the
Eddington limit. Building upon the standard CAK formalism for
line-driven winds, the presentation here summarizes recent work on how
the lateral structuring, or porosity, of a medium can moderate continuum
driving, and lead to a much stronger mass loss that, as inferred for the
giant eruption of eta Carina, approaches the “photon tiring” limit. A
particular focus is the role of rapid (near-critical) stellar rotation
in inducing an equatorial gravity darkening, with the associated polar
brightening then driving both a higher polar mass flux and higher polar
flow speed, a configuration that fits naturally with the inferred mass
distribution and bipolar shape of the eta Carinae Homunculus nebula.

---------------------------------------------------------
Title: The Rigidly Rotating Magnetosphere of σ Orionis E
Authors: Townsend, R. H. D.; Owocki, S. P.; Groote, D.
2005ApJ...630L..81T Altcode: 2005astro.ph..3668T
We characterize the observed variability of the magnetic helium-strong
star σ Ori E in terms of a recently developed rigidly rotating
magnetosphere model. This model predicts the accumulation
of circumstellar plasma in two corotating clouds, situated
in magnetohydrostatic equilibrium at the intersections between the
magnetic and rotational equators. We find that the model can reproduce
well the periodic modulations observed in the star's light curve,
Hα emission-line profile, and longitudinal field strength, confirming
that it furnishes an essentially correct, quantitative description of
the star's magnetically controlled circumstellar environment.

---------------------------------------------------------
Title: Chandra HETGS Multiphase Spectroscopy of the Young Magnetic
O Star θ1 Orionis C
Authors: Gagné, Marc; Oksala, Mary E.; Cohen, David H.; Tonnesen,
Stephanie K.; ud-Doula, Asif; Owocki, Stanley P.; Townsend, Richard
H. D.; MacFarlane, Joseph J.
2005ApJ...628..986G Altcode: 2005astro.ph..4296G
We report on four Chandra grating observations of the oblique magnetic
rotator θ1 Ori C (O5.5 V), covering a wide range of viewing
angles with respect to the star's 1060 G dipole magnetic field. We
employ line-width and centroid analyses to study the dynamics of
the X-ray-emitting plasma in the circumstellar environment, as well
as line-ratio diagnostics to constrain the spatial location, and
global spectral modeling to constrain the temperature distribution and
abundances of the very hot plasma. We investigate these diagnostics as
a function of viewing angle and analyze them in conjunction with new
MHD simulations of the magnetically channeled wind shock mechanism
on θ1 Ori C. This model fits all the data surprisingly
well, predicting the temperature, luminosity, and occultation of the
X-ray-emitting plasma with rotation phase.

---------------------------------------------------------
Title: 2D simulations of the line-driven instability in hot-star
winds. II. Approximations for the 2D radiation force
Authors: Dessart, L.; Owocki, S. P.
2005A&A...437..657D Altcode: 2005astro.ph..3514D
We present initial attempts to include the multi-dimensional nature of
radiation transport in hydrodynamical simulations of the small-scale
structure that arises from the line-driven instability in hot-star
winds. Compared to previous 1D or 2D models that assume a purely
radial radiation force, we seek additionally to treat the lateral
momentum and transport of diffuse line-radiation, initially here
within a 2D context. A key incentive is to study the damping effect
of the associated diffuse line-drag on the dynamical properties of
the flow, focusing particularly on whether this might prevent lateral
break-up of shell structures at scales near the lateral Sobolev angle of
ca. 1^o. Based on 3D linear perturbation analyses that show a viscous
diffusion character for the damping at these scales, we first explore
nonlinear simulations that cast the lateral diffuse force in the simple,
local form of a parallel viscosity. We find, however, that the resulting
strong damping of lateral velocity fluctuations only further isolates
azimuthal zones, leading again to azimuthal incoherence down to the
grid scale. To account then for the further effect of lateral mixing
of radiation associated with the radial driving, we next explore models
in which the radial force is azimuthally smoothed over a chosen scale,
and thereby show that this does indeed translate to a similar scale for
the resulting density and velocity structure. Accounting for both the
lateral line-drag and the lateral mixing in a more self-consistent way
thus requires a multi-ray computation of the radiation transport. As
a first attempt, we explore further a method first proposed by Owocki
(1999), which uses a restricted 3-ray approach that combines a radial
ray with two oblique rays set to have an impact parameter p &lt;
Rast within the stellar core. From numerical simulations
with various grid resolutions (and p), we find that, compared to
equivalent 1-ray simulations, the high-resolution 3-ray models show
systematically a much higher lateral coherence. This first success
in obtaining a lateral coherence of wind structures in physically
consistent 2D simulations of the radiative instability motivates future
development of more general multi-ray methods that can account for
transport along directions that do not intersect the stellar core.

---------------------------------------------------------
Title: Radiatively Driven Surface Flows in Close Hot-Star Binaries
Authors: Gayley, K. G.; Onifer, A. J.; Parsons, J.; Townsend, R.;
Owocki, S. P.
2005AAS...206.4004G Altcode: 2005BAAS...37..494G
The radiative equilibrium condition at the surface of stars in close
binaries requires that there be no net flux normal to the surface, but
within a skin depth there will be net flux transverse to the surface. We
investigate the possible consequences of such a transverse flux, and
show for a plausible range in the uncertain transverse viscosity that
surface flows of order the sound speed are possible. The observable
consequences of such flows are an alteration of the rotationally
broadened line shapes, in a manner consistent with the so-called
“Struve-Sahade effect” in He I resonance lines. The first author
would like to acknowledge funding from the National Science Foundation
(AST 00-98155).

---------------------------------------------------------
Title: Inferring hot-star-wind acceleration from Line Profile
Variability
Authors: Dessart, L.; Owocki, S. P.
2005A&A...432..281D Altcode: 2004astro.ph.11288D
The migration of profile sub-peaks identified in time-monitored optical
emission lines of Wolf-Rayet (WR) star spectra provides a direct
diagnostic of the dynamics of their stellar winds via a measured
Δ vLOS/Δ t, a line-of-sight velocity change per unit
time. Inferring the associated wind acceleration scale from such an
apparent acceleration then relies on the adopted intrinsic velocity
of the wind material at the origin of this variable pattern. Such a
characterization of the Line Emission Region (LER) is in principle
subject to inaccuracies arising from line optical depth effects and
turbulence broadening. In this paper, we develop tools to quantify
such effects and then apply these to reanalyze the LER properties of
time-monitored WR stars. We find that most program lines can be fitted
well with a pure optically thin formation mechanism, that the observed
line-broadening is dominated by the finite velocity extent of the
LER, and that the level of turbulence inferred through Line Profile
Variability (lpv) has only a minor broadening effect in the overall
profile. Our new estimates of LER velocity centroids are systematically
shifted outwards closer to terminal velocity compared to previous
determinations, now suggesting WR-wind acceleration length scales β
R* of the order of 10-20 R⊙, a factor of a few
smaller than previously inferred. Based on radiation-hydrodynamics
simulations of the line-driven-instability mechanism, we compute
synthetic lpv for Ciii5696 Å for WR 111. The results match well the
measured observed migration of 20-30 m s-2, equivalent to β
R* ∼ 20 R⊙. However, our model stellar radius
of 19 R⊙, typical of an O-type supergiant, is a factor 2-10
larger than generally expected for WR core radii. Such small radii leave
inferred acceleration scales to be more extended than expected from
dynamical models of line driving, which typically match a “beta”
velocity law v(r)=v∞ (1-R*/r)β,
with β ≈ 1-2; but the severity of the discrepancy is substantially
reduced compared to previous analyses. We conclude with a discussion
of how using lines formed deeper in the wind would provide a stronger
constraint on the key wind dynamics in the peak acceleration region,
while also potentially providing a diagnostic on the radial variation of
wind clumping, an issue that remains crucial for reliable determination
of O-star mass loss rates.

---------------------------------------------------------
Title: A rigidly rotating magnetosphere model for circumstellar
emission from magnetic OB stars
Authors: Townsend, R. H. D.; Owocki, S. P.
2005MNRAS.357..251T Altcode: 2004astro.ph..8565T; 2005MNRAS.tmp...29T
We present a semi-analytical approach for modelling circumstellar
emission from rotating hot stars with a strong dipole magnetic field
tilted at an arbitrary angle to the rotation axis. By assuming the
rigid-field limit in which material driven (e.g. in a wind outflow)
from the star is forced to remain in strict rigid-body corotation,
we are able to solve for the effective centrifugal-plus-gravitational
potential along each field line, and thereby identify the location of
potential minima where material is prone to accumulate. Applying basic
scalings for the surface mass flux of a radiatively driven stellar
wind, we calculate the circumstellar density distribution that obtains
once ejected plasma settles into hydrostatic stratification along field
lines. The resulting accumulation surface resembles a rigidly rotating,
warped disc, tilted such that its average surface normal lies between
the rotation and magnetic axes. Using a simple model of the plasma
emissivity, we calculate time-resolved synthetic line spectra for the
disc. Initial comparisons show an encouraging level of correspondence
with the observed rotational phase variations of Balmer-line emission
profiles from magnetic Bp stars such as σ Ori E.

---------------------------------------------------------
Title: A pseudo-planar, periodic-box formalism for modelling the
outer evolution of structure in spherically expanding stellar winds
Authors: Runacres, M. C.; Owocki, S. P.
2005A&A...429..323R Altcode: 2004astro.ph..5315R
We present an efficient technique to study the 1D evolution of
instability-generated structure in winds of hot stars out to very
large distances (∼ 1000 stellar radii). This technique makes use of
our previous finding that external forces play little rôle in the
outer evolution of structure. Rather than evolving the entire wind,
as is traditionally done, the technique focuses on a representative
portion of the structure and follows it as it moves out with the
flow. This requires the problem to be formulated in a moving reference
frame. The lack of Galilean invariance of the spherical equations of
hydrodynamics is circumvented by recasting them in a pseudo-planar
form. By applying the technique to a number of problems we show that it
is fast and accurate, and has considerable conceptual advantages. It is
particularly useful to test the dependence of solutions on the Galilean
frame in which they were obtained. As an illustration, we show that,
in a one-dimensional approximation, the wind can remain structured
out to distances of more than 1300 stellar radii from the central star.

---------------------------------------------------------
Title: Chandra X-ray Imaging and Spectroscopy of the Sigma Orionis
Cluster and its Massive OB Stars
Authors: Skinner, S.; Cohen, D.; Gagne, M.; Owocki, S.; Townsend, R.
2004AAS...20510517S Altcode: 2004BAAS...36.1520S
The sigma Orionis cluster is an excellent target for studying the
X-ray properties of a rich population of young stars with ages of a few
million years viewed through low intervening extinction. The cluster
population spans the entire stellar mass spectrum from massive OB stars
to low-mass brown dwarfs. We present results of a sensitive Chandra
observation of the sigma Orionis cluster acquired with ACIS-S/HETG. We
use the newly acquired spectra to test hot star X-ray emission models
based on the wind shock paradigm. The grating spectrum of sigma
Ori AB (O9.5V + B) is dominated by low-temperature emission lines
originating in cool plasma (kT &lt; 1 keV). No significant asymmetries
or Doppler shifts are seen, but the brightest lines do reveal slight
excess broadening (albeit well below that expected for lines formed far
out in the wind at the terminal speed of 1100 km/s). These properties,
along with weak forbidden lines, are more suggestive of line formation
closer to the star where the wind has not yet reached terminal speed. In
sharp contrast, the ACIS-S CCD spectrum of the magnetic B star sigma
Ori E (B2Vp) shows multi-temperature structure with a hot component
at kT &gt; 2 keV and its emission is variable. The hotter plasma is
inconsistent with classical (non-magnetic) wind shock models. It could
originate in an as yet unseen late-type companion, but we discuss an
alternate interpretation in terms of magnetically confined wind shocks
and summarize related MHD model development for hot stars. This
work was supported by NASA / SAO grant GO3-4007A.

---------------------------------------------------------
Title: A Porosity-Length Formalism for Photon-Tiring-limited Mass
Loss from Stars above the Eddington Limit
Authors: Owocki, Stanley P.; Gayley, Kenneth G.; Shaviv, Nir J.
2004ApJ...616..525O Altcode: 2004astro.ph..9573O
We examine radiatively driven mass loss from stars near and above the
Eddington limit. Building on the standard CAK theory of driving by
scattering in an ensemble of lines with a power-law distribution of
opacity, we first show that the formal divergence of such line-driven
mass loss as a star approaches the Eddington limit is actually limited
by the “photon tiring” associated with the work needed to lift
material out of the star's gravitational potential. We also examine
such tiring in simple continuum-driven models in which a specified
outward increase in opacity causes a net outward acceleration above the
radius where the generalized Eddington parameter exceeds unity. When
the density at this radius implies a mass loss too close to the tiring
limit, the overall result is flow stagnation at a finite radius. Since
escape of a net steady wind is precluded, such circumstances are
expected to lead to extensive variability and spatial structure. After
briefly reviewing convective and other instabilities that also can be
expected to lead to extensive structure in the envelope and atmosphere
of a star near or above the Eddington limit, we investigate how the
porosity of such a structured medium can reduce the effective coupling
between the matter and radiation. Introducing a new “porosity-length”
formalism, we derive a simple scaling for the reduced effective opacity
and use this to derive an associated scaling for the porosity-moderated,
continuum-driven mass-loss rate from stars that formally exceed the
Eddington limit. For a simple super-Eddington model with a single
porosity length that is assumed to be on the order of the gravitational
scale height, the overall mass loss is similar to that derived in
previous porosity models, given roughly by L*/a*c
(where L* is the stellar luminosity and c and a*
are the speed of light and the atmospheric sound speed). This is much
higher than is typical of line-driven winds but is still only a few
percent of the tiring limit. To obtain still stronger mass loss that
approaches observationally inferred values near this limit, we draw
on an analogy with the power-law distribution of line-opacity in
the standard CAK model of line-driven winds and thereby introduce
a “power-law-porosity” model in which the associated structure
has a broad range of scales. We show that for power indices
αp&lt;1, the mass-loss rate can be enhanced over the
single-scale model by a factor that increases with the Eddington
parameter as Γ-1+1/αp. For lower αp
(~0.5-0.6) and/or moderately large Γ (&gt;3-4), such models lead to
mass-loss rates that approach the photon-tiring limit. Together with
the ability to drive quite fast outflow speeds (of order the surface
escape speed), the derived, near-tiring-limited mass loss offers a
potential dynamical basis to explain the observationally inferred
large mass loss and flow speeds of giant outbursts in η Carinae and
other luminous blue variable stars.

---------------------------------------------------------
Title: FUSE Observations of Variability in Hot-Star Winds
Authors: Fullerton, A. W.; Massa, D. L.; Prinja, R. K.; Howarth,
I. D.; Willis, A. J.; Owocki, S. P.
2004astro.ph.10082F Altcode:
New constraints on the large-scale wind structures responsible for
discrete absorption components are obtained from far-ultraviolet
time-series observations of O supergiants in the Large Magellanic Cloud.

---------------------------------------------------------
Title: MHD Simulation of Magnetically Confined Wind Shock (MCWS)
Model for Hot Star X-ray Emission
Authors: Owocki, Stanley
2004cxo..prop.4011O Altcode:
We propose to use 3D dynamical simulations to model the X-ray
spectrum from early-type stars with magnetic fields that channel their
radiatively driven stellar winds into strong, Magnetically Confined
Wind Shocks (MCWS). Building on our recent success in applying the MCWS
model for interpreting the Chandra spectrum of the slowly rotating,
moderately magnetic hot star theta-1 Ori C, our development will now
focus on accounting for cases with much more rapid rotation and/or a
much stronger degree of magnetic confinement. The results will yield
predicted X-ray spectra for a range of rotation periods, wind mass
loss rates, field strengths and orientations. This will provide an
important new tool for interpreting X-ray observations of early-type
stars with relatively hard X-ray spectra.

---------------------------------------------------------
Title: A Steady, Radiative-Shock Method for Computing X-Ray Emission
from Colliding Stellar Winds in Close, Massive-Star Binaries
Authors: Antokhin, I. I.; Owocki, S. P.; Brown, J. C.
2004ApJ...611..434A Altcode:
We present a practical, efficient, semianalytic formalism for computing
steady state X-ray emission from radiative shocks between colliding
stellar winds in relatively close (orbital period up to order tens of
days) massive-star, binary systems. Our simplified approach idealizes
the individual wind flows as smooth and steady, ignoring the intrinsic
instabilities and associated structure thought to occur in such
flows. By also suppressing thin-shell instabilities for wind-collision
radiative shocks, our steady state approach avoids the extensive
structure and mixing that has thus far precluded reliable computation of
X-ray emission spectra from time-dependent hydrodynamical simulations
of close-binary, wind-collision systems; but in ignoring the unknown
physical level of such mixing, the luminosity and hardness of X-ray
spectra derived here represent upper limits to what is possible for a
given set of wind and binary parameters. A key feature of our approach
is the separation of calculations for the small-scale shock-emission
from the ram-pressure-balance model for determining the large-scale,
geometric form of the wind-wind interaction front. Integrating the
localized shock emission over the full interaction surface and using
a warm-absorber opacity to take account of attenuation by both the
smooth wind and the compressed, cooled material in the interaction
front, the method can predict spectra for a distant observer at any
arbitrary orbital inclination and phase. We illustrate results for a
sample selection of wind, stellar, and binary parameters, providing
both full X-ray light curves and detailed spectra at selected orbital
phases. The derived spectra typically have a broad characteristic
form, and by synthetic processing with the standard XSPEC package,
we demonstrate that they simply cannot be satisfactorily fitted with
the usual attenuated single- or two-temperature thermal-emission
models. We conclude with a summary of the advantages and limitations
of our approach and outline its potential application for interpreting
detailed X-ray observations from close, massive-star binary systems.

---------------------------------------------------------
Title: On the Steady Nature of Line-Driven Disk Winds
Authors: Pereyra, Nicolas A.; Owocki, Stanley P.; Hillier, D. John;
Turnshek, David A.
2004ApJ...608..454P Altcode: 2003astro.ph.11268P
We perform an analytic investigation of the steady nature of line-driven
disk winds, independent of hydrodynamic simulations. Our motive is to
determine whether line-driven disk winds can account for the wide/broad
UV resonance absorption lines seen in cataclysmic variables (CVs)
and quasi-stellar objects (QSOs). In both CVs and QSOs, observations
generally indicate that the absorption arising in the outflowing winds
has a steady velocity structure on timescales exceeding years (for CVs)
and decades (for QSOs). However, published results from hydrodynamic
simulations of line-driven disk winds are mixed, with some researchers
claiming that the models are inherently unsteady, while other models
produce steady winds. The analytic investigation presented here shows
that the line-driven disk winds can be steady if the accretion disk is
steady and capable of locally supplying the corresponding mass flow. In
particular, we show that a gravitational force initially increasing
along the wind streamline, which is characteristic of disk winds, does
not imply an unsteady wind. The steady nature of line-driven disk winds
is consistent with the one-dimensional streamline disk wind models of
Murray and collaborators and the 2.5-dimensional time-dependent models
of Pereyra and collaborators. This paper emphasizes the underlying
physics behind the steady nature of line-driven disk winds using
mathematically simple models that mimic the disk environment.

---------------------------------------------------------
Title: Rotation and Mass Ejection: the Launching of Be-Star Disks
(Invited Review)
Authors: Owocki, S. P.
2004IAUS..215..515O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Effects of Field-Aligned Rotation on the Magnetically
Channeled Line-Driven Winds
Authors: Ud-Doula, A.; Owocki, S.
2004IAUS..215..525U Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Chandra HETGS Multi-phase Spectroscopy of the Young Magnetic
O Star Theta 1 Ori C
Authors: Oksala, M.; Gagne, M.; Cohen, D.; Tonnesen, S.; ud-Doula,
A.; Owocki, S.; MacFarlane, J.
2004AAS...204.6214O Altcode: 2004BAAS...36..774O
Chandra High-Energy Grating spectra obtained at four rotational phases
of θ 1 Ori C (O6-7 V), corresponding to four different
viewing angles with respect to the magnetic axis, are used to constrain
the temperature, spatial location, and kinematics of the hot plasma
on this very young hot star with a strong (1100 G) dipole field. The
plasma is moving, but only at speeds of a few 100 km s-1,
much slower than the terminal wind velocity. It is close to the star
(within 1.5 R* of the surface) and hot (peak temperature 30 MK). We
analyze these diagnostics in conjunction with new MHD simulations of
the magnetically channeled wind shock mechanism on θ 1
Ori C. This model fits all the data surprisingly well, reproducing
the very high temperatures, relatively narrow and unshifted lines,
and the near-star source location.

---------------------------------------------------------
Title: Be-star rotation: how close to critical?
Authors: Townsend, R. H. D.; Owocki, S. P.; Howarth, I. D.
2004MNRAS.350..189T Altcode: 2003astro.ph.12113T
We argue that, in general, observational studies of Be-star rotation
have paid insufficient attention to the effects of equatorial gravity
darkening. We present new line-profile calculations that emphasize
the insensitivity of line width to rotation for fast rotators. Coupled
with a critical review of observational procedures, these calculations
suggest that the observational parameter v sini may systematically
underestimate the true projected equatorial rotation velocity,
ve sini, by some tens of per cent for rapid rotators. A
crucial implication of this work is that Be stars may be rotating
much closer to their critical velocities than is generally supposed,
bringing a range of new processes into contention for the elusive
physical mechanism responsible for the circumstellar disc thought to
be central to the Be phenomenon.

---------------------------------------------------------
Title: Kinematic model inversions of hot star recurrent DAC data -
tests against dynamical CIR models
Authors: Krtička, J.; Barrett, R. K.; Brown, J. C.; Owocki, S. P.
2004A&A...417.1039K Altcode: 2004astro.ph..1383K
The Discrete Absorption Components (DACs) commonly observed in
the ultraviolet lines of hot stars have previously been modelled
by dynamical simulations of Corotating Interaction Regions (CIRs)
in their line-driven stellar winds. Here we apply the kinematic DAC
inversion method of Brown et al. to the hydrodynamical CIR models
and test the reliability of the results obtained. We conclude that
the inversion method is able to recover valuable information on the
velocity structure of the mean wind and to trace movement of velocity
plateaux in the hydrodynamical data, though the recovered density
profile of the stream is correct only very near to the stellar surface.

---------------------------------------------------------
Title: An XMM-Newton observation of the massive binary HD 159176
Authors: De Becker, M.; Rauw, G.; Pittard, J. M.; Antokhin, I. I.;
Stevens, I. R.; Gosset, E.; Owocki, S. P.
2004A&A...416..221D Altcode: 2004astro.ph..2663D
We report the analysis of an XMM-Newton observation of the close binary
HD 159176 (O7 V + O7 V). The observed LX/Lbol
ratio reveals an X-ray luminosity exceeding by a factor ∼7 the
expected value for X-ray emission from single O-stars, therefore
suggesting a wind-wind interaction scenario. EPIC and RGS spectra are
fitted consistently with a two temperature mekal optically thin thermal
plasma model, with temperatures ranging from ∼2 to 6×106
K. At first sight, these rather low temperatures are consistent with
the expectations for a close binary system where the winds collide
well before reaching their terminal velocities. We also investigate
the variability of the X-ray light curve of HD 159176 on various short
time scales. No significant variability is found and we conclude that
if hydrodynamical instabilities exist in the wind interaction region of
HD 159176, they are not sufficient to produce an observable signature in
the X-ray emission. Hydrodynamic simulations using wind parameters from
the literature reveal some puzzling discrepancies. The most striking one
concerns the predicted X-ray luminosity which is one or more orders of
magnitude larger than the observed one. A significant reduction of the
mass loss rate of the components compared to the values quoted in the
literature alleviates the discrepancy but is not sufficient to fully
account for the observed luminosity. Because hydrodynamical models are
best for the adiabatic case whereas the colliding winds in HD 159176 are
most likely highly radiative, a totally new approach has been envisaged,
using a geometrical steady-state colliding wind model suitable for
the case of radiative winds. This model successfully reproduces the
spectral shape of the EPIC spectrum, but further developments are still
needed to alleviate the disagreement between theoretical and observed
X-ray luminosities. Based on observations with XMM-Newton, an ESA
Science Mission with instruments and contributions directly funded by
ESA Member States and the USA (NASA).

---------------------------------------------------------
Title: Inference of hot star density stream properties from data
on rotationally recurrent DACs
Authors: Brown, J. C.; Barrett, R. K.; Oskinova, L. M.; Owocki, S. P.;
Hamann, W. -R.; de Jong, J. A.; Kaper, L.; Henrichs, H. F.
2004A&A...413..959B Altcode:
The information content of data on rotationally periodic
recurrent discrete absorption components (DACs) in hot star
wind emission lines is discussed. The data comprise optical
depths τ(w,φ) as a function of dimensionless Doppler velocity
w=(Δλ/λ0)(c/v&amp;infy) and of time expressed
in terms of stellar rotation angle φ. This is used to study the
spatial distributions of density, radial and rotational velocities, and
ionisation structures of the corotating wind streams to which recurrent
DACs are conventionally attributed. The simplifying assumptions
made to reduce the degrees of freedom in such structure distribution
functions to match those in the DAC data are discussed and the problem
then posed in terms of a bivariate relationship between τ(w,φ) and
the radial velocity vr(r), transverse rotation rate Ω(r)
and density ρ(r,φ) structures of the streams. The discussion applies
to cases where: the streams are equatorial; the system is seen edge
on; the ionisation structure is approximated as uniform; the radial
and transverse velocities are taken to be functions only of radial
distance but the stream density is allowed to vary with azimuth. The
last kinematic assumption essentially ignores the dynamical feedback
of density on velocity and the relationship of this to fully dynamical
models is discussed. The case of narrow streams is first considered,
noting the result of Hamann et al. (\cite{Ham01}) that the apparent
acceleration of a narrow stream DAC is higher than the acceleration
of the matter itself, so that the apparent slow acceleration of DACs
cannot be attributed to the slowness of stellar rotation. Thus DACs
either involve matter which accelerates slower than the general wind
flow, or they are formed by structures which are not advected with the
matter flow but propagate upstream (such as Abbott waves). It is then
shown how, in the kinematic model approximation, the radial speed
of the absorbing matter can be found by inversion of the apparent
acceleration of the narrow DAC, for a given rotation law. The
case of broad streams is more complex but also more informative. The
observed τ(w,φ) is governed not only by vr(r) and Ω(r)
of the absorbing stream matter but also by the density profile across
the stream, determined by the azimuthal (φ0) distribution
function F0(φ0) of mass loss rate around the
stellar equator. When F0(φ0) is fairly wide
in φ0, the acceleration of the DAC peak τ(w,φ) in w
is generally slow compared with that of a narrow stream DAC and the
information on vr(r), Ω(r) and F0(φ0)
is convoluted in the data τ(w,φ). We show that it is possible,
in this kinematic model, to recover by inversion, complete information
on all three distribution functions vr(r), Ω(r) and
F0(φ0) from data on τ(w,φ) of sufficiently
high precision and resolution since vr(r) and Ω(r) occur
in combination rather than independently in the equations. This
is demonstrated for simulated data, including noise effects, and is
discussed in relation to real data and to fully hydrodynamic models. Figures \ref{fig:results1}, \ref{fig:results2}, \ref{fig:results7},
\ref{fig:results8}, \ref{fig:results13}, \ref{fig:results14},
\ref{fig:rotation1} and \ref{fig:rotation2} are only available in
electronic form at http://www.edpsciences.org

---------------------------------------------------------
Title: The Effect of Magnetic Field Tilt and Divergence on the Mass
Flux and Flow Speed in a Line-driven Stellar Wind
Authors: Owocki, Stanley P.; ud-Doula, Asif
2004ApJ...600.1004O Altcode: 2003astro.ph.10176O
We carry out an extended analytic study of how the tilt and
faster-than-radial expansion from a magnetic field affect the mass
flux and flow speed of a line-driven stellar wind. A key motivation
is to reconcile results of numerical MHD simulations with previous
analyses that had predicted nonspherical expansion would lead to a
strong speed enhancement. By including finite-disk correction effects,
a dynamically more consistent form for the nonspherical expansion,
and a moderate value of the line-driving power index α, we infer
more modest speed enhancements that are in good quantitative agreement
with MHD simulations and also are more consistent with observational
results. Our analysis also explains simulation results that show the
latitudinal variation of the surface mass flux scales with the square
of the cosine of the local tilt angle between the magnetic field and
the radial direction. Finally, we present a perturbation analysis of
the effects of a finite gas pressure on the wind mass-loss rate and
flow speed in both spherical and magnetic wind models, showing that
these scale with the ratio of the sound speed to surface escape speed,
a/vesc, and are typically 10%-20% compared to an idealized,
zero-gas-pressure model.

---------------------------------------------------------
Title: Stellar wind mechanisms and instabilities
Authors: Owocki, S.
2004EAS....13..163O Altcode:
I review driving mechanisms for stellar winds, using first the example
of the coronal, pressure-driven solar wind, but then focussing mainly
on radiation-pressure driven winds from hot, luminous stars. For
the latter, I review the central role of line-opacity as a coupling
between matter and radiation, emphasizing how the Doppler shift of
an accelerating wind outflow exposes the strong line opacity to a
substantial continuum flux, and thus allows the line force to sustain
the outward acceleration against gravity. Through the CAK formalism that
assumes a power-law distribution of line-opacity, I derive the mass
loss rate and wind velocity law, and discuss how these are altered
by various refinements like a finite-disk correction, ionization
variations in opacity, and a non-zero sound speed. I also discuss how
multiline scattering in Wolf-Rayet (WR) winds can allow them to exceed
the single scattering limit, for which the wind and radiative momenta
are equal. Through a time-dependent perturbation analysis, I show how
the line-driving leads to a fast, inward “Abbott-wave” mode for long
wavelength perturbations, and a strong Line-Deshadowing-Instability at
short wavelengths, summarizing also 1D and 2D numerical simulations
of the nonlinear evolution of this instability. I next discuss how
rapid stellar rotation alters the latitudinal variation of mass
loss and flow speed, and how this depends on treatment of gravity
darkening, nonradial line forces, and “bi-stability” shifts in
ionization. Finally, I conclude with a discussion of the large mass
loss epochs of Luminous Blue Variable (LBV) stars, and how these might
be modeled via super-Eddington, continuum driving moderated by the
“porosity” associated with extensive spatial structure.

---------------------------------------------------------
Title: Two-dimensional simulations of the line-driven instability
in hot-star winds
Authors: Dessart, L.; Owocki, S. P.
2003A&A...406L...1D Altcode:
We report initial results of two-dimensional simulations of the
nonlinear evolution of the line-driven instability (LDI) in hot-star
winds. The method is based on the Smooth Source Function (SSF)
formalism for nonlocal evaluation of the radial line-force, implemented
separately within each of a set of radiatively isolated azimuthal
grid zones. The results show that radially compressed “shells”
that develop initially from the LDI are systematically broken up
by Rayleigh-Taylor or thin-shell instabilities as these structures
are accelerated outward. Through radial feedback of backscattered
radiation, this leads ultimately to a flow structure characterized by
nearly complete lateral incoherence, with structure extending down
to the lateral grid scale, which here corresponds to angle sizes of
order a fifth of a degree. We briefly discuss the implications for
interpreting various observational diagnostics of wind structure, but
also emphasize the importance of future extensions to include lateral
line-drag effects of diffuse radiation, which may set a minimum lateral
scale for break-up of flow structure.

---------------------------------------------------------
Title: X-Ray Emission-Line Profile Modeling of O Stars: Fitting
a Spherically Symmetric Analytic Wind-Shock Model to the Chandra
Spectrum of ζ Puppis
Authors: Kramer, Roban H.; Cohen, David H.; Owocki, Stanley P.
2003ApJ...592..532K Altcode: 2002astro.ph.11550K
X-ray emission-line profiles provide the most direct insight into the
dynamics and spatial distribution of the hot, X-ray-emitting plasma
above the surfaces of OB stars. The O supergiant ζ Puppis shows
broad, blueshifted, and asymmetric line profiles, generally consistent
with the wind-shock picture of OB star X-ray production. We model
the profiles of eight lines in the Chandra HETGS spectrum of this
prototypical hot star. The fitted lines indicate that the plasma is
distributed throughout the wind starting close to the photosphere,
that there is significantly less attenuation of the X-rays by the
overlying wind than is generally supposed, and that there is not a
strong trend in wind absorption with wavelength.

---------------------------------------------------------
Title: On multicomponent effects in stellar winds of stars at
extremely low metallicity
Authors: Krtička, J.; Owocki, S. P.; Kubát, J.; Galloway, R. K.;
Brown, J. C.
2003A&A...402..713K Altcode: 2003astro.ph..3026K
We calculate multicomponent line-driven wind models of stars at
extremely low metallicity suitable for massive first generation
stars. For most of the models we find that the multicomponent wind
nature is not important for either wind dynamics or for wind temperature
stratification. However, for stars with the lowest metallicities we
find that multicomponent effects influence the wind structure. These
effects range from pure heating to possible fallback of the nonabsorbing
wind component. We present a simple formula for the calculation of
metallicity for which the multicomponent effects become important. We
show that the importance of the multicomponent nature of winds of
low metallicity stars is characterised not only by the low density of
driving ions, but also by lower mass-loss rate.

---------------------------------------------------------
Title: High-Resolution Chandra Spectroscopy of τ Scorpii: A
Narrow-Line X-Ray Spectrum from a Hot Star
Authors: Cohen, David H.; de Messières, Geneviève E.; MacFarlane,
Joseph J.; Miller, Nathan A.; Cassinelli, Joseph P.; Owocki, Stanley
P.; Liedahl, Duane A.
2003ApJ...586..495C Altcode: 2002astro.ph.11412C
Long known to be an unusual early-type star by virtue of its hard
and strong X-ray emission, τ Scorpii poses a severe challenge to the
standard picture of O-star wind-shock X-ray emission. The Chandra HETGS
spectrum now provides significant direct evidence that this B0.2 star
does not fit this standard wind-shock framework. The many emission
lines detected with the Chandra gratings are significantly narrower
than what would be expected from a star with the known wind properties
of τ Sco, although they are broader than the corresponding lines
seen in late-type coronal sources. While line ratios are consistent
with the hot plasma on this star being within a few stellar radii of
the photosphere, from at least one He-like complex there is evidence
that the X-ray-emitting plasma is located more than a stellar radius
above the photosphere. The Chandra spectrum of τ Sco is harder and
more variable than those of other hot stars, with the exception of
the young magnetized O star θ1 Ori C. We discuss these
new results in the context of wind, coronal, and hybrid wind-magnetic
models of hot-star X-ray emission.

---------------------------------------------------------
Title: X-ray emission line profile modeling of hot stars
Authors: Kramer, Roban H.; Tonnesen, Stephanie K.; Cohen, David H.;
Owocki, Stanley P.; ud-Doula, Asif; MacFarlane, Joseph J.
2003RScI...74.1966K Altcode: 2002astro.ph.12313K
The launch of high-spectral-resolution x-ray telescopes (Chandra,
XMM) has provided a host of new spectral line diagnostics for the
astrophysics community. In this paper we discuss Doppler-broadened
emission line profiles from highly supersonic outflows of massive
stars. These outflows, or winds, are driven by radiation pressure and
carry a tremendous amount of kinetic energy, which can be converted to
x rays by shock-heating even a small fraction of the wind plasma. The
unshocked, cold wind is a source of continuum opacity to the x rays
generated in the shock-heated portion of the wind. Thus the emergent
line profiles are affected by transport through a two-component,
moving, optically thick medium. While complicated, the interactions
among these physical effects can provide quantitative information
about the spatial distribution and velocity of the x-ray-emitting and
absorbing plasma in stellar winds. We present quantitative models of
both a spherically-symmetric wind and a wind with hot plasma confined
in an equatorial disk by a dipole magnetic field.

---------------------------------------------------------
Title: FUSE observations of stellar wind variability in the LMC
supergiant Sk -67deg166 (O4 If+)
Authors: Fullerton, Alexander W.; Massa, Derck L.; Prinja, Raman K.;
Howarth, Ian D.; Willis, Allan J.; Owocki, Stanley P.
2003IAUS..212..182F Altcode:
We describe FUSE time-series observations of stellar wind variability
in Sk -67deg166 (HDE 269698), a nitrogen-rich O4 supergiant in the LMC.

---------------------------------------------------------
Title: Radiatively driven winds: shaping bipolar LBV nebulae
Authors: Dwarkadas, Vikram V.; Owocki, Stanley P.
2003IAUS..212..172D Altcode:
Massive stars which are fast rotators can give rise to asymmetric
winds. These winds may result in the formation of aspherical wind-blown
nebulae. In this work the theory of radiatively-driven winds from
massive stars is used to model the formation of bipolar nebulae around
LBV stars.

---------------------------------------------------------
Title: Overloaded and fractured stellar winds
Authors: Feldmeier, Achim; Oskinova, Lida M.; Hamann, Wolf-Rainer;
Owocki, Stanley P.
2003IAUS..212...56F Altcode:
We discuss the connection between wind overloading and discrete
absorption components in P-Cygni line profiles from O-type
stars. Overloading can create horizontal plateaus in the radial wind
speed that cause the extra absorption in the line profile. The upstream
propagation speed of these velocity plateaus is analyzed. The second
part of the paper deals with X-ray emission from O-type stars. X-ray
line profiles observed with Chandra and XMM are often symmetric,
contrary to what is expected for lines from a homogeneous wind. We
discuss the influence on line symmetry of photon escape channels in
a strongly clumped wind.

---------------------------------------------------------
Title: Clumps and shocks in the outer winds of hot stars
Authors: Runacres, Mark C.; Owocki, Stanley P.
2003IAUS..212..226R Altcode:
We present a moving periodic box technique to study the outer-wind
evolution of instability-generated structure in hot-star winds. This
has considerable computational and conceptual advantages

---------------------------------------------------------
Title: Dynamical simulations of magnetically channeled line-driven
stellar winds
Authors: Ud-Doula, Asif; Owocki, Stanley P.
2003IAUS..212..247U Altcode:
We present numerical magnetohydrodynamic simulations of the effect of
stellar dipole magnetic fields on line-driven wind outflows from hot,
luminous stars. Unlike previous fixed-field analyses, the simulations
here take full account of the dynamical competition between field
and flow, and thus apply to a full range of magnetic field strength,
and within both closed and open magnetic topologies. A key result
is that the overall degree to which the wind is influenced by the
field depends largely on a single, dimensionless, `wind magnetic
confinement parameter', η* = Beq2
R*2/{dot M} v∞, which characterizes
the ratio between magnetic field energy density and kinetic energy
density of the wind. For weak confinement η* &lt;= 1,
the field is fully opened by the wind outflow, but nonetheless for
confinements as small as η* = 1/10 can have a significant
back-influence in enhancing the density and reducing the flow speed
near the magnetic equator. For stronger confinement η*
&gt; 1, the magnetic field remains closed over a limited range of
latitude and height about the equatorial surface, but eventually is
opened into a nearly radial configuration at large radii. Within closed
loops, the flow is channeled toward loop tops into shock collisions
that are strong enough to produce hard X-rays, with the stagnated
material then pulled by gravity back onto the star in quite complex
and variable inflow patterns. Within open field flow, the equatorial
channeling leads to oblique shocks that are again strong enough to
produce X-rays, and also lead to a thin, dense, slowly outflowing
`disk' at the magnetic equator. The polar flow is characterized by a
faster-than-radial expansion that is more gradual than anticipated
in previous 1D flow-tube analyses, and leads to a much more modest
increase in terminal speed (&lt; 30%), consistent with observational
constraints. Overall, the results here provide a dynamical groundwork
for interpreting many types of observations, e.g. UV line-profile
variability; red-shifted absorption or emission features; enhanced
density-squared emission; and X-ray emission, that might be associated
with perturbation of hot-star winds by surface magnetic fields.

---------------------------------------------------------
Title: Instabilities in massive stars
Authors: Owocki, Stanley P.
2003IAUS..212..281O Altcode:
A defining property of massive stars is the dominant, dynamical role
played by radiation throughout the stellar interior, atmosphere,
and wind. Associated with this radiation hydrodynamics are several
distinct kinds of instabilities that can lead to convection in
both core and envelope, clumping in atmosphere and wind outflow,
and perhaps even the dramatic mass loss outbursts associated with
Luminous Blue Variable phases. Here I review these instabilities with
emphasis on basic physical properties of radiative driving. I draw
on two specific examples of dynamical instability, namely the strong
instability associated with line-driving of a stellar wind outflow,
and the global stellar instabilities associated with approaching or
exceeding a modified Eddington limit. I conclude with a brief mention
of recent ideas on the role of stellar rotation in the shaping of
bipolar LBV outbursts.

---------------------------------------------------------
Title: The Effects of Magnetic Fields on Line-Driven Hot-Star Winds
Authors: Ud-Doula, A.; Owocki, S.
2003ASPC..305..343U Altcode: 2003astro.ph.10180U; 2003mfob.conf..343U
This talk summarizes results from recent MHD simulations of the role
of a dipole magnetic field in inducing large-scale structure in the
line-driven stellar winds of hot, luminous stars. Unlike previous
fixed-field analyses, the MHD simulations here take full account of the
dynamical competition between the field and the flow. A key result is
that the overall degree to which the wind is influenced by the field
depends largely on a single, dimensionless `wind magnetic confinement
parameter', $\eta_\ast (= B_{eq}^2 R_{\ast}^2/\dot{M} v_\infty$),
which characterizes the ratio between magnetic field energy density and
kinetic energy density of the wind. For weak confinement, $\eta_\ast
\le 1$, the field is fully opened by wind outflow, but nonetheless,
for confinement as small as $\eta_\ast=1/10$ it can have significant
back-influence in enhancing the density and reducing the flow speed
near the magnetic equator. For stronger confinement, $\eta_\ast &gt;
1$, the magnetic field remains closed over limited range of latitude
and height above the equatorial surface, but eventually is opened
into nearly radial configuration at large radii. Within the closed
loops, the flow is channeled toward loop tops into shock collisions
that are strong enough to produce hard X-rays. Within the open field
region, the equatorial channeling leads to oblique shocks that are
again strong enough to produce X-rays and also lead to a thin, dense,
slowly outflowing “disk” at the magnetic equator.

---------------------------------------------------------
Title: Emission-line-profile variability as a contraint on the
structure and dynamics of hot star winds
Authors: Dessart, Luc; Owocki, Stanley P.
2003IAUS..212..168D Altcode:
We present theoretical calculations of emission-line-profile
variability (LPV), based on radiation hydrodynamics simulations of
the infamous radiative instability of hot star winds. We demonstrate
that spherically symmetric wind structures (shells) cannot account for
the observed profile variability at line center. Hence, we resort to
a model that breaks-up the wind volume into a number of independent
star-centered cones. The essential approximation made here is that
each of these cones can be described by a structure calculated with
a one-dimensional (1D) radiation hydrodynamics model. Such pseudo-3D
`patch'-method leads to a satisfactory reproduction of the fundamental
characteristics of LPV observed in O-type and Wolf-Rayet star optical
spectra: the low-level fluctuations in the profile centre region, a
migration of variable sub-peaks from line center to edge, that mimics
the underlying wind acceleration. Our method highlights the correlation
between the velocity scale of profile sub-peaks at line center and the
lateral extent of wind structures, while at line edge it reflects the
intrinsic radial velocity dispersion of emitting clumps. However, our
model fails to reproduce the increase in this characteristic velocity
scale from line center to edge, which we believe is a shortcoming of
our purely 1D hydrodynamics approach.

---------------------------------------------------------
Title: Radiatively Driven Winds and the Formation of Bipolar
Planetary Nebulae
Authors: Dwarkadas, V. V.; Owocki, S.
2003IAUS..209..467D Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Magnetic Spin-Up of Line-Driven Stellar Winds
Authors: Owocki, S.; Ud-Doula, A.
2003ASPC..305..350O Altcode: 2003mfob.conf..350O; 2003astro.ph.10179O
We summarize recent 2D MHD simulations of line-driven stellar winds
from rotating hot-stars with a dipole magnetic field aligned to the
star's rotation axis. For moderate to strong fields, much wind outflow
is initially along closed magnetic loops that nearly corotate as a
solid body with the underlying star, thus providing a torque that
results in an effective angular momentum spin-up of the outflowing
material. But instead of forming the “magnetically torqued disk”
(MTD) postulated in previous phenemenological analyses, the dynamical
simulations here show that material trapped near the tops of such
closed loops tends either to fall back or break out, depending on
whether it is below or above the Keplerian corotation radius. Overall
the results raise serious questions about whether magnetic torquing
of a wind outflow could naturally result in a Keplerian circumstellar
disk. However, for very strong fields, it does still seem possible to
form a %“magnetically confined, centrifugally supported, rigid-body
disk”, centrifugally supported, “magnetically rigid disk” (MRD),
in which the field not only forces material to maintain a rigid-body
rotation, but for some extended period also holds it down against
the outward centrifugal force at the loop tops. We argue that such
rigid-body disks seem ill-suited to explain the disk emission from Be
stars, but could provide a quite attractive paradigm for circumstellar
emission from the magnetically strong Bp and Ap stars.

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Title: X-ray Emission Line Profile Modeling of O stars: zeta Puppis
as a Wind-Shock Source
Authors: Kramer, R. H.; Cohen, D. H.; Owocki, S. P.
2002AAS...20111305K Altcode: 2002BAAS...34.1284K
The origin of X-ray emission lines associated with O- and B-type stars
is the subject of continuing controversy. Leading theories point to
shocks in the star's massive, radiation-driven winds as the source of
the X-rays. In recent years Chandra has taken high-resolution X-ray
spectra of several hot stars, revealing broad and often asymmetric
line profiles. At the same time, theoretical calculations of line
profiles from spherically-symmetric shock distributions in the wind
have shown the potential to produce similar profile shapes. We have
parameterized a spherically-symmetric, distributed-shock wind model
and fit it to several lines from the O4f star ζ Puppis. Our fits
suggest lower than expected X-ray continuum opacities, and onset of
shock formation within a few stellar radii.

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Title: Radiatively Driven Winds and the Shaping of Bipolar Luminous
Blue Variable Nebulae
Authors: Dwarkadas, Vikram V.; Owocki, Stanley P.
2002ApJ...581.1337D Altcode:
Nebulae around luminous blue variable (LBV) stars are often
characterized by a bipolar, prolate form. In the standard interpretation
of the generalized interacting stellar winds model, this bipolar form
is attributed to an asymmetry in the density structure of the ambient
medium. However, there is limited observational evidence to suggest
that such an asymmetric medium is present in most LBV nebulae. In this
work we use scaling relations derived from the theory of radiatively
driven winds to model the outflows from LBV stars, taking account
of stellar rotation and the associated latitudinal variation of the
stellar flux due to gravity darkening. We show that, for a star rotating
close to its critical speed, the decrease in effective gravity near
the equator and the associated decrease in the equatorial wind speed
results naturally in a bipolar, prolate interaction front, even for a
spherically symmetric ambient medium. Moreover, when gravity darkening
is included, the resulting density of the outburst is also strongest
over the prolate poles. We discuss the implications of these results
for the formation of windblown nebulae in general.

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Title: Analysis of Doppler-Broadened X-ray Emission Line Profiles
from Hot Stars
Authors: Cohen, D. H.; Kramer, R. H.; Owocki, S. P.
2002hrxs.confE...9C Altcode:
We show how X-ray emission arising within an accelerating, expanding
medium that also contains a source of continuum absorption generates
line profiles of a characteristic shape. A simple, spherical wind
model based on this picture provides good fits to the Chandra HETGS
spectrum of the prototypical O star, Zeta Pup. We discuss the model,
the fitting procedure and the determination of confidence limits on the
model parameters, and our initial results for this star. The derived
fit parameters are consistent with a generic wind-shock scenario for
Zeta Pup, but there are several surprising aspects of the results,
including a lower-than-expected mean wind optical depth and a nearly
complete lack of wavelength dependence of the results.

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Title: X-ray Line Profiles of Magnetically Confined Hot-Star Winds
Authors: Tonnesen, S. K.; Cohen, D. H.; Owocki, S. P.; ud-Doula, A.;
Gagne, M.; Oksala, M.
2002AAS...20111301T Altcode: 2002BAAS...34.1284T
With the discovery of a strong magnetic field on θ 1
Orionis C, the magnetically confined wind shock model has become a
leading explanation for X-ray emission from some O and B stars. This
model involves a dipole field that channels the stellar wind from both
poles of the star towards the equator where the resulting shocks heat
the wind to X-ray emitting temperatures. As the shocked wind cools,
it feeds an equatorial disk that can be opaque to X-rays. In this
poster we describe synthetic line profiles from both analytic models
and magnetohydrodynamic numerical simulations of magnetically confined
hot star winds. We discuss the effect of the closed vs. open field
regions on line shape and width, and also discuss effects of absorption
arising from the equatorial cooling disk and the cool polar wind. For
a star with a tilted dipole, our viewing angle to the magnetically
confined wind structure changes as the star rotates, making the line
profiles phase-dependent. We compare our synthesized line profiles and
band-pass fluxes to phase-resolved Chandra spectra of θ 1
Orionis C. This work is supported by the NASA, under grant NAS8-39073.

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Title: Wavelet analysis of instability-generated line profile
variations in hot-star winds
Authors: Dessart, L.; Owocki, S. P.
2002A&A...393..991D Altcode:
We investigate whether instability-generated structure of line-driven
stellar winds can account for the emission line profile variability
(LPV) observed in hot star spectra. In a previous paper, we introduced
a three-dimensional (3D) “patch” method to compute the temporal
evolution of the wind emissivity, based on 1D radiation hydrodynamics
simulations. Here we apply a wavelet analysis to these synthetic LPVs,
allowing a direct comparison with observations analysed in the same
way, with particular focus on the characteristic velocity scale of
LPVs at various frequency locations within the line profile. Wavelet
analyses of observed LPV generally show this scale to increase from 50
to 100-200 km s-1 from line-centre to edge. We argue here
that the characteristic sub-peak broadening is dominated at line-centre
by the lateral spatial extent of wind structures, while at line-edge it
is controlled by their intrinsic radial velocity dispersion. We find
that the wavelet transforms of synthetic LPV yield characteristic
widths that are comparable to observed values at line-centre, but
much narrower at line-edges. We thus conclude that the patch size
of 3 deg assumed here provides a reasonable representation of the
lateral coherence length associated with observed LPV, but that the
1D instability models that form the basis of the patch method have
too low a radial velocity dispersion to reproduce the characteristic
widths observed at line edge. We discuss how the latter limitation
might be overcome by inclusion of radial velocity shear, and also
outline possible approaches to developing multi-dimensional instability
simulations that could account for such shear effects.

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Title: Dynamical Simulations of Magnetically Channeled Line-driven
Stellar Winds. I. Isothermal, Nonrotating, Radially Driven Flow
Authors: ud-Doula, Asif; Owocki, Stanley P.
2002ApJ...576..413U Altcode: 2002astro.ph..1195U
We present numerical magnetohydrodynamic (MHD) simulations of
the effect of stellar dipole magnetic fields on line-driven wind
outflows from hot, luminous stars. Unlike previous fixed-field
analyses, the simulations here take full account of the dynamical
competition between field and flow and thus apply to a full
range of magnetic field strength and within both closed and open
magnetic topologies. A key result is that the overall degree
to which the wind is influenced by the field depends largely on
a single, dimensionless “wind magnetic confinement parameter”
η* (=B2eqR2*/
Mv∞), which characterizes the ratio between magnetic
field energy density and kinetic energy density of the wind. For
weak confinement, η*&lt;=1, the field is fully opened
by the wind outflow, but nonetheless, for confinements as small
as η*=1/10 it can have a significant back-influence in
enhancing the density and reducing the flow speed near the magnetic
equator. For stronger confinement, η*&gt;1, the magnetic
field remains closed over a limited range of latitude and height about
the equatorial surface, but eventually is opened into a nearly radial
configuration at large radii. Within closed loops, the flow is channeled
toward loop tops into shock collisions that are strong enough to produce
hard X-rays, with the stagnated material then pulled by gravity back
onto the star in quite complex and variable inflow patterns. Within
open field flow, the equatorial channeling leads to oblique shocks
that are again strong enough to produce X-rays and also lead to a
thin, dense, slowly outflowing “disk” at the magnetic equator. The
polar flow is characterized by a faster-than-radial expansion that is
more gradual than anticipated in previous one-dimensional flow tube
analyses and leads to a much more modest increase in terminal speed
(less than 30%), consistent with observational constraints. Overall,
the results here provide a dynamical groundwork for interpreting many
types of observations-e.g., UV line profile variability, redshifted
absorption or emission features, enhanced density-squared emission,
and X-ray emission-that might be associated with perturbation of
hot-star winds by surface magnetic fields.

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Title: Hydrodynamic Models of QSO Disk Winds
Authors: Hillier, D. J.; Pereyra, N. A.; Turnshek, D. A.; Owocki, S. P.
2002AAS...200.0509H Altcode: 2002BAAS...34R.648H
We present hydrodynamic time-dependent 2.5D models of line-driven
accretion disk winds in QSOs. We assume that the wind originates
from a standard Shakura-Sunyaev accretion disk. It is assumed that
the X-ray emission orginates at the center of the disk so that the
gas above the disk is partially shielded from the X-rays by the disk
itself. The X-ray emission plays a crucial role in the dynamics of
the wind by photoionizing gas in certain regions such that in these
regions the line-radiation force is negligible. From the results of
the hydrodynamic models, and assuming single scattering, we calculate
theoretical CIV resonance line profiles. We attempt to confirm the
results of Murray et al. (1995), who through 1D dynamical models showed
that the disk wind scenario was consistent with a unified picture of
BAL (Broad Absorption Line) and non-BAL QSOs, in which the existence of
broad absorption lines was dependent on viewing angle. We find that the
disk wind scenario may account for the detached absorption troughs and
the multiple absorption troughs observed in the CIV line of many BAL
QSOs. This scenario is also consistent with the X-ray-weak character
of BAL QSOs. For the models presented in this work we use a black hole
mass of M = 109 M⊙ and disk luminosity of L =
1047 erg ; s-1. We are currently studying our
models by varying the values of the force multiplier parameters and
varying the height of X-ray source at the disk center. We are also
studying possible physical mechanisms to account for the UV emission
lines within the accretion disk scenario. This work is funded by the
National Science Foundation Grant AST-0071193.

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Title: Ion Runaway Instability in Low-Density, Line-driven Stellar
Winds
Authors: Owocki, Stanley P.; Puls, Joachim
2002ApJ...568..965O Altcode:
We examine the linear instability of low-density, line-driven stellar
winds to runaway of the heavy minor ions when the drift speed of these
ions relative to the bulk, passive plasma of hydrogen and helium
approaches or exceeds the plasma thermal speed. We first focus on
the surprising results of recent steady state, two-component models,
which indicate that the limited Coulomb coupling associated with
suprathermal ion drift leads not to an ion runaway, but instead to a
relatively sharp shift of both the ion and passive fluids to a much
lower outward acceleration. Drawing on analogies with subsonic outflow
in the solar wind, we provide a physical discussion of how this lower
acceleration is the natural consequence of the weaker frictional
coupling, allowing the ion line driving to maintain its steady
state balance against collisional drag with a comparatively shallow
ion velocity gradient. However, we then carry out a time-dependent,
linearized stability analysis of these two-component steady solutions
and thereby find that, as the ion drift increases from sub- to
suprathermal speeds, a wave mode characterized by separation between
the ion versus passive plasma goes from being strongly damped to being
strongly amplified. Unlike the usual line-driven flow instability of
high-density, strongly coupled flows, this ion separation instability
occurs even in the long-wavelength Sobolev limit, although with only
a modest spatial growth rate. At shorter wavelengths, the onset of
instability occurs for ion drift speeds that are still somewhat below
the plasma thermal speed and, moreover, generally has a very large
spatial growth. For all wavelengths, however, the temporal growth
rate exceeds the already rapid growth of line-driven instability by a
typical factor of ~100, corresponding to the mass-density ratio between
the bulk plasma and the driven minor ions. We further show that this
ion separation mode has an inward propagation speed that is strongly
enhanced (at its maximum by a similar factor of ~100) over the usual
“Abbott wave” speed of a fully coupled, line-driven flow, implying
that in the context of this separation mode, the entire domain of any
steady state solution can be considered as “subcritical.” Finally,
we note that, despite the extremely rapid linear growth rate, further
analyses and/or simulations will be needed to determine whether the
nonlinear evolution of this instability should lead to true ion runaway
or instead might perhaps be limited by damping from two-stream plasma
instabilities.

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Title: Emission profile variability in hot star winds. A pseudo-3D
method based on radiation hydrodynamics simulations
Authors: Dessart, L.; Owocki, S. P.
2002A&A...383.1113D Altcode:
We present theoretical calculations of emission line profile variability
based on hot star wind structure calculated numerically using radiation
hydrodynamics simulations. A principal goal is to examine how well
short-time-scale variations observed in wind emission lines can be
modelled by wind structure arising from small-scale instabilities
intrinsic to the line-driving of these winds. The simulations here
use a new implementation of the Smooth Source Function formalism
for line-driving within a one-dimensional (1D) operation of the
standard hydrodynamics code ZEUS-2D. As in previous wind instability
simulations, the restriction to 1D is necessitated by the computational
costs of non-local integrations needed for the line-driving force;
but we find that naive application of such simulations within an
explicit assumption of spherically symmetric structure leads to an
unobserved strong concentration of profile variability toward the line
wings. We thus introduce a new “patch method” for mimicking a full
3D wind structure by collecting random sequences of 1D simulations
to represent the structure evolution along radial rays that extend
over a selectable patch-size of solid angle. We provide illustrative
results for a selection of patch sizes applied to a simulation with
standard assumptions that govern the details of instability-generated
wind structure, and show in particular that a typical model with a
patch size of about 3 deg can qualitatively reproduce the fundamental
properties of observed profile variations. We conclude with a discussion
of prospects for extending the simulation method to optically thick
winds of Wolf-Rayet (WR) stars, and for thereby applying our “patch
method” to dynamical modelling of the extensive variability observed
in wind emission lines from these WR stars.

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Title: Mass Loss and Magnetospheres: X-rays from Hot Stars and Young
Stellar Objects
Authors: Gagné, M.; Cohen, D.; Owocki, S.; Ud-Doula, A.
2002ASPC..262...31G Altcode: 2002heus.conf...31G; 2001astro.ph..9090G
The discovery of strong X-ray emission from hot stars was one one of
the early surprises of the Einstein mission. Although wind shocks may
produce most of the soft X-rays on some O stars, some young OB stars
show variable hard X-ray emission that cannot be explained by standard
instability-driven wind shocks. I present recent efforts to synthesize
X-ray spectra of magnetically confined wind shocks. On the other end
of the H-R diagram, X-ray flares on low-mass young stellar objects
exhibit plasma temperatures &gt;50 MK, variable column densities, and
very high emission measures, suggesting enormous magnetic loops. In
high-mass and low-mass young stars, rotation, magnetospheres and disks
may play a key role in the X-ray emission process.

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Title: The Link between Radiation-Driven Winds and Pulsation in
Massive Stars (invited paper)
Authors: Owocki, S. P.; Cranmer, S. R.
2002ASPC..259..512O Altcode: 2002rnpp.conf..512O; 2002IAUCo.185..512O
No abstract at ADS

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Title: The outer evolution of instability-generated structure in
radiatively driven stellar winds
Authors: Runacres, M. C.; Owocki, S. P.
2002A&A...381.1015R Altcode:
We investigate stochastic structure in hot-star winds. The structure
(i.e. inhomogeneities such as clumps and shocks) is generated
by the instability of the line driving mechanism in the inner
wind. It is self-excited in the sense that it persists even in the
absence of explicit perturbations. The evolution of structure as it
moves out with the flow is quantified by the radial dependence of
statistical properties such as the clumping factor and the velocity
dispersion. We find that structure evolves under the influence of
two competing mechanisms. Dense clumps pressure-expand into the
rarefied gas that separates them, but this expansion is counteracted
by supersonic collisions among the clumps, which tend to compress
them further. Because of such ongoing collisions, clumps can survive
over an extended region out of pressure equilibrium with the rarefied
surrounding gas. Moreover, the line-driving force has little rôle in
maintaining the structure beyond about 20-30 R*, implying
that the outer evolution can be simplified as a pure gasdynamical
problem. In modelling the distant wind structure we find it is necessary
to maintain a relatively fine constant grid spacing to resolve the
often quite narrow dense clumps. We also find that variations in the
heating and cooling, particularly the “floor” temperature to which
shock-compressed gas is allowed to cool, can affect both the density and
temperature variation. Finally, we find that increasing the value of the
line-driving cut-off parameter kappamax can significantly
enhance the level of flow structure. Overall, the results of our
work suggest that structure initiated in the inner wind acceleration
region can survive to substantial distances ( ~100 R*),
and thus can have an important influence on observational diagnostics
(e.g. infrared and radio emission) formed in the outer wind.

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Title: Numerical Simulations of Magnetically Confined Line Driven
Winds
Authors: ud-Doula, A.; Owocki, S.
2001AAS...19913507U Altcode: 2001BAAS...33.1506U
We present fully dynamic numerical magnetohydrodynamic (MHD) simulations
of line-driven winds from hot stars with assumed dipolar magnetic fields
at the stellar surface. The magnetic fields can guide the wind outflows
from higher latitudes towards the magnetic equator causing them to
collide there with speeds of hundreds of km s-1. This may
lead to significant equatorial density enhancements and wind shocks
may heat up the gas to temperatures high enough to produce X-rays. Our
results show that the governing parameter for how much the wind is
influenced by the field is the `magnetic confinement wind number',
η * ( = B2 R*2/{˙ M}
v∞ ), which characterizes the ratio between magnetic field
energy density and kinetic energy density of the wind. We find that
for η * of the order of unity or greater, magnetic fields
can channel or confine the winds, and lead to significant spatial and
temporal variability of wind properties. Such characteristics contrast
with the classical view of steady and spherical stellar winds from
hot stars, and indeed provide a potential model for the X-ray emission
and UV wind line variability often detected from such massive stars.

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Title: Radiatively Driven Winds and the Shaping of Bipolar LBV Nebulae
Authors: Dwarkadas, V. V.; Owocki, S. P.
2001AAS...19913509D Altcode: 2001BAAS...33Q1507D
Nebulae around Luminous Blue Variable (LBV) stars are characterized
by asymmetric, often bipolar shapes. A classic example is the
Homunculus nebula around the massive star η Carinae. In the standard
interpretation of the generalized interacting stellar winds models,
the asymmetry in shape is attributed to an asymmetry in the density
structure of the ambient medium. However the observational evidence
does not support this. In this work we use scaling relations derived
from the theory of radiatively driven winds to model the outflows from
LBV stars. Rotation of the star, and the latitudinal variation of the
stellar flux due to gravity darkening is taken into account. It is
shown that a star rotating close to its critical velocity will emit
a wind whose velocity is higher at the poles than the equator, which
can give rise to an asymmetric, bipolar wind-blown nebula. Inclusion
of gravity darkening shifts the relative density toward the poles,
but does not change the overall shape of the interaction front. We
discuss the implications of these results for the general formation
of wind-blown nebulae. VVD is supported by NASA grant NAG5-3530, and
by a grant from NASA administered by the American Astronomical Society.

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Title: X-ray Emission Line Profile Modeling of Hot Stars
Authors: Kramer, R. H.; Reed, J. E.; Cohen, D. H.; Owocki, S. P.
2001AAS...19913513K Altcode: 2001BAAS...33.1507K
With the high spectral resolution afforded by the Chandra spectrometers,
it is now possible to observe directly the Doppler shift of a wind
outflow in X-ray emission lines from hot stars. This provides an
important and relatively unambiguous diagnostic for addressing the
decades-old mystery of X-ray production in such stars. We have developed
a broadly applicable general model of wind-based X-ray line emission
in an expanding medium, which we apply to new Chandra HETGS spectra of
several hot stars. This model includes both continuum and line opacity,
as well as velocity- and density-gradient effects. In this paper,
we report on the properties of the model and the results of fits to
the prototypical O supergiant, zeta Pup, as well as several other OB
stars. Only in some cases can these high-resolution Chandra spectra
be explained in the context of the standard wind-shock paradigm. This
work was supported by NASA grant NAG5-10088 and by the Delware Space
Grant Consortium.

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Title: Chandra Emission Line Diagnostics of the Unusual Hot Star
tau Scorpii
Authors: de Messieres, G. E.; Cardamone, C.; Cohen, D. H.; MacFarlane,
J. J.; Owocki, S. P.; ud-Doula, A.
2001AAS...19913512D Altcode: 2001BAAS...33R1507D
It is generally believed that O and early-B stars produce X-rays via
shocks embedded in their highly supersonic stellar winds. However,
several lines of evidence point to other mechanisms operating in at
least some unusual OB stars (those that are perhaps young, magnetized,
cooler, or have circumstellar disks). The B0 V star tau Sco has long
been known to be unusual in several ways, and to have harder and
stronger X-ray emission than most hot stars. With the unprecedented
spectral resolution provided by the Chandra gratings, we can begin to
quantitatively assess the different X-ray production mechanisms that
have been proposed for this unusual star. We report on line ratio
analyses (of density, temperature, and local UV radiation field),
as well as line widths and centroids, in order to discriminate among
the various models. The notable results are that the copius very hot
(kT &gt; 1 keV) plasma on this star is situated one or two stellar
radii above the photosphere, and that it is quite stationary with
respect to the star, in contrast to the appreciable wind velocity seen
in UV lines. We discuss these new results in the context of several
types of models, including those based on line-force instability,
cloud-infall, magnetically confined wind shocks, and coronal magnetic
reconnection. This work was partially funded by NASA grant number
NAG5-10088 and by the Keck Northeast Astronomy Consortium.

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Title: X-Ray Line Profiles from Parameterized Emission within an
Accelerating Stellar Wind
Authors: Owocki, Stanley P.; Cohen, David H.
2001ApJ...559.1108O Altcode: 2001astro.ph..1294O
Motivated by recent detections by the XMM and Chandra satellites of
X-ray line emission from hot, luminous stars, we present synthetic
line profiles for X-rays emitted within parameterized models of a
hot-star wind. The X-ray line emission is taken to occur at a sharply
defined comoving-frame resonance wavelength, which is Doppler-shifted
by a stellar wind outflow parameterized by a “β” velocity law,
v(r)=v∞(1-R*/r)β. Above some
initial onset radius Ro for X-ray emission, the radial
variation of the emission filling factor is assumed to decline as
a power law in radius, f(r)~r-q. The computed emission
profiles also account for continuum absorption within the wind,
with the overall strength characterized by a cumulative optical
depth τ*. In terms of a wavelength shift from line
center scaled in units of the wind terminal speed v∞,
we present normalized X-ray line profiles for various combinations
of the parameters β, τ*, q, and Ro and also
including the effect of instrumental and/or macroturbulent broadening as
characterized by a Gaussian with a parameterized width σ. We discuss
the implications for interpreting observed hot-star X-ray spectra,
with emphasis on signatures for discriminating between “coronal”
and “wind-shock” scenarios. In particular, we note that in profiles
observed so far the substantial amount of emission longward of line
center will be difficult to reconcile with the expected attenuation
by the wind and stellar core in either a wind-shock or coronal model.

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Title: Line Forces in Keplerian Circumstellar Disks and Precession
of Nearly Circular Orbits
Authors: Gayley, K. G.; Ignace, R.; Owocki, S. P.
2001ApJ...558..802G Altcode: 2001astro.ph..3408G
We examine the effects of optically thick line forces on orbiting
circumstellar disks, such as occur around Be stars. For radially
streaming radiation (e.g., as from a point source), line forces
are effective only if there is a strong radial velocity gradient,
as occurs, for example, in a line-driven stellar wind. However, we
emphasize here that, within an orbiting disk, the radial shear of the
azimuthal velocity leads to strong line-of-sight velocity gradients
along nonradial directions. As such, we show that, in the proximity of a
stellar surface extending over a substantial cone angle, the nonradial
components of stellar radiation can impart a significant line force
to such a disk, even in the case of purely circular orbits with no
radial velocity. Given the highly supersonic nature of orbital velocity
variations, we use the Sobolev approximation for the line transfer,
extending to the disk case the standard CAK formalism developed
for line-driven winds. We delineate the parameter regimes for which
radiative forces might alter disk properties; but even when radiative
forces are small, we analytically quantify higher-order effects in the
linear limit, including the precession of weakly elliptical orbits. We
find that optically thick line forces, both radial and azimuthal, can
have observable implications for the dynamics of disks around Be stars,
including the generation of either prograde or retrograde precession in
slightly eccentric orbits. However, our analysis here suggests a net
retrograde effect, in apparent contradiction with observed long-term
variations of violet/red line profile asymmetries from Be stars, which
are generally thought to result from prograde propagation of a one-arm,
disk-oscillation mode. We also conclude that radiative forces may
alter the dynamical properties at the surface of the disk where disk
winds originate, and in the outer regions far from the star, and may
even make low-density disks vulnerable to being blown off completely.

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Title: X-ray Line Spectroscopy of Hot Stars
Authors: Cohen, David H.; Owocki, Stanley P.
2001tysc.confE.182C Altcode:
Chandra grating spectroscopy has provided a completely new view of high
energy processes occurring on massive, hot stars. The initial results
from high-resolution spectral observations of a small number of hot
stars have been surprising, to say the least. The fast stellar winds
of hot stars make these sources one of the few classes of astrophysical
objects for which Chandra can make spectrally resolved measurements of
line profiles. The observed line profiles vary significantly in both
width and shape among the hot stars thus far observed. These results,
along with line-ratio diagnostics of density and proximity to the
photospheric UV field, are difficult to understand within the framework
of any single theory of X-ray production on hot stars. In this talk,
I will review the current observational results of high resolution
Chandra spectroscopy of hot stars, and will discuss the implications
for coronal, wind shock, and hybrid models of hot star X-rays.

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Title: FUSE Observations of Stellar Wind Variability in {Sk -67°166}
Authors: Fullerton, A. W.; Massa, D. L.; Howarth, I. D.; Owocki,
S. P.; Prinja, R. K.; Willis, A. J.
2000AAS...197.0802F Altcode: 2000BAAS...32R1405F; 2000AAS...197..802F
We present results from an 18-day campaign to monitor stellar wind
variability in {Sk -67°166} (HDE 269698), an O4 If+ star in the Large
Magellanic Cloud, with the Far Ultraviolet Spectroscopic Explorer
(FUSE) satellite. Optical depth enhancements that progress from blue
to red through the absorption trough are prominent in all unsaturated
P Cygni profiles, particularly the resonance doublets of {S 4} and
{P 5}. Related variability is evident in the resonance lines of {S 6}
and {O 6}. The variations are qualitatively similar to those observed
in the {Si 4} wind lines of the Galactic supergiant ζ Puppis [O4
I(n)f] during a 16-day monitoring campaign with IUE. However, the FUSE
observations contain more diagnostic information about the nature of
the structures responsible for the observed variability. In particular,
the relative amplitudes of the variations in {S 4} and {S 6} provide
the first empirical constraint on the ionization equilibrium of these
structures in an O star wind, while the variability of {O 6} traces the
distribution of very hot gas. This work is 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.

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Title: Radiative Forces in Be Disks: Precession of Nearly Circular
Orbits
Authors: Gayley, K.; Ignace, R.; Owocki, S.
2000AAS...197.6001G Altcode: 2000BAAS...32.1502G
We apply the standard CAK treatment of partially optically thick
line forces to an orbiting disk around a B-type star, to delineate
the parameter regimes where the radiative force may be expected to
alter the global disk properties. Also, even when these radiative
forces are small, we quantify the potentially important ramifications
for higher-order perturbations such as one-arm mode confinement and
precession of nearly circular orbits. We find that optically thick
radiative forces, both radial and azimuthal, should be expected to be
present in a Keplerian Be disk, and may have far-reaching implications
for dynamical simulations over mode-precession timescales. Although our
analysis is limited to a linear treatment of nearly circular orbits,
we speculate that radiative forces may even control the saturation and
nonlinear precession of observed large-amplitude one-arm modes. This
work was supported in part by NASA grants NAG5-3530 and NAG5-4065.

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

---------------------------------------------------------
Title: Radiatively Driven Stellar Winds from Hot Stars
Authors: Owocki, S.
2000eaa..bookE1887O Altcode:
A stellar wind is the continuous, supersonic outflow of matter from a
star. Among the most massive stars—which tend also to be the hottest
and most luminous—the winds can be very strong, with important
consequences both for the star's own evolution as well as for the
surrounding interstellar medium. Such hot-star winds are understood
to be driven by the pressure of the star's emitted radiation....

---------------------------------------------------------
Title: Radiative Torque and Partial Spin-Down of Winds from Rotating
Hot Stars
Authors: Gayley, K. G.; Owocki, S. P.
2000ApJ...537..461G Altcode:
We examine the degree to which the azimuthal component of the
line-driving force can remove angular momentum from the equatorial wind
of a rapidly rotating hot star, using a straightforward extension of
the standard CAK formalism. We illustrate how even in a wind that is
azimuthally symmetric, such a net azimuthal line force results from the
prograde/retrograde velocity gradient asymmetries that are inherent
to a non-rigidly rotating outflow. In particular, we show that the
sense of the associated line torque always acts against the rotation
whenever (as is generally the case) the azimuthal velocity falls below
the linear outward increase (vφ~r) associated with rigid
rotation. Through a parameter study based on two-dimensional numerical
hydrodynamical simulations, we find that the net loss of wind angular
momentum is significant but generally quite moderate, about 30%-40%,
for a wide range of conditions. We then present an extensive analytic
analysis that further illuminates the physical nature of the wind
spin-down and its robust net magnitude. This emphasizes the inherent
dynamical feedback between line driving and flow acceleration, which
allows the radiative force to effectively amplify the Coriolis force
in the rotating frame, and so cause the rotation speed to decrease even
more steeply with radius than required to conserve angular momentum. A
general conclusion is that, while the moderate net spin-down of the
wind is not likely to have a major impact on the overall wind dynamics,
it should be observable from emission line diagnostics, and that doing
so would provide an independent test of line-driven wind theory.

---------------------------------------------------------
Title: Stellar Winds APS Poster, Encyclopedia Article Available
Authors: Owocki, Stanley P.
2000BeSN...34...38O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book Review: Introduction to stellar winds / Cambridge U
Press, 1999
Authors: Owocki, S. P.
2000SSRv...91..719O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Outer Wind Evolution of Instability-Generated Clumped Structure
in Hot Star Winds
Authors: Owocki, S. P.; Runacres, M. C.; Cohen, D. H.
2000ASPC..204..183O Altcode: 2000tiaf.conf..183O
No abstract at ADS

---------------------------------------------------------
Title: EUV/X-ray Emission and the Thermal and Ionization Structure
of B Star Winds
Authors: Cohen, D. H.; Cassinelli, J. P.; Macfarlane, J. J.; Owocki,
S. P.
2000ASPC..204...65C Altcode: 2000tiaf.conf...65C; 2000astro.ph..8351C
We discuss the EUV and X-ray properties of B stars, focusing on
$\epsilon$ CMa (B2 II) which is the only star with both emission
lines and a photospheric continuum detected with EUVE. We explore the
modest effects of the photospheric EUV continua on the wind, as well as
the much stronger effects of the short-wavelength EUV and soft X-ray
emission lines. Attenuation of the EUV and soft X-ray emission by the
wind plays an important role, and leads to the reprocessing of X-rays
via He$^+$ ionization and the Bowen mechanism in the wind. Finally,
we explore some of the new diagnostics that will shortly become
available with the next generation of high spectral resolution X-ray
telescopes. All of this analysis is presented in the context of a
two component stellar wind--a dense component (clumps) that contains
most of the mass but fills a negligible fraction of the volume, and
a rarefied component that fills most of the volume but accounts for
only a small fraction of the mass.

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

---------------------------------------------------------
Title: Working Group on Hot Massive Stars (Groupe De Travail Sur
Les Etoiles Massives Chaudes)
Authors: Eenens, Philippe; Cassinelli, Joseph; Conti, Peter; Garmany,
Catharine; van der Hucht, Karel; Kudritzki, Rolf; Lamers, Henny;
Maeder, André; Moffat, Anthony; Owocki, Stanley
2000IAUTA..24..176E Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Effect of Instability-Generated Clumping on Wind Compressed
Disk Inhibition
Authors: Owocki, S. P.; Cohen, D. H.
2000ASPC..214..621O Altcode: 2000bpet.conf..621O; 2000IAUCo.175..621O
No abstract at ADS

---------------------------------------------------------
Title: The First Extrasolar Measurement of Stellar He II and O III
Bowen Fluorescence Emission in the EUV: A New Diagnostic of Hot Star
Wind Conditions Applied to ɛ Canis Majoris (B 2 II)
Authors: Cohen, David H.; Macfarlane, Joseph J.; Cassinelli, Joseph
P.; Owocki, Stanley P.
1999APS..DPP.FO106C Altcode:
The B bright giant ɛ CMa is one of only two OB stars observable with
the Extreme Ultraviolet Explorer (EUVE) spectrometers (between 70
Åand 760ÅOne of the most interesting aspects of this unique spectrum
is the presence of strong emission lines at 304 ÅHe II Lyman-α) and
374 ÅO III) due to the Bowen fluorescence mechanism. In this process,
the He II 304 Åline pumps a resonance line of O III and the subsequent
radiative decay yields several UV emission lines between 3000 Åand
4000 ÅThese lines are observed in nebulae, AGN, and novae, but the
final O III transition leads to emission near 400 Åwhich had never
before been seen in any astrophysical object outside of the solar
system. The Bowen emission lines are formed in the radiation-driven
stellar wind of ɛ CMa, as is shown by the Doppler-broadened 304 Åline
profile. Our modeling indicates that the He II 304 Åline is sensitive
to the X-ray emission in the wind of ɛ CMa, due to the importance of
X-rays photoionization in controlling the ionization of helium. We
also explore the temperature-sensitivity of the 374 ÅÅand 435 ÅO
III lines, and use the non-detection of the latter two lines to place
an upper limit on the wind temperature.

---------------------------------------------------------
Title: A Simple Scaling Analysis of X-Ray Emission and Absorption
in Hot-Star Winds
Authors: Owocki, Stanley P.; Cohen, David H.
1999ApJ...520..833O Altcode: 1999astro.ph..1250O
We present a simple analysis of X-ray emission and absorption for
hot-star winds, designed to explore the natural scalings of the
observed X-ray luminosity with wind and stellar properties. We
show that an exospheric approximation, in which all of the
emission above the optical depth unity radius escapes the wind,
reproduces very well the formal solution for radiation transport
through a spherically symmetric wind. Using this approximation
we find that the X-ray luminosity LX scales naturally
with the wind density parameter Ṁ/v∞, obtaining
LX~(Ṁ/v∞)2 for optically thin
winds, and LX~(Ṁ/v∞)1+s for
optically thick winds with an X-ray filling factor that varies in radius
as f~rs. These scalings with wind density contrast with the
commonly inferred empirical scalings of X-ray luminosity LX
with bolometric luminosity LBol. The empirically derived
linear scaling of LX~LBol for thick winds can,
however, be reproduced through a delicate cancellation of emission
and absorption, if one assumes modest radial fall-off in the X-ray
filling factor (s~-0.25 or s~-0.4, depending on details of the secondary
scaling of wind density with luminosity). We also explore the nature
of the X-ray spectral energy distribution in the context of this
model and find that the spectrum is divided into a soft, optically
thick part and a hard, optically thin part. Finally, we conclude
that the energy-dependent emissivity must have a high-energy cutoff,
corresponding to the maximum shock energy, in order to reproduce the
general trends seen in X-ray spectral energy distributions of hot stars.

---------------------------------------------------------
Title: Charge states of C and O from coronal holes: Non-Maxwellian
distribution vs. unequal ion speeds
Authors: Owocki, S. P.; Ko, Y. -K.
1999AIPC..471..263O Altcode: 1999sowi.conf..263O
The Solar Wind Ion Composition Spectrometer (SWICS) on board Ulysses
has compiled an extensive collection of ion charge state measurements in
high-speed-wind streams. These provide important diagnostic constraints
for the acceleration region of the large south polar coronal hole in
which these charge states were “frozen-in.” Initial analyses of these
data have inferred that the coronal electron distribution may deviate
modestly from a Maxwellian (1) or that the coronal outflow speeds
of heavy ions may vary with the ion mass (2) Here we apply a simple
freezing-in approximation to examine the robustness and uniqueness of
these inferences. In particular, we emphasize that careful attention
to the ionization states of both Oxygen and Carbon provides the best
potential diagnostic for a non-Maxwellian distribution of coronal
electrons, since the similarity in their overall rate coefficients
suggests a similar freezing-in location, while differences in their
(comparatively high) ionization potentials provide a differential
sensitivity to a high-energy electron tail. We also note the possibility
that the freezing-in of the ionization state of these elements may
begin in the underlying transition region of their source coronal hole.

---------------------------------------------------------
Title: Line-driven Ablation and Wind Tilting by External Irradiation
Authors: Gayley, K. G.; Owocki, S. P.; Cranmer, S. R.
1999ApJ...513..442G Altcode:
The directional variation of the velocity gradient in a supersonic,
radiatively accelerated flow gives an effectively anisotropic
character to the line-scattering process. This leads to surprising
consequences in source geometries that are more complex than isolated
nonrotating stars. In this paper we explore the wind dynamics from
a planar slab atmosphere that is irradiated by an external oblique
source, within the framework of standard Castor, Abbott, &amp; Klein
(CAK) wind theory. We show that the presence of externally incident
radiation can be surprisingly effective at tilting the flow away
from the vertical. Even more surprising is our conclusion that such
illumination should often enhance the mass loss and can even induce
outflow from a surface with no intrinsic radiation source. We examine
the physical causes of such “line-driven ablation” and discuss the
potential implications for modeling line-driven flows in massive-star
binary and accretion-disk systems.

---------------------------------------------------------
Title: Dynamics and variability of winds in WR+OB binaries
Authors: Gayley, K. G.; Owocki, S. P.
1999IAUS..193..168G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Dynamics and variability of winds from single Wolf-Rayet stars
Authors: Owocki, S. P.; Gayley, K. G.
1999IAUS..193..157O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Line-driven Stellar Winds: The Dynamical Role of Diffuse
Radiation Gradients and Limitations to the Sobolev Approach
Authors: Owocki, S. P.; Puls, J.
1999ApJ...510..355O Altcode:
Line-driven stellar winds from hot, luminous OB stars have been most
extensively modeled as smooth, steady state, supersonic outflows for
which a local, Sobolev line-transfer treatment is used to compute
the line-driving force. In this paper we apply a recently developed,
nonlocal escape-integral source function (EISF) method for computing
the line force toward time-dependent simulations. In accord with
previous linear perturbation analyses, the initially most unstable
flow fluctuations in numerical simulations here exhibit an outward
phase propagation characterized by a positive correlation between
velocity and density variations. However, such outward-mode waves
quickly saturate by self-shadowing effects at a relatively low
amplitude. Thus, much as in previous instability simulations, the
nonlinear wind structure is still dominated by reverse shocks that
arise from the somewhat slower (but less easily saturated) growth of
inward-mode waves with an anticorrelated velocity-density structure. An
unexpected result involves the important role that the diffuse,
scattered radiation field--ignored in a Sobolev approach--plays in the
dynamics around the wind sonic point. In particular, we find that the
strong asymmetry in the forward and backward escape probabilities near
the sonic point induces a marked depression in the scattering source
function in this region. The resulting inwardly directed diffuse line
force can significantly alter the mean wind conditions inferred from
steady-wind models using the conventional Sobolev approach. We discuss
the implications of these results and consider in particular why these
effects have been overlooked in previous wind analyses.

---------------------------------------------------------
Title: Turbulence in Line-Driven Stellar Winds
Authors: Owocki, Stanley
1999intu.conf...79O Altcode:
Hot, luminous OB stars have strong stellar winds driven by the
line-scattering of the star's continuum radiation. This line-driving
mechanism is understood to be highly unstable to small-scale
perturbations. I will review efforts to simulate the nonlinear
evolution of this instability using radiation hydrodynamics simulation
codes. Because the usual local, Sobolev treatment for the line-force
does not apply, a major challenge has been to develop computationally
tractable methods for approximating the inherently non-local radiative
transfer in the large number of wind-driving lines. Results of 1-D
simulations generally show development of a highly compressible,
stochastic wind structure dominated by strong reverse shocks and dense
shells that arise from amplification of inward-mode radiatively-modified
acoustic modes with anti-correlated velocity and density. In 2-D and
3-D, linear analysis predicts that lateral variations in velocity should
be strongly damped by the "line-drag" effect of the diffuse radiation
scattered with the line resonance, suggesting possible suppression of
classical Rayleigh-Taylor modes for lateral breakup of wind structure. I
will summarize methods and preliminary results of recent efforts toward
2-D simulation of the nonlinear wind structure. An overall goal is
to develop connections with studies of highly compressible turbulent
structure in other physical and astrophysical contexts.

---------------------------------------------------------
Title: Co-Rotating Interaction Regions in 2D Hot-Star Wind Models
with Line-Driven Instability
Authors: Owocki, Stanley P.
1999LNP...523..294O Altcode: 1999vnss.conf..294O; 1999IAUCo.169..294O
I review simulations of Co-rotating Interaction Regions (CIRs) in
line-driven stellar winds. Previous CIR models have been based on a
local, Sobolev treatment of the line-force, which effectively suppresses
the strong, small-scale instability intrinsic to line-driving. Here I
describe a new "3-ray-aligned-grid" method for computing the nonlocal,
smooth-source-function line-force in 2D models that do include this
line-driven instability. Preliminary results indicate that key overall
features of large-scale CIRs can be quite similar in both Sobolev
and non-Sobolev treatments, if the level of instability-generated
wind structure is not too great. However, in certain models wherein
the unstable self-excitation of wind variability penetrates back to
the wind base, the stochastic, small-scale structure can become so
dominant that it effectively disrupts any large-scale, CIR pattern.

---------------------------------------------------------
Title: ISO-PHOT observations of Wolf-Rayet winds
Authors: Runacres, M. C.; Blomme, R.; Vyverman, K.; Cohen, M.;
Leitherer, C.; Owocki, S. P.; Haas, M.
1999IAUS..193...96R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Non-spherical Radiation-Driven Wind Models
Authors: Puls, Joachim; Petrenz, Peter; Owocki, Stanley P.
1999LNP...523..131P Altcode: 1999IAUCo.169..131P; 1999vnss.conf..131P
The present state of modelling radiatively driven stellar winds
from rapidly rotating stars is reviewed. Various processes affecting
the actual, still controversial wind structure are highlighted, in
particular non-radial line-forces and gravity darkening, and useful
scaling relations are provided. The importance of accounting for
consistent NLTE line-forces depending both on the actual density
structure and radiation field (as function of latitude and radius)
is stressed, and some independent test calculations confirming earlier
numerical results are reported.

---------------------------------------------------------
Title: Line-Driven Ablation by External Irradiation
Authors: Gayley, Kenneth G.; Owocki, Stanley P.; Cranmer, Steven R.
1999LNP...523..151G Altcode: 1999vnss.conf..151G; 1999IAUCo.169..151G
The Sobolev approximation for supersonic flows creates an effective
opacity distribution that is nonisotropic, because the line-of-sight
velocity gradient is different in different directions. To better
understand the importance of this phenomenon in a simplified geometry,
we consider line-driven flows in the plane-parallel zero-sound-speed
limit, and solve for the wind driven by radiation with an arbitrary
angular distribution. One conclusion, surprising at first glance, is
that the acceleration component normal to the surface is independent
of both the strength and angular profile of the driving radiation
field. The flow tilt and overall mass-loss rate do depend on the
character of the radiation field. Also interesting is that mass loss
through a surface may be generated or enhanced by irradiation that
originates above the surface.

---------------------------------------------------------
Title: Line-Driven Instability
Authors: Feldmeier, Achim; Owocki, Stanley
1998Ap&SS.260..113F Altcode:
Line-driven winds are subject to a strong radiation-hydrodynamic
instability. We discuss the linear stability analysis and numerical
simulations of the fully developed wind structure. The latter show
sequences of strong reverse shocks, and two different families of clouds
which mutually collide. Possible applications are the X-ray emission
from O stars and the formation of dense clouds in broad absorption
line quasars.

---------------------------------------------------------
Title: Mass Loss from Rotating Hot-stars: Inhibition of Wind
Compressed Disks by Nonradial Line-forces
Authors: Owocki, S. P.; Cranmer, S. R.; Gayley, K. G.
1998Ap&SS.260..149O Altcode:
We review the dynamics of radiatively driven mass loss from rapidly
rotating hot-stars. We first summarize the angular momentum conservation
process that leads to formation of a Wind Compressed Disk(WCD) when
material from a rapidly rotating star is driven gradually outward
in the radial direction. We next describe how stellar oblateness
and asymmetries in the Sobolev line-resonance generally leads to
nonradialcomponents of the driving force is a line-driven wind,
including an azimuthal spin-down force acting against the sense of
the wind rotation, and a latitudinal force away from the equator. We
summarize results from radiation-hydrodynamical simulations showing
that these nonradial forces can lead to an effective suppressionof
the equatorward flow needed to form a WCD, as well as a modest
(∼ 25%) spin-downof the wind rotation. Furthermore, contrary to
previous expectations that the wind mass flux should be enhanced by the
reduced effective gravity near the equator, we show here that gravity
darkening effects can actually lead to a reducedmass loss, and thus
lower density, in the wind from the equatorial region. Finally, we
examine the equatorial bistability model, and show that a sufficiently
strong jump in wind driving parameters can, in principle, overcome the
effect of reduced radiative driving flux, thus still allowing moderate
enhancements in density in an equatorial, bistability zone wind.

---------------------------------------------------------
Title: Line-Driven Ablation and Wind Tilting by External Irradiation
Authors: Gayley, K.; Owocki, S.; Cranmer, S.
1998AAS...192.2603G Altcode: 1998BAAS...30..850G
Sobolev opacity in a hot-star wind preferentially scatters photons that
are incident along the direction of steepest velocity gradient. This
non-isotropic response can rotate the force vector relative to the
direction of net radiative flux, in a manner analogous to the way a
non-isotropic sail and keel can allow a boat to sail upwind. For hot
star binaries, the curious feedback between the radiative forces and
the flows they drive allows for counter-intuitive self-consistent
solutions. For example, we show that illumination that is purely
external to a reflecting radiative-equilibrium atmosphere can ablate
a highly tilted and fast wind, loosely reminiscent of “tacking”
in the sailing analogy. The conclusion is that whenever the radiation
source geometry is complicated, the non-isotropic nature of Sobolev
opacity must be carefully accounted for to obtain even a qualitative
understanding of the atmospheric response. Thus CAK theory continues
to reveal new surprises even in its most basic formulation.

---------------------------------------------------------
Title: Latitudinal Dependence of Radiatively Driven Mass Loss from
Rapidly Rotating Hot-Stars
Authors: Owocki, S. P.; Cranmer, S. R.; Gayley, K. G.
1998ASSL..233..205O Altcode: 1998best.work..205O
No abstract at ADS

---------------------------------------------------------
Title: Effects of Gravity Darkening on Radiatively Driven Mass Loss
from Rapidly Rotating Stars
Authors: Owocki, S. P.; Gayley, K. G.; Cranmer, S. R.
1998ASPC..131..237O Altcode: 1998phls.conf..237O
No abstract at ADS

---------------------------------------------------------
Title: Spectral Diagnostics of Blue Stars with Winds
Authors: Puls, J.; Kudritzki, R. -P.; Santolaya-Rey, A. E.; Herrero,
A.; Owocki, S. P.; McCarthy, J. K.
1998ASPC..131..245P Altcode: 1998phls.conf..245P
No abstract at ADS

---------------------------------------------------------
Title: Modelling Variability in Hot-Star Winds
Authors: Owocki, S. P.
1998cvsw.conf..325O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: ISOPHOT Observations of Early-type Stars
Authors: Blomme, R.; Runacres, M. C.; Vyverman, K.; Cohen, M.;
Leitherer, C.; Owocki, S. P.; Haas, M.
1998Ap&SS.255..145B Altcode: 1997Ap&SS.255..145B
No abstract at ADS

---------------------------------------------------------
Title: Metallicity Dependence of Stellar Outflows and Their
Variability
Authors: Puls, J.; Springmann, U.; Owocki, S. P.
1998cvsw.conf..389P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Wind variability of B supergiants. III. Corotating spiral
structures in the stellar wind of HD 64760.
Authors: Fullerton, A. W.; Massa, D. L.; Prinja, R. K.; Owocki, S. P.;
Cranmer, S. R.
1997A&A...327..699F Altcode:
Fourier analysis of two spectroscopic time series obtained with the
IUE observatory confirm that the ultraviolet stellar wind profiles of
HD 64760 (B0.5 Ib) are periodically variable. The periodic component
consists of modulations that extend over most of the P Cygni absorption
trough, and can frequently be traced through the emission lobe. The
modulations coexist with variations due to the propagation of discrete
absorption components, but there does not seem to be a direct link
between these two types of variability. In a long time series obtained
in 1995 January during the IUE MEGA Campaign, the modulations in
the P Cygni profiles of the Si III, Si IV, C IV, and N V resonance
lines were dominated by two sinusoidal variations with semi-amplitudes
between ~5-10% of the continuum flux and periods of 1.202+/-0.004 and
2.44+/-0.04days. The weak emission-lobe variability was predominantly
due to the 2.4-day modulation. In the absorption trough, the ratio
of the amplitude of the 1.2-day modulation to the amplitude of the
2.4-day modulation increased systematically as a function of ionization
potential. For both periods, the distribution of the phase constant with
position in the absorption trough exhibited a maximum near -710km/s,
and decreased symmetrically toward larger and smaller velocities. There
was a systematic decrease in the value of the maximum phase between
Si IV and N V. Only the 2.4-day period was present in a shorter time
series obtained in 1993 March, when its amplitude was nearly twice its
1995 value and it was more concentrated toward smaller velocities in
the absorption trough. There is no clear evidence for phase bowing in
the 1993 data. Since the 2.4- and 1.2-day periods are approximately
a half and a quarter of the estimated rotational period of HD 64760,
respectively, we interpret the modulations in terms of 2 (1993) and 4
(1995) broad, corotating circumstellar structures that modulate the
optical depth of the stellar wind. The bowed distribution of phase
implies that the structures are azimuthally extended, probably
spiral-shaped arms, and we develop a kinematic interpretation of
the projected velocity associated with the phase turnover in terms
of the degree of bending of the spirals. We derive a value for the
exponent governing the radial expansion of the wind of β=~1, which
is in good agreement with the canonical value for smooth, spherically
symmetric winds and suggests that the spiral structures are long-lived
perturbations through which material flows. The systematic phase lag
associated with higher ions suggests that they are preferentially
located along the inner, trailing edge of the spiral, as expected
if the structures are formed by the collision of fast and slow winds
originating from equally-spaced longitudinal sectors of the stellar
surface. Although a photospheric process is implicated in the origin
of these structures, it is not clear that magnetic fields or nonradial
pulsations could readily account for the switch between 2- and 4-equally
spaced surface patches that evidently occurred between 1993 and 1995.

---------------------------------------------------------
Title: Sudden Radiative Braking in Colliding Hot-Star Winds
Authors: Gayley, K. G.; Owocki, S. P.; Cranmer, S. R.
1997ApJ...475..786G Altcode:
Hot, massive stars have strong stellar winds, and in hot-star binaries
these winds can undergo violent collision. Because such winds are
thought to be radiatively driven, radiative forces may also play an
important role in moderating the wind collision. However, previous
studies have been limited to considering how such forces may inhibit
the initial acceleration of the companion stellar wind. In this paper
we analyze the role of an even stronger radiative braking effect,
whereby the primary wind is rather suddenly decelerated by the radiative
momentum flux it encounters as it approaches a bright companion. We
further show that the braking location and velocity law along the
line of centers between the stars can be approximated analytically
using a simple one-dimensional analysis. The results of this analysis
agree well with a detailed two-dimensional hydrodynamical simulation
of the wind collision in the WR + O binary V444 Cygni and demonstrate
that radiative braking can significantly alter the bow-shock geometry
and reduce the strength of the wind collision. We then apply
the derived analytic theory to a set of 14 hot-star binary systems,
and conclude that radiative braking is likely to be of widespread
importance for wind-wind collisions in WR + O binaries with close
to medium separation, D &lt;~ 100 R⊙. It may also be
important in other types of hot-star binaries that exhibit a large
imbalance between the component wind strengths.

---------------------------------------------------------
Title: Can the Line-Driven Instability Form BAL QSO Clouds?
Authors: Feldmeier, A.; Norman, C.; Pauldrach, A.; Owocki, S.; Puls,
J.; Kaper, L.
1997ASPC..128..258F Altcode: 1997meag.conf..258F
No abstract at ADS

---------------------------------------------------------
Title: ThePhysics of Stellar Winds Near the Eddington Limit
Authors: Owocki, S. P.; Gayley, K. G.
1997ASPC..120..121O Altcode: 1997lbv..conf..121O
No abstract at ADS

---------------------------------------------------------
Title: Far Infrared ISO Observations of RD 160529
Authors: Blomme, R.; Runacres, M. C.; Vyverman, K.; Cohen, M.;
Leitherer, C.; Owocki, S. P.
1997ASPC..120...41B Altcode: 1997lbv..conf...41B
No abstract at ADS

---------------------------------------------------------
Title: Inhibition FO Wind Compressed Disk Formation by Nonradial
Line-Forces in Rotating Hot-Star Winds
Authors: Owocki, S. P.; Cranmer, S. R.; Gayley, K. G.
1996ApJ...472L.115O Altcode:
We investigate the effects of nonradial line forces on the formation of
a "wind-compressed disk" (WCD) around a rapidly rotating B star. Such
nonradial forces can arise both from asymmetries in the line resonances
in the rotating wind and from rotational distortion of the stellar
surface. They characteristically include a latitudinal force component
directed away from the equator and an azimuthal force component
acting against the sense of rotation. Here we present results from
radiation-hydrodynamical simulations showing that these nonradial
forces can lead to an effective suppression of the equatorward
flow needed to form a WCD as well as a modest (~20%) spin-down of
the wind rotation. Furthermore, contrary to previous expectations
that the wind mass flux should be enhanced by the reduced effective
gravity near the equator, we show here that gravity darkening effects
can actually lead to a reduced mass loss, and thus lower density,
in the wind from the equatorial region. Overall, the results here
thus imply a flow configuration that is markedly different from that
derived in previous models of winds from rotating early-type stars. In
particular, a major conclusion is that equatorial wind compression
effects should be effectively suppressed in any radiatively driven
stellar wind for which, as in the usual CAK formalism, the driving
includes a significant component from optically thick lines. This
presents a serious challenge to the WCD paradigm as an explanation for
disk formation around Be and other rapidly rotating hot stars thought
to have CAK-type, line-driven winds.

---------------------------------------------------------
Title: The Incidence and Origin of Rotational Modulation in B
Supergiant Winds
Authors: Massa, D.; Prinja, R. K.; Fullerton, A. W.; Owocki, S. P.;
Cranmer, S. R.
1996AAS...189.9615M Altcode: 1996BAAS...28.1401M
We report the results of a 30 day IUE time series (with a mean sampling
of ~ 3 times a day) of wind variability in two B supergiants with
typical projected rotational velocities. The implied rotation periods
for the program stars are &lt;= 18.6 days for one and &lt;= 27 days
for the other. The wind variability in the more rapidly rotating
supergiant clearly shows cyclical behavior with a period of ~ 7.7
days. The modulation is most clearly seen at low velocity in the low
ions (C ii lambda lambda 1335, Al iii lambda lambda 1860, and the Si
iii lambda lambda 1300 triplets), demonstrating a photospheric origin
of the disturbances. Furthermore, since the period of the variability
is roughly half of the most probable rotation period of the star,
we attribute the variability to rotational modulation of its wind
by two distinct, equidistant surface features. We note, however,
that there is also complex substructure to the modulation which is
unresolved at our temporal sampling rate. The more slowly rotating
supergiant does not show distinctly repeating structures in its wind
lines, but there is an indication that a single feature is repeating
on the same time scale as its rotation period. When considered in
context with previous observations of a rapidly rotating supergiant,
the current results indicate that wind variability in B supergiants
is intimately linked to the presence of surface features on these stars.

---------------------------------------------------------
Title: Effect of Gravity Darkening on Bistable Winds in B[e] Stars
Authors: Owocki, S.; Gayley, K.
1996AAS...189.4807O Altcode: 1996BAAS...28.1337O
The circumstellar envelopes B[e] stars are thought to have enhanced
density near the equatorial plane. Lamers and Pauldrach (1991, A&amp;A,
244, L5) proposed that this could be produced from a "bistability
mechanism" in the radiatively driven wind, by which the lower effective
temperature in rotationally gravity-darkened equatorial regions leads
to an abrupt shift in the wind ionization, and an associated enhanced
efficiency in the radiative driving. Here we describe how this picture
is modified by taking self-consistent account of the reduced radiative
flux associated with the reduced equatorial effective temperature. The
general result is a reduction in the equatorial density enhancement, and
thus a potentially significant moderation of the net bistability effect.

---------------------------------------------------------
Title: Sudden radiative braking in colliding hot-star winds.
Authors: Gayley, K. G.; Owocki, S. P.; Cranmer, S. R.
1996RMxAC...5...68G Altcode:
When two hot-star winds collide, their interaction centers at the
point where the momentum fluxes balance. However, in WR+O systems, the
imbalance in the corporeal momentum fluxes may be extreme enough to
preclude a standard head-on wind/wind collision. On the other hand,
an important component of the total momentum flux in radiatively
driven winds is carried by photons. Thus, if the wind interaction
region has sufficient scattering opacity, it can reflect stellar
photons and cause important radiative terms to enter the momentum
balance. This radiative input would result in additional braking of
the wind. We use a radiative-hydrodynamics calculation to show that
such radiative braking can be an important effect in many types of
colliding hot-star winds. Characterized by sudden deceleration of the
stronger wind in the vicinity of the weak-wind star, it can allow a wind
ram balance that would otherwise be impossible in many WR+O systems
with separations less than a few hundred solar radii. It also greatly
weakens the shock strength and the encumbent X ray production. We
demonstrate the significant features of this effect using V444 Cygni
as a characteristic example. We also derive a general analytic theory
that applies to a wide class of binaries, yielding simple predictions
for when radiative braking should play an important role.

---------------------------------------------------------
Title: The Impact of Pulsations and Waves on Hot-Star Wind Variability
Authors: Cranmer, S. R.; Massa, D.; Owocki, S. P.
1996AAS...188.5907C Altcode: 1996BAAS...28R.918C
Hot luminous stars (O, B, W-R) are observed to have strong and variable
stellar winds, and many classes of these stars are also inferred
to pulsate radially or nonradially. It has been suspected for some
time that these oscillations can induce periodic modulations in the
surrounding stellar wind and produce observational signatures in, e.g.,
ultraviolet P Cygni line profiles. However, the fact that most low-order
and low-degree oscillation modes are evanescent in the photosphere
(i.e., damping exponentially instead of propagating sinusoidally)
presents a problem to the survival of significant wave amplitude in
the wind. We find, though, that the presence of an accelerating wind
can provide the necessary impetus for evanescent modes to effectively
“tunnel” their way out of the interior. First, in the subsonic, or
near-static wind, the reference frame of the temporal oscillations is
itself beginning to propagate, and this implies that a small degree
of group velocity is imparted to the evanescent waves. Second, in
the supersonic wind, the density no longer falls off exponentially,
but much more slowly, so the effective scale height grows much
larger. Frequencies previously evanescent here no longer “see”
as much of an underlying density gradient, and are free to propagate
nearly acoustically. We model the propagation of oscillations into
a hot-star wind via a numerical radiation-hydrodynamics code, and
we find that evanescence is indeed not a hindrance to producing
wind variability correlated with stellar pulsations. Preliminary
models of strong (nonlinear) radial wind oscillations of the beta
Cephei variable BW Vulpeculae show good agreement between observed
and modeled base “radial velocity curves” and wind-contaminated UV
profile variability. We are currently applying this general modeling
technique to other systems, especially those which rotate rapidly
and exhibit nonradial oscillations (e.g., zeta Puppis and HD 64760,
extensively observed by the IUE MEGA project).

---------------------------------------------------------
Title: The impact of pulsations and waves on hot-star wind
variability.
Authors: Cranmer, S. R.; Massa, D.; Owocki, S. P.
1996BAAS...28Q.918C Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Book Review: Instability and variability of hot-star winds /
Kluwer, 1995
Authors: Moffat, A. F. J.; Owocki, S. P.; Fullerton, A. W.; St-Louis,
N.
1996SSRv...76..375M Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Nonlocal Escape-Integral Approximations for the Line Force
in Structured Line-driven Stellar Winds
Authors: Owocki, S. P.; Puls, J.
1996ApJ...462..894O Altcode:
We develop a nonlocal, integral escape-probability formalism for
approximating both the direct and diffuse line force in a structured,
radiatively driven stellar wind. Our approach represents a direct
generalization of the local Sobolev escape-probability methods commonly
applied in smooth, steady wind models. It naturally incorporates
previous nonlocal force methods based on pure-absorption or smooth
source function (S SF) approximations for the line transport. However,
it also leads to the development of a new "escape-integral source
function" (EISF) method, which, for the first time, takes into
account the dynamical effects of gradients in the perturbed source
function. Perturbation analyses, formulated here in terms of the
perturbed escape probability, demonstrate how key aspects of the linear
wind instability, including line-drag and phase-propagation reversals,
are incorporated in the various nonlocal force approximations. The
methods here thus provide the basis for further, more complete
simulations of the nonlinear wind structure resulting from this strong
line-driven flow instability.

---------------------------------------------------------
Title: Inhibition of Wind Compressed Disk Formation by Nonradial
Line-Forces
Authors: Owocki, S.; Gayley, K.; Cranmer, S.
1996AAS...188.3801O Altcode: 1996BAAS...28..881O
We investigate the effects of nonradial line-forces on the formation of
a “Wind Compressed Disk” (WCD) around a rapidly rotating B-star. Such
nonradial forces can arise from both asymmetries in the line resonances
in the rotating wind, as well as from rotational distortion of the
stellar surface. They characteristically include an azimuthal force
component acting against the sense of rotation, and a latitudinal force
component directed away from the equator. Here we present results from
radiation-hydrodynamical simulations showing that these nonradial
forces can lead to a significant spin-down of the wind rotation, as
well as an effective suppression of the equatorward flow needed to
form a WCD. The qualitative sense of these effects can be understood
from simple physical arguments and analytic test cases, though further
testing and analysis is still needed to confirm their quantitative
importance. Nonetheless, these results indicate that nonradial force
components can effectively inhibit equatorial wind compression in
a line-driven outflow. If confirmed, these effects would seriously
undermine the WCD paradigm as an explanation for disk formation around
Be and other rapidly rotating hot stars with line-driven stellar winds.

---------------------------------------------------------
Title: Hydrodynamical Simulations of Corotating Interaction Regions
and Discrete Absorption Components in Rotating O-Star Winds
Authors: Cranmer, Steven R.; Owocki, Stanley P.
1996ApJ...462..469C Altcode: 1995astro.ph..8004C
We present two-dimensional hydrodynamical simulations of corotating
interaction regions (CIRs) in the wind from a rotating 0 star,
together with resulting synthetic line profiles showing discrete
absorption components (DACs). For computational tractability, we use a
local, Sobolev treatment of the radiative force, which suppresses the
small-scale instability intrinsic to line driving but still allows us to
model the dynamics of large-scale wind structure. As a first step toward
modeling the wind response to large-scale base perturbations (e.g., from
surface magnetic fields or nonradial pulsations), the structure here is
explicitly induced by localized increases or decreases in the radiative
force, as would result from a bright or dark "star spot" near the star's
equator. We find that bright spots with enhanced driving generate
high-density, low-speed streams, while dark spots generate low-density,
high-speed streams. CIRs form where fast material collides with slow
material; e.g., at the leading (trailing) edge of a stream from a dark
(bright) spot, often steepening into shocks. The unperturbed supersonic
wind obliquely impacts the high-density CIR and sends back a nonlinear
signal that takes the form of a sharp propagating discontinuity ("kink"
or "plateau") in the radial velocity gradient. In the wind's comoving
frame, these features propagate inward at the fast characteristic speed
derived by Abbott for radiatively modified acoustic waves, but because
this is generally only slightly less than the outward wind speed, the
features evolve only slowly outward in the star's frame. We find that
these slow kinks, rather than the CIRs themselves, are more likely to
result in DACs in the absorption troughs of unsaturated P Cygni line
profiles. Because the hydrodynamic structure settles to a steady state
in a frame corotating with the star, the more tightly spiraled kinks
sweep by an observer on a longer timescale than material moving with
the wind itself. This is in general accord with observations showing
slow apparent accelerations for DACs.

---------------------------------------------------------
Title: Sudden Radiative Braking in Colliding Hot-Star Winds
Authors: Gayley, K.; Owocki, S.; Cranmer, S.
1996AAS...188.6016G Altcode: 1996BAAS...28Q.922G
Hot, massive stars have strong stellar winds, and in hot-star binaries
these winds can undergo violent collision. Because such winds are
thought to be radiatively driven, radiative forces may also play an
important role in moderating the wind collision. However, previous
studies have been limited to considering how such forces may inhibit
the initial acceleration of the companion stellar wind. In this
poster we describe the role of an even stronger radiative braking
effect, whereby the primary wind is rather suddenly decelerated by
the radiative momentum flux it encounters as it approaches a bright
companion. We show that the braking location and velocity law along
the line of centers between the stars can be approximated analytically
using a simple one-dimensional analysis. The results of this analysis
agree well with a detailed two-dimensional hydrodynamical simulation of
the wind collision in the WR+O binary V444 Cygni, and demonstrate that
radiative braking can significantly alter the bow-shock geometry and
reduce the strength of the wind collision. We also apply the derived
analytic theory to a set of 14 hot-star binary systems, and conclude
that radiative braking is likely to be of widespread importance for
wind-wind collisions in WR+O binaries with close to medium separation,
D &lt;= 100 Rsun. It may also be important in other types of
hot-star binaries that exhibit a large imbalance between the component
wind strengths.

---------------------------------------------------------
Title: Evidence for Wind Attenuation and a Multitemperature Plasma
in the Combined EUVE and ROSAT Observations of epsilon Canis Majoris
(B2 II)
Authors: Cohen, D. H.; Cooper, R. G.; Macfarlane, J. J.; Owocki,
S. P.; Cassinelli, J. P.; Wang, P.
1996ApJ...460..506C Altcode:
We use both EUVE and ROSAT data sets to test three general , wind shock,
and external-for the production of the observed high-energy emission
from the B giant, η CMa (B2 II). Because of the very low interstellar
opacity along its line of sight, η CMa is the only early-type star that
has strong emission lines detected with the EUVE spectrometers. The
line spectrum provides the first solid observational evidence that
the emission is thermal. Theoretical EUV spectra based upon two-
temperature model fits to the ROSA T data predict too much flux,
especially in the iron line complex near 175 Å. We use progressively
more complex models until we are able to achieve a fit to the combined
data sets. We find that both a temperature distribution in the emitting
plasma and some attenuation of the EUV and soft X-ray emission by the
ionized stellar wind must be included in the models. The model fitting
indicates that only 13% to 21% of the emission-line complex near 175 Å
escapes the wind. This amount is consistent with the wind shock model,
in which the emitting material is distributed throughout the stellar
wind. It is much more absorption than is predicted by the external
source model, where all of the emitting material is at radii beyond
the cold stellar wind. And it is significantly less absorption than
is expected in the coronal model, given what is known about the star's
mass-loss rate. The derived temperature distribution and wind filling
factor of hot gas are also qualitatively consistent with our numerical
simulations of wind shocks. We conclude that although the observed flux
from η CMa in the interval 54 eV &lt; E &lt; 100 eV is approximately
the same as that above 100 eV, because of wind attenuation the total
generated radiation in then EUV band between 54 eV and 100 eV is 5
times greater than that in the X-ray region.

---------------------------------------------------------
Title: On flow phenomena that emit X-rays in hot star winds.
Authors: Feldmeier, A.; Puls, J.; Kudritzki, R. P.; Pauldrach,
A. W. A.; Owocki, S. P.; Reile, C.; Palsa, R.
1996rftu.proc...29F Altcode:
The X-ray emission from O stars may originate from instability-generated
shocks in their stellar winds. Previous numerical simulations that
assumed the wind to be isothermal could only draw limited conclusions
concerning this emission. The authors present new calculations including
the energy transfer in the wind. They confirm that up to a few stellar
radii, radiative cooling is efficient, i.e., shock cooling zones are
short compared with dynamical lengths. At larger radii, however, the
wind structure changes drastically because all shocks are destroyed
quickly due to a broadening of their cooling zones. The authors discuss
the following flow phenomena as possible sources of X-rays: (1) inner
radiative shocks; (2) shock merging; (3) outer adiabatic shocks;
(4) leftover hot gas from shock destruction; and (5) an outer corona.

---------------------------------------------------------
Title: Sudden radiative braking in colliding hot-star winds.
Authors: Gayley, K.; Owocki, S. P.; Cranmer, S. R.
1996BAAS...28..922G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Importance of Radiative Braking for the Wind Interaction
in the Close WR+O Binary V444 Cygni
Authors: Owocki, S. P.; Gayley, K. G.
1995ApJ...454L.145O Altcode:
We describe radiation-hydrodynamical simulations of the wind interaction
in the close WR+O binary V444 Cygni, with special emphasis on the
potential role of the O-star light in decelerating the approaching
massive WR wind. We demonstrate that such radiative braking can
significantly alter the strength and overall geometry of the wind
interaction, leading, for example, to a substantially wider opening
angle for the wind bow shock. It can also cause the X-ray production
to fall far below previous theoretical estimates based on collision
of the two winds at their terminal speeds. We further find that the
importance of radiative braking in this system depends crucially on
the effectiveness of the WR wind line opacity in reflecting O-star
light. This suggests that observational estimates of quite gross
system characteristics, like the bow-shock opening angle, can be used
to infer the degree of radiative braking, and so provide a useful new
contraint for line-driving models of WR winds.

---------------------------------------------------------
Title: Periodic Variations in Ultraviolet Spectral Lines of the
B0.5 Ib Star HD 64760: Evidence for Corotating Wind Streams Rooted
in Surface Variations
Authors: Owocki, Stanley P.; Cranmer, Steven R.; Fullerton,
Alexander W.
1995ApJ...453L..37O Altcode:
We discuss recently observed periodic modulations in the UV wind lines
of the B-type supergiant HD 64760, with a focus on the peculiar,
upwardly bowed shape seen in isoflux contours of the absorption
variations plotted against velocity and time. We show that this
qualitative impression of bowed contours is quantitatively confirmed by
a peak in the phase for the associated periodic variation at very nearly
the same line position as the apparent bow minimum. The bowed shape
is significant because it indicates that wind variations evolve both
blueward and redward, i.e., toward both larger and smaller line-of-sight
velocities. We show here, however, that these characteristics arise
naturally from absorption by strictly accelerating corotating wind
streams seen in projection against the stellar disk. The quite good
agreement obtained with the observed profile variations provides strong
evidence for corotating stream modulations in this wind.

---------------------------------------------------------
Title: Shocks and Shells in Hot Star Winds
Authors: Feldmeier, A.; Puls, J.; Reile, C.; Pauldrach, A. W. A.;
Kudritzki, R. P.; Owocki, S. P.
1995Ap&SS.233..293F Altcode:
Radiation-driven winds of hot, massive stars showvariability in UV and
optical line profiles on time scales of hours to days.Shock heating
of wind material is indicated by the observed X-ray emission. We
present time-dependent hydrodynamical models of these winds, where
flowstructures originate from a strong instability of the radiative
driving. Recent calculations (Owocki 1992) of the unstable growth
of perturbations were restricted by the assumptions of 1-D spherical
symmetry and isothermality of the wind. We drop the latter assumption
and include the energy transfer in the wind. This leads to a severe
numerical shortcoming, whereby all radiative cooling zones collapse
and the shocks become isothermal again. We propose a method to hinder
this collapse. Calculations for dense supergiant winds then show: (1)
The wind consists of a sequence of narrow and dense shells, which are
enclosed by strong reverse shocks (with temperatures of 106
to 107 K) on their starward facing side. (2) Collisions of
shells are frequent up to 6 to 7 stellar radii. (3) Radiative cooling
is efficient only up to 4 to 6R *. Beyond these radii,
cooling zones behind shocks become broad and alter the wind structure
drastically: all reverse shocks disappear, leaving regions ofpreviously
heated gas.

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

---------------------------------------------------------
Title: Momentum Deposition in Wolf-Rayet Winds: Nonisotropic Diffusion
with Effectively Gray Opacity
Authors: Gayley, Kenneth G.; Owocki, Stanley P.; Cranmer, Steven R.
1995ApJ...442..296G Altcode:
We derive the velocity and mass-loss rate of a steady state Wolf-Rayet
(WR) wind, using a nonisotropic diffusion approximation applied to
the transfer between strongly overlapping spectral lines. Following
the approach of Friend &amp; Castor (1983), the line list is assumed
to approximate a statistically parameterized Poisson distribution in
frequency, so that photon transport is controlled by an angle-dependent,
effectively gray opacity. We show the nonisotropic diffusion
approximation yields good agreement with more accurate numerical
treatments of the radiative transfer, while providing analytic insight
into wind driving by multiple scattering. We illustrate, in particular,
that multiple radiative momentum deposition does not require that
photons be repeatedly reflected across substantial distances within
the spherical envelope, but indeed is greatest when photons undergo a
nearly local diffusion, e.g., through scattering by many lines closely
spaced in frequency. Our results reiterate the view that the so-called
'momentum problem' of Wolf-Rayet winds is better characterized as
an 'opacity problem' of simply identifying enough lines. One way of
increasing the number of thick lines in Wolf-Rayet winds is to transfer
opacity from saturated to unsaturated lines, yielding a steeper opacity
distribution than that found in OB winds. We discuss the implications
of this perspective for extending our approach to W-R wind models
that incorporate a more fundamental treatment of the ionization and
excitation processes that determine the line opacity. In particular,
we argue that developing statistical descriptions of the lines to
allow an improved effective opacity for the line ensemble would offer
several advantages for deriving such more fundamental W-R wind models.

---------------------------------------------------------
Title: The Effect of Oblateness and Gravity Darkening on the Radiation
Driving in Winds from Rapidly Rotating B Stars
Authors: Cranmer, Steven R.; Owocki, Stanley P.
1995ApJ...440..308C Altcode:
We calculate the radiative driving force for winds around rapidly
rotating oblate B stars, and we estimate the impact these forces should
have on the production of a wind compressed disk. The effects of limb
darkening, gravity darkening, oblateness, and an arbitrary wind velocity
field are included in the computation of vector 'oblate finite disk'
(OFD) factors, which depend on both radius and colatitude in the
wind. The impact of limb darkening alone, with or without rotation,
can increase the mass loss by as much as 10% over values computed using
the standard uniformly bright spherical finite disk factor. For rapidly
rotating stars, limb darkening makes 'sub-stellar' gravity darkening the
dominant effect in the radial and latitudinal OFD factors, and lessens
the impact of gravity darkening at other visible latitudes (nearer
to the oblate limb). Thus, the radial radiative driving is generally
stronger over the poles and weaker over the equator, following the
gravity darkening at these latitudes. The nonradial radiative driving
is considerably smaller in magnitude than the radial component, but
is directed both away from the equatorial plane and in a retrograde
azimuthal direction, acting to decrease the effective stellar rotation
velocity. These forces thus weaken the equatorward wind compression
compared to wind models computed with nonrotating finite disk factors.

---------------------------------------------------------
Title: The non-isotropic diffusion approximation in Wolf-Rayet winds
Authors: Gayley, K. G.; Owocki, S. P.
1995IAUS..163..158G Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The dynamics of Wolf-Rayet winds (Invited)
Authors: Owocki, S. P.; Gayley, K. G.
1995IAUS..163..138O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Hydrodynamical Simulations of Co-Rotating Interaction Regions
and Discrete Absorption Components in Rotating O-Star Winds
Authors: Cranmer, S. R.; Owocki, S. P.
1994AAS...185.8003C Altcode: 1994BAAS...26.1446C
We present 2D hydrodynamical simulations of co-rotating stream
structure in the winds from rotating O-stars, together with resulting
synthetic line profiles showing discrete absorption components
(DAC's). The azimuthal variation is induced by a local increase
or decrease in the radiative driving force, as would arise from a
“star spot” in the equatorial plane. Since much of the emergent
wind structure seems independent of the exact method of perturbation,
we expect similar morphology in winds perturbed by localized magnetic
fields or non-radial pulsations. Because the radiative force depends
on the local rate of mass loss, bright spots with enhanced driving
generate high-density, low-velocity streams, while dark spots generate
low-density, high-velocity streams. Co-rotating interaction regions
(CIR's) form where fast material collides with slow material -- e.g. at
the leading (trailing) edge of a stream from a dark (bright) spot,
often steepening into shocks. The asymmetric wind also generates sharp
propagating discontinuities (“kinks”) in the radial velocity gradient,
which travel inward in the co-moving frame at the radiative-acoustic
characteristic speed, and slowly outward in the star's frame. We find
that these slow kinks, rather than the CIR's themselves, are more
likely to result in high-opacity DAC's in the absorption troughs of
unsaturated P Cygni line profiles. Because the hydrodynamic structure
settles to a steady state in a frame co-rotating with the star, the
more tightly-spiraled kinks sweep by an observer on a longer timescale
than material moving with the wind itself. This is in general accord
with observations showing slow apparent accelerations for DAC's.

---------------------------------------------------------
Title: Instability and variability of hot-star winds
Authors: Moffat, Anthony F. J.; Owocki, Stanley P.; Fullerton, Alex
W.; St-Louis, Nicole
1994Ap&SS.221.....M Altcode:
There are many unanswered questions in the area of hot-star wind
instability and its observable manifestations. The workshop on the
instability and variability of hot-star winds discussed many of these
questions. The key issue was how time-dependent structures observed
in hot-star winds relate to radiative and other instabilities. Further
questions concerned the role of turbulence and the nature of its driver,
and the effect of stellar rotation, pulsation, and magnetic fields on
time-dependent phenomena in hot-star winds. Also discussed was the
impact of stellar wind variability on the deprivation of mass-loss
rates, on stellar evolution, and on momentum/energy deposition in
the interstellar medium. For individual titles, see A95-78603 through
A95-78639.

---------------------------------------------------------
Title: Theory Review: Line-Driven Instability and Other Causes of
Structure and Variability in Hot-Star Winds
Authors: Owocki, Stanley P.
1994Ap&SS.221....3O Altcode:
The winds of the hot, luminous, OB stars are driven by the
line-scattering of the star's continuum radiation flux. Several kinds
of observational evidence indicate that such winds are highly structured
and variable. This paper will review possible theoretical causes of such
wind structure. For relatively small-scale, stochastic variability, I
review the role of the strong intrinsic instability of the line-driving
process itself. For larger scale structure, I describe recent efforts
to examine how disturbances from the underlying, rotating star can be
translated outward into propagating features in the wind.

---------------------------------------------------------
Title: 2-D Hydrodynamical Simulations of Wind Compressed Disks
(Abstract)
Authors: Owocki, S. P.; Cranmer, S. R.; Blondin, J. M.
1994Ap&SS.221..455O Altcode:
We present results of 2-D hydrodynamical simulations of a radiatively
driven stellar wind from a rapidly rotating Be-star. These generally
confirm predictions of the semi-analytic “Wind-Compressed-Disk”
model recently proposed by Bjorkman and Cassinelli to explain the
circumstellar disks inferred observationally to exist around such
rapidly rotating stars. However, our numerical simulations are able to
incorporate several important effects not accounted for in the simple
model, including a dynamical treatment of the outward radiative driving
and gas pressure, as well as a rotationally distorted, oblate stellar
surface. This enables us to model quantitatively the compressed wind
and shock that forms the equatorial disk. The simulation results thus
do differ in several important details from the simple model, showing,
for example, an inner diskinflow not possible in the heuristic approach
of assuming a fixed outward velocity law. There is also no evidence
for the predicted detachment of the disk that arises in the fixed
outflow picture. The peak equatorward velocity in the dynamical models
is furthermore about a factor of two smaller than the analytically
predicted value of ∼ 50% the stellar equatorial rotation speed. As a
result, the dynamical disks are somewhat weaker than predicted, with a
wider opening angle, lower disk/pole density ratio, and smaller shock
velocity jump (each by roughly the same factor of two).

---------------------------------------------------------
Title: X-Ray Emission in Wind Instability Simulations
Authors: Cooper, R. G.; Owocki, S. P.
1994Ap&SS.221..427C Altcode:
We estimate X-ray emission by shock-heated regions in hot star winds,
using temperature and density profiles calculated by time-dependent
dynamical models; the shocks result from the instability of the
line scattering force that drives the wind. For main sequence late O
and early B stars, the model X-ray flux is generally well below the
observed flux, though the shape of the model spectrum is approximately
consistent with observations. For the early O supergiantζ Pup, the
model spectral shape again agrees with observation; the total flux
predicted by models is well above the observed flux, though significant
uncertainties remain.

---------------------------------------------------------
Title: Synthesis of Line Profiles from Models of Structured Winds
Authors: Puls, J.; Feldmeier, A.; Springmann, U. W. E.; Owocki, S. P.;
Fullerton, A. W.
1994Ap&SS.221..409P Altcode:
On the basis of a a careful analysis of resonance line formation
(both for singlets and doublets) in structured winds, presenttime
dependent models of the line driven winds of hot stars (Owocki et al.,
this volume; Feldmeier, this volume) are shown to be able to explain
a number of observational features with respect to variability and
structure: they are (in principle) able to reproduce theblack andbroad
troughs (without any artificial “turbulence velocity”) and the
“blue edge variability” observed in saturated resonance lines;
they might explain the “long lived narrow absorption components”
often observed in unsaturated lines at high velocities; they predict a
relation between the “edge velocity” of UV-lines and the radiation
temperature of the observed X-ray emission. As a first example of the
extent to which theoretical models can be constrained by comparisons
between observations and profiles calculated by spectrum synthesis from
structured winds, we show here that models with deep-seated onset of
structure formation (≳ 1.1R * ) produce resonance lines
which agreequalitatively with observational findings; in contrast,
the here presented models with structure formation only well out in
the wind (≳ 1.6R * ) fail in this respect.

---------------------------------------------------------
Title: Summary comments
Authors: Owocki, S. P.
1994Ap&SS.221..491O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: 1-D Models of Induced Density Enhancements in Hot-Star Winds
Authors: Owocki, S. P.; Fullerton, A. W.; Puls, J.
1994Ap&SS.221..437O Altcode:
We present a 1-D dynamical model of large-scale flow structures
induced in a hot-star wind by an initial density perturbation in the
inner wind. The resulting wind response is very complex, but includes
strong density enhancements that propagate slowly outward through the
wind. These density structures exhibit a very slow outward acceleration
reminiscent of the discrete absorption components frequently observed
in unsaturated UV lines formed in hot-star winds.

---------------------------------------------------------
Title: Acceleration Efficiency in Line-driven Flows
Authors: Gayley, Kenneth G.; Owocki, Stanley P.
1994ApJ...434..684G Altcode:
We reexamine the physics of flow driving by line scattering of a
continuum radiation source to determine the degree to which such line
scattering can heat as well as accelerate the flow. Within the framework
of the Sobolev theory for line transfer, we argue that the finite
thermal width of the line scattering profile can lead to a significant
'Doppler heating' via photon frequency redistribution within a Sobolev
resonance layer. Quantitative computation of this heating shows,
however, that it is largely canceled by a corresponding cooling by the
diffuse radiation. The resulting reduction in net Doppler heating or
cooling means that the overall effect is only of limited importance in
the energy balance of line-driven stellar winds. Through simple scaling
relations, we compare the effect to other competing heating or cooling
terms, including the ion-drag frictional heating recently discussed
by Springmann and Pauldrach. We also provide a physical explanation of
the unexpected cooling effect, and comment that its near cancellation
of the anticipated heating provides another example of the tendency
for ideal Sobolev theory to apply to a higher order than expected.

---------------------------------------------------------
Title: Two-dimensional Hydrodynamical Simulations of Wind-compressed
Disks around Rapidly Rotating B Stars
Authors: Owocki, Stanley P.; Cranmer, Steven R.; Blondin, John M.
1994ApJ...424..887O Altcode:
We use a two-dimensional piecewise parabolic method (PPM) code to
simulate numerically the hydrodynamics of a radiation-driven stellar
wind from a rapidly rotating Be star. The results generally confirm
predictions of the semianalytic 'wind-compressed disk' model recently
proposed by Bjorkman and Cassinelli to explain the circumstellar
disks inferred observationally to exist around such rapidly rotating
stars. However, this numerical simulation is able to incorporate
several important effects not accounted for in the simple model,
including a dynamical treatment of the outward radiative driving and
gas pressure, as well as a rotationally distorted, oblate stellar
surface. This enables us to model quantitatively the compressed wind
and shock that forms the equatorial disk. The simulation results thus
do differ in several important details from the simple method, showing,
for example, an inner disk inflow not possible in the heuristic approach
of assuming a fixed outward velocity law. There is also no evidence
for the predicted detachment of the disk that arises in the fixed
outflow picture. The peak equatorward velocity in the dynamical models
is furthermore about a factor of 2 smaller than the lytically predicted
value of approximately 50% of the stellar equatorial rotation speed. As
a result, the dynamical disks are somewhat weaker than predicted,
with a wider opening angle, lower disk/pole density ratio, and smaller
shock velocity jump. The principal cause of these latter differences
appears to be an artificially strong equatorward drift of the subsonic
outflow in the original analytic model. Much better agreement with the
dynamical results can be obtained, however, from a slightly modified,
analytic wind-compression model with a more detailed specification of
the fixed wind outflow and a lower boundary set to the sonic radius
along a rotationally oblate stellar surface. Hence, despite these
detailed differences, the general predicted effect of disk formation
by wind compression toward the equator is substantially confirmed.

---------------------------------------------------------
Title: The Basic Physics of Hot-Star Winds
Authors: Owocki, S. P.
1994IAUS..162..475O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Two-dimensional hydrodynamical simulations of wind-compressed
disks around rapidly rotating B-stars
Authors: Owocki, S. P.; Cranmer, S. R.; Blondin, J. M.
1994IAUS..162..469O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: On the synthesis of resonance lines in dynamical models of
structured hot-star winds.
Authors: Puls, J.; Owocki, S. P.; Fullerton, A. W.
1993A&A...279..457P Altcode:
We examine basic issues involved in synthesizing resonance-line
profiles from 1-D, dynamical models of highly structured hot-star
winds. Although these models exhibit extensive variations in density
as well as velocity, the density scale length is still typically much
greater than the Sobolev length. The line transfer is thus treated
using a Sobolev approach, as generalized by Rybicki &amp; Hummer (1978)
to take proper account of the multiple Sobolev resonances arising from
the nonmonotonic velocity field. The resulting reduced-lambda-matrix
equation describing nonlocal coupling of the source function is solved
by iteration, and line profiles are then derived from formal solution
integration using this source function. Two more approximate methods
that instead use either a stationary or a structured, local source
function yield qualitatively similar line-profiles, but are found to
violate photon conservation by 10% or more. The full results suggest
that such models may indeed be able to reproduce naturally some of the
qualitative properties long noted in observed UV line profiles, such as
discrete absorption components in unsaturated lines, or the blue-edge
variability in saturated lines. However, these particular models do not
yet produce the black absorption troughs commonly observed in saturated
lines, and it seems that this and other important discrepancies (e.g.,
in acceleration time scale of absorption components) may require
development of more complete models that include rotation and other
2-D and/or 3-D effects.

---------------------------------------------------------
Title: Fluctuations at the blue edge of saturated wind lines in IUE
spectra of O-type stars
Authors: Owocki, Stanley P.; Fullerton, Alex
1993dunw.rept.....O Altcode:
We examine basic issues involved in synthesizing resonance-line
profiles from 1-D, dynamical models of highly structured hot-star
winds. Although these models exhibit extensive variations in density
as well as velocity, the density scale length is still typically much
greater than the Sobolev length. The line transfer is thus treated
using a Sobolev approach, as generalized by Rybicki &amp; Hummer (1978)
to take proper account of the multiple Sobolev resonances arising from
the nonmonotonic velocity field. The resulting reduced-Lambda-matrix
equation describing nonlocal coupling of the source function is solved
by iteration, and line profiles and then derived from formal solution
integration using this source function. The more appropriate methods
that instead use either a stationary or a structured, local source
function yield qualitatively similar line-profiles, but are found to
violate photon conservation by 10 percent or more. The full results
suggest that such models may indeed be able to reproduce naturally
some of the qualitative properties long noted in observed UV line
profiles, such as discrete absorption components in unsaturated
lines, or the blue-edge variability in saturated lines. However,
these particular models do not yet produce the black absorption
troughs commonly observed in saturated lines, and it seems that this
and other important discrepancies (e.g., in acceleration time scale
of absorption components) may require development of more complete
models that include rotation and other 2-D and/or 3-D effects.

---------------------------------------------------------
Title: Radiation Driven Winds of Hot Stars - some Remarks on
Stationary Models and Spectrum Synthesis in Time-Dependent
Simulations. (Ludwig Biermann Award Lecture 1992)
Authors: Puls, J.; Pauldrach, A. W. A.; Kudritzki, R. -P.; Owocki,
S. P.; Najarro, F.
1993RvMA....6..271P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Radiation driven winds of hot stars: Some remarks on stationary
models and spectrum synthesis in time-dependent simulations
Authors: Puls, Joachim; Pauldrach, Adalbert W. A.; Kudritzki,
Rolf-Peter; Owocki, Stanley P.; Najarro, Francisco
1992STIN...9421578P Altcode:
The basic concept of some stationary models and the way in which these
models are used for and in how far they influence the determination of
stellar and wind parameters of massive hot stars are discussed. However,
as follows directly from the assumption of stationarity, these
models are inherently incapable of describing a number of additional
observational features, which immediately show that the nonstationary
aspects of the wind must be of significant importance. A number of
these features are discussed: the soft X-ray emission; the discrete
absorption components; the temporal variability of the wind lines,
where the blue edges are varying most significantly while the red
emission part remains relatively constant; the electron scattering
wings of recombination lines in Wolf-Rayet stars; the black troughs
in saturated P-Cygni profiles; the variability of optical lines; and
the nonthermal radio emission observed in massive stars. The basic
picture resulting from the present dynamical models is discussed.

---------------------------------------------------------
Title: 2-D Hydrodynamical Simulations of the Wind-Compressed-Disk
Model for Be Stars
Authors: Owocki, S.; Cranmer, S.; Blondin, J.
1992AAS...181.1903O Altcode: 1992BAAS...24.1150O
We use a 2-D PPM code to simulate numerically the hydrodynamics
of a radiation-driven stellar wind from a rapidly rotating
B-star. The results generally confirm predictions of a semi-analytic
“Wind-Compressed-Disk" model recently proposed by Bjorkman and
Cassinelli to explain the circumstellar disks inferred observationally
to exist around Be stars. However, this numerical simulation is able to
incorporate several important effects not accounted for in the simple
model, including a dynamical treatment of the outward radiative driving
and gas pressure. This enables us to model quantatively the compressed
wind and shock that forms the equatorial disk. The simulation results
thus do differ in several important details from the simple model,
showing, for example, cases of inner disk inflow not possible in the
heuristic approach of assuming a fixed outward velocity law. This
poster paper will present a detailed comparison of the analytic and
numerical models.

---------------------------------------------------------
Title: Time-Dependent Models of X-Ray Emission from Shocks in
Radiatively Driven Stellar Winds (Contributed Poster)
Authors: Cooper, B. C.; Owocki, S. P.
1992ASPC...22..281C Altcode: 1992nvos.work..281C
No abstract at ADS

---------------------------------------------------------
Title: Instabilities in Hot-Star Winds: Basic Physics and Recent
Developments (Invited Paper)
Authors: Owocki, S. P.
1992ASPC...22..273O Altcode: 1992nvos.work..273O
No abstract at ADS

---------------------------------------------------------
Title: Can Nonstationary Velocity Plateaus Account for Slowly Moving
Discrete Absorption Components? (Contributed Poster)
Authors: Fullerton, A. W.; Owocki, S. P.
1992ASPC...22..177F Altcode: 1992nvos.work..177F
No abstract at ADS

---------------------------------------------------------
Title: Instabilities in hot-star winds: Basic physics and recent
developments
Authors: Owocki, Stanley P.
1992LNP...401..393O Altcode: 1992aets.conf..393O
The winds of the hot, luminous, OB stars are driven by the
line-scattering of the star's continuum radiation flux. Several kinds
of observational evidence indicate that such winds are highly structured
and variable, and it seems likely that a root cause of this variability
is the strong instability of the line-driving mechanism. This
paper reviews the basic physics of the linear instability and
summarizes results from numerical simulations of its nonlinear
evolution. Particular emphasis is placed on the dynamical importance
of the diffuse, scattered radiation field, and on recent methods for
incorporating such scattering effects into the numerical simulations. I
also summarize recent preliminary results on synthetic UV line, Ha,
IR continuum spectra in dynamical wind models with extensive structure.

---------------------------------------------------------
Title: X ray emission from dynamical shock models in hot-star winds
Authors: Owocki, Stanley P.
1991dunw.rept.....O Altcode:
The principal aim of this project was to determine whether x ray
emission from instability-generated shocks in dynamical models of highly
unstable hot-star winds could explain the x ray flux spectrum observed
from such hot stars by Einstein and other x ray satellites. Our initial
efforts focused on extending the earlier isothermal simulations of wind
instabilities to include an explicit treatment of the energy balance
between shock heating and simplified radiative cooling. It was found,
however, that direct resolution of cooling regions behind shocks is
often impractical, and thus additional, indirect methods for determining
this shock x ray emission were also developed. The results indicate
that the reverse shocks that dominate simple 1-D instability models
typically have too little material undergoing a strong shock to produce
the observed x ray emission. Other models with more strongly driven
variability from the wind base sometimes show high-speed collisions
between relatively dense clumps, and in these instances the computed
x ray flux spectrum matches the observed spectrum quite well. This
suggests that collisions between relatively large scale wind streams
of different speeds may be more suited to producing the observed x
rays than the reverse shocks arising from small-scale instabilities.

---------------------------------------------------------
Title: The Effect of Viscosity on Steady Transonic Flow with a Nodal
Solution Topology
Authors: Owocki, Stanley P.; Zank, Gary P.
1991ApJ...368..491O Altcode:
The effect of viscosity on a steady, transonic flow for which the
inviscid limit has a nodal solution topology near the critical point
is investigated. For the accelerating case, viscous solutions tend to
repel each other, so that a very delicate choice of initial conditions
is required to prevent them from diverging. Only the two critical
solutions extend to arbitrarily large distances into both the subsonic
and supersonic flows. For the decelerating case, the solutions tend
to attract, and so an entire two-parameter family of solutions now
extends over large distances. The general effect of viscosity on the
solution degeneracy of a nodal topology is thus to reduce or limit
it for the accelerating case and to enhance it for the decelerating
case. The astrophysical implications of these findings are addressed.

---------------------------------------------------------
Title: Instabilities in Line-driven Stellar Winds. V. Effect of an
Optically Thick Continuum
Authors: Owocki, Stanley P.; Rybicki, George B.
1991ApJ...368..261O Altcode:
Earlier analyses of the linear instability of line-driven stellar winds
are extended to the case, relevant to Wolf-Rayet stars, in which the
continuum remains optically thick well above the sonic point. It
is found that an optically thick flow driven by pure scattering
lines is stabilized by the drag effect of the diffuse, scattered
radiation. However, even a relatively small photon destruction
probability can cause a flow with continuum optical thickness much
greater than 1 to remain unstable, with a given growth rate. The
implications of these results for the variability characteristics of
winds from Wolf-Rayet stars are briefly discussed.

---------------------------------------------------------
Title: Stellar Winds from Massive Stars: the Influence of X-Rays on
the Dynamics
Authors: Stevens, I.; Cooper, G.; Owocki, S.
1991IAUS..143..318S Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Theory of Intrinsic Variability in Hot-Star Winds (review)
Authors: Owocki, S. P.
1991IAUS..143..155O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: A Smooth Source Function Method for Including Scattering in
Radiatively Driven Wind Simulations
Authors: Owocki, S. P.
1991ASIC..341..235O Altcode: 1991sabc.conf..235O
No abstract at ADS

---------------------------------------------------------
Title: The Steady State Solutions of Radiatively Driven Stellar
Winds for a Non-Sobolev, Pure Absorption Model
Authors: Poe, C. H.; Owocki, S. P.; Castor, J. I.
1990ApJ...358..199P Altcode:
The steady state solution topology for absorption line-driven flows is
investigated for the condition that the Sobolev approximation is not
used to compute the line force. The solution topology near the sonic
point is of the nodal type with two positive slope solutions. The
shallower of these slopes applies to reasonable lower boundary
conditions and realistic ion thermal speed v(th) and to the Sobolev
limit of zero of the usual Castor, Abbott, and Klein model. At finite
v(th), this solution consists of a family of very similar solutions
converging on the sonic point. It is concluded that a non-Sobolev,
absorption line-driven flow with a realistic values of v(th) has no
uniquely defined steady state. To the extent that a pure absorption
model of the outflow of stellar winds is applicable, radiatively driven
winds should be intrinsically variable.

---------------------------------------------------------
Title: The steady-state solutions of radiatively driven stellar
winds for a non-Sobolev, pure-absorption model.
Authors: Poe, Clint H.; Owocki, Stanley P.; Castor, John I.
1990ASPC....7..278P Altcode: 1990phls.work..278P
The authors summarize here the reasons for the statement: to the
extent that a pure-absorption model is applicable, radiatively driven
stellar winds have no well defined steady state. Using the non-Sobolev,
pure-absorption radiation force from Owocki et al., they find that
the solution topology at the sonic point is a node, not a saddle or
"x" as in the solar case. The number of transonic solutions increases
from one unique solution for the "x" type to a range of solutions for
the node type topology. Thus, in the pure-absorption approximation,
line driven winds can have a range of possible mass-loss rates and
terminal velocities.

---------------------------------------------------------
Title: Instabilities in Line-driven Stellar Winds. IV. Linear
Perturbations in Three Dimensions
Authors: Rybicki, G. B.; Owocki, S. P.; Castor, J. I.
1990ApJ...349..274R Altcode:
Nonradial wave propagation in line-driven stellar winds is analyzed
including both finite disk effects and the line-drag effect of scattered
line radiation. Within the local (WKB) analysis the results apply to
wavelengths both longer and shorter than the Sobolev length. The finite
disk causes short-wavelength waves with lateral velocity polarization
to be unstable in the idealized case of pure absorption; however, the
growth rates are smaller than for radially polarized waves, and the
instability is damped by a relatively small amount of scattering. Hence,
in realistic stellar winds, where the driving is primarily by scattering
lines, perturbations with an arbitrary mixture of lateral and radial
polarizations at the wind base should quickly become nearly radially
polarized farther out in the wind. The implications of these results
are discussed, both for interpretation of observational signatures of
wind structure and for theoretical calculations aimed at modeling the
nonlinear evolution of wind instabilities.

---------------------------------------------------------
Title: Applicability of steady models for hot-star winds.
Authors: Owocki, Stanley P.; Poe, Clint H.; Castor, John I.
1990ASPC....7..283O Altcode: 1990phls.work..283O
Non-Sobolev models of radiatively driven stellar winds based on
a pure-absorption approximation do not have a well-defined steady
state. Here the authors examine the implications of this for flow
time-dependence, showing that, under such circumstances, instabilities
in the flow attain an absolute character that leads to intrinsic
variability. In this case, steady solutions are inherently inapplicable
because they do not represent physically realizable states. However,
for actual hot-star winds, driving is principally by scattering, not
pure-absorption. In practice, the relatively weak force associated
with slight asymmetries in the diffuse, scattered radiation field may
play a crucial role in breaking the solution degeneracy and in reducing
the instability from an absolute to an advective character.

---------------------------------------------------------
Title: Winds from Hot Stars.
Authors: Owocki, S. P.
1990RvMA....3...98O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Physics of Instabilities in Radiatively Driven Stellar Winds
Authors: Owocki, S. P.; Castor, J. I.; Rybicki, G. B.
1989ASSL..157..291O Altcode: 1989IAUCo.113..291O; 1989plbv.coll..291O
No abstract at ADS

---------------------------------------------------------
Title: Time-dependent Models of Radiatively Driven Stellar
Winds. I. Nonlinear Evolution of Instabilities for a Pure Absorption
Model
Authors: Owocki, Stanley P.; Castor, John I.; Rybicki, George B.
1988ApJ...335..914O Altcode:
The authors describe results of numerical radiation-hydrodynamics
simulations of the nonlinear evolution of instabilities in radiatively
driven stellar winds. The wind is idealized as a spherically
symmetric, isothermal flow driven by pure absorption of stellar
radiation in a fixed ensemble of spectral lines. The simulations
indicate that there is a strong tendency for the unstable flow to
form rather sharp rarefactions in which the highest speed material
has very low density. The growth of wave perturbations thus remains
nearly exponential well beyond the linear regime, until the waves
are kinematically steepened into strong shocks. The strongest shocks
here are reverse shocks that arise to decelerate high-speed, rarefied
flow as it impacts slower material that has been compressed into
dense shells. The subsequent wind evolution shows a slow decay of the
shocks and the gradual thermal decompression and interaction of the
dense shells.

---------------------------------------------------------
Title: Absolute Instability as a Cause of Intrinsic Variablity in
Line-Driven Stellar Winds
Authors: Owocki, S. P.; Poe, C. H.; Castor, J. I.
1988BAAS...20.1013O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Steady Solutions for a Radiation-Driven Stellar Wind Not
Based on the Sobolev Approximation
Authors: Poe, C. H.; Owocki, S. P.; Castor, J. I.
1988BAAS...20.1012P Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Time-Dependent Mass Loss from Hot Stars With and Without
Radiative Driving
Authors: Castor, John I.; Owocki, Stanley P.; Rybicki, George B.
1988ASSL..148..229C Altcode: 1988pmls.conf..229C
A numerical hydrodynamics code is used to investigate two aspects of
the winds of hot stars. The first is the question of the instability
of the massive radiatively-driven wind of an O star that is caused
by the line shape mechanism: modulation of the radiation force by
velocity fluctuations. The evolution of this instability is studied in
a model O star wind, and is found to lead to wave structures that are
compatible with observations of wind instabilities. The other area of
investigation is of main-sequence B star winds.

---------------------------------------------------------
Title: Shock Formation from the Nonlinear Evolution of Instabilities
in Line-Driven Stellar Winds
Authors: Owocki, S. P.; Castor, J. I.; Rybicki, G. B.
1987BAAS...19..702O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Shock Formation from the Nonlinear Evolution of Instabilities
in Line-Driven Stellar Winds
Authors: Owocki, S.; Castor, J. I.; Rybicki, G. B.
1987sowi.conf..177O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Nonlinear dynamics of instabilities in line-driven stellar
winds
Authors: Owocki, S. P.; Castor, J. I.; Rybicki, G. B.
1987ASSL..136..269O Altcode: 1987ilet.work..269O
The authors have been developing a numerical radiation-hydrodynamics
program in order to study the nonlinear evolution of instabilities in
line-driven winds from luminous, early-type stars. Initial tests of
the code indicate that the velocity structure of nonlinear pulses in
such a wind may be quite different than assumed in previous analyses.

---------------------------------------------------------
Title: Instabilities in Line-driven Stellar Winds. III. Wave
Propagation in the Case of Pure Line Absorption
Authors: Owocki, S. P.; Rybicki, G. B.
1986ApJ...309..127O Altcode:
The spatial and temporal evolution of small-amplitude velocity
perturbations is examined in the idealized case of a stellar wind that
is driven by pure line absorption of the star's continuum radiation. It
is established that the instability in the supersonic region is of the
advective type relative to the star, but of the absolute type relative
to the wind itself. It is also shown that the inward propagation of
information in such a wind is limited to the sound speed, in contrast
to the theory of Abbott, which predicts inward propagation faster than
sound. This apparent contradiction is resolved through an extensive
discussion of the analytically soluble case of zero sound speed.

---------------------------------------------------------
Title: A Time-Dependent Line-Driven Wind Model Not Based on the
Sobolev Approximation
Authors: Owocki, S. P.; Castor, J. I.; Rybicki, G. B.
1986BAAS...18..953O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Role of Nonclassical Electron Transport in the Lower
Solar Transition Region
Authors: Owocki, S. P.; Canfield, R. C.
1986ApJ...300..420O Altcode:
One problem in solar physics is concerned with an understanding of the
observed brightness of the quiet solar atmosphere in spectral lines
which are formed in the lower solar transition region. The present
paper has the objective to examine the possibility that the observed
line emission results from nonclassical electron transport effects
which are associated with the inherently steep temperature gradients
in the solar transition region. The height variation of the electron
temperature is parameterized to enable correspondence with a variety
of one-dimensional constant pressure transition region models. The
models include empirical models, theoretical models, and the constant
classical heat fluxx model used by Shoub (1983). The electron velocity
distribution function is considered along with the effect on collisional
excitation and ionization rates, and effects on heat transport.

---------------------------------------------------------
Title: Instabilities in line-driven stellar winds. II - Effect
of scattering.
Authors: Owocki, S. P.; Rybicki, G. B.
1985ApJ...299..265O Altcode:
An earlier analysis (Owocki and Rybicki) of the linear instability
of line-driven stellar winds is extended to take proper account of
the dynamical effect of scattered radiation. The principal findings
are as follows: (1) the drag effect of the mean scattered radiation
does indeed greatly reduce the contribution of scattering lines to the
instability at the very base of the wind, but the instability growth
rate associated with such lines rapidly increases as the flow moves
outward from the base, reaching more than 50 percent of the growth
rate for pure absorption lines within a stellar radius of the surface,
and eventually reaching 80 percent of that rate at large radii; (2)
perturbations in the scattered radiation field may be important for the
propagation of wind disturbances, but they have little effect on the
wind instability; (3) the contribution of a strongly shadowed line to
the wind instability is often reduced compared to that of an unshadowed
line, but its effect is not one of damping in the outer parts of the
wind. The primary conclusion derived from these results is thus that,
even when all scattering effects are taken into account, the bulk of
the flow in a line-driven stellar wind is still highly unstable.

---------------------------------------------------------
Title: Effect of scattering on instabilities in line-driven stellar
winds.
Authors: Owocki, S. P.; Rybicki, G. B.
1985NASCP2358..221O Altcode: 1985onhm.rept..221O
Line driven O-B stellar winds are unstable to perturbations of short
spatial wavelength and the growth rates for such instabilities are very
rapid. The nature of the nonlinear development of this instability
is unknown, but might possibly be one of blobs of gas driven through
ambient gas or a quasiregular train of outward moving shocks. In either
case the resulting dissipation of mechanical energy might explain the
observed anomalous heating n O-B stars as evidenced by their X-ray
emission and high ionization state. It also might explain the observed
fine structure of the absorption lines and their time variability. The
driving due to the absorption of the stellar continuum flux was
considered and the effects of the diffuse, scattered radiation field
were neglected. It was shown that under certain special conditions
the effect of scattering could reduce the instability growth rate to
zero. A stability analysis that includes scattering, but that uses
the more physically realistic assumption of complete redistribution
instead of coherent scattering, and that includes the effects of
transverse velocity gradients, which become important as the flow
moves away from the stellar surface is presented. It is found that
the instability is eliminated right at the base of the wind, but
that as the flow moves outward the instability rate rapidly becomes
equal to a substantial fraction of the calculated value, the fraction
asymptotically reaching 80% at large radii. Since this still implies
many e folds in a characteristic outflow time, the primary conclusion
that these winds are highly unstable is unchanged.

---------------------------------------------------------
Title: Effect of Scattering on the Instability of Radiation-Driven
Stellar Winds
Authors: Rybicki, G. B.; Owocki, S. P.
1984BAAS...16R.993R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Role of Non-Classical Transport in the Formation of the
Ly-α Temperature Plateau
Authors: Owocki, S. P.; Canfield, R. C.; McClymont, A. N.
1984BAAS...16..992O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Instabilities in line-driven stellar winds. I. Dependence on
perturbation wavelength.
Authors: Owocki, S. P.; Rybicki, G. B.
1984ApJ...284..337O Altcode:
An analytical study is presented of the stability of absorption
line-driven flows, such as found in stellar winds, in the presence
of small-amplitude disturbances. A generalized calculation of the
perturbed direct extinction force is performed and the evolution of
the perturbation into the nonlinear regime and the dynamical results
of perturbation in the scattered radiation field are examined. An
expression is derived for the wavenumber variation of the perturbed line
force from the milieu of nonoverlapping lines which have a power-law
distribution in opacity. A linear dispersion analysis is carried out
to model the growth and propagation of radiative-acoustic waves in
absorption line-driven flows, which are found unstable to perturbations
that may be smaller than the Sobolev length. No damping mechanism was
found that would eliminate the absorption line-driven flows.

---------------------------------------------------------
Title: On the Formation of Temperature Plateaus in the Solar
Transition Region
Authors: Owocki, S. P.; McClymont, A. N.; Canfield, R. C.
1984BAAS...16..729O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Effect of Scattered Radiation on the Instability of Hot
Star Winds
Authors: Rybicki, G. B.; Owocki, S. P.
1984BAAS...16Q.725R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The solar wind ionization state as a coronal temperature
diagnostic
Authors: Owocki, S. P.; Holzer, T. E.; Hundhausen, A. J.
1983ApJ...275..354O Altcode:
The 'frozen' solar wind ionization state within a few solar radii
of the photosphere suggests that ion measurements at 1 AU may yield
information on the electron temperature conditions at the base of
the coronal expansion. The freezing-in process is examined in light
of traditional assumptions as to coronal expansion, where electron
temperature decreases monotonically with height, the bulk flow of all
charge states of a given ion species are equal to the proton speed,
and the ion outflow is spherically symmetric. The consequences of
the relaxation of these assumptions include the underestimation of
the magnitude of a temperature maximum occurring near the freezing-in
radius. Because it is associated with high speed, low density flow,
an areal divergence that is faster than that in a spherical outflow
lowers the ionization state freezing-in level relative to that which
is typical in spherically symmetric expansion.

---------------------------------------------------------
Title: The effect of a coronal shock wave on the solar wind ionization
state
Authors: Owocki, S. P.; Hundhausen, A. J.
1983ApJ...274..414O Altcode:
In connection with studies of solar wind ionization state freezing, it
is shown that, by using a Lagrangian approach of following individual
fluid parcels, the techniques used previously for calculating
ionization state variations in a steady state case can be extended
straightforwardly to time-varying flows. The specific ionization
state calculations presented are for a relatively simple picture
of time-dependent coronal flow, based on a well-known model of a
self-similar shock wave propagating through the corona. Time-dependent
ionization effects for the sudden transition between two otherwise
steady flows are likely to be limited to a narrow range of gas
parcels which, having been shocked within the coronal freezing-in
radius, pass a fixed interplanetary observer in an interval of a
few tens of minutes. The amplitude of any rise in interplanetary
ionization temperature associated with the coronal shock is likely
to be considerably smaller than the jump in electron temperature that
actually occurs in the corona.

---------------------------------------------------------
Title: Interpreting the solar wind ionization state.
Authors: Owocki, S. P.
1983NASCP.2280.623O Altcode: 1983sowi.conf..623O
The ionization state of the solar coronal expansion is frozen within a
few solar radii of the solar photosphere, and spacecraft measurements of
the solar wind heavy ion charge state can therefore yield information
about coronal conditions (e.g., electron temperature). Previous
interpretations of the frozen-in ionization state have always
assumed that in the coronal freezing-in region, (1) all heavy ions
flow at the same bulk speed as protons, (2) the electron velocity
distribution function is Maxwellian, and (3) conditions vary in
space but not in time. The consequences of relaxing these assumptions
for the interpretation of solar wind charge state measurements are
examined. It is found that: (1) the temperature inferred by traditional
interpretation of the interplanetary ionization state overestimates
(underestimate) the actual coronal electron temperature if higher
ion charge stages flow systematically faster (slower) than lower
stages at the coronal freezing radius; (2) temperatures inferred
from relative abundance measurements of ion-charge-stages with high
ionization potentials moderately overestimate the actual coronal
electron temperature if the high-energy tail of the coronal electron
velocity distribution is enhanced relative to a Maxwellian distribution;
(3) the propagation of a disturbance, e.g., a shock wave, through the
corona can strongly affect the frozen-in charge state, but only over
a time (a few times ten minutes) corresponding to the coronal transit
time for the disturbance.

---------------------------------------------------------
Title: Instabilities in Line-Driven Stellar Winds
Authors: Rybicki, G. B.; Owocki, S. P.
1983BAAS...15..968R Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The effect of a non-Maxwellian electron distribution on oxygen
and iron ionization balances in the solar corona
Authors: Owocki, S. P.; Scudder, J. D.
1983ApJ...270..758O Altcode:
Analytic expressions are derived for ionization and recombination
rates in a parameterized non-Maxwellian electron velocity distribution
with an enhanced high-energy tail. These expressions are then used in
investigating the effect of such an enhancement in the high-energy tail
of the coronal electron velocity distribution on the oxygen and iron
ionization balances, O(+6) - O(+7) and Fe(+11) - Fe(+12). Relative to a
Maxwellian of the same mean electron energy, the degree of ionization
allowed by such a distribution is found to be either unchanged or
slightly decreased for iron but often substantially increased for
oxygen. The greater sensitivity of oxygen ionization balance to the
high-energy distribution tail derives from the higher oxygen ionization
threshold energy. It is noted that the electron temperature inferred
from a measurement of the oxygen ionization ratio, O(+6)/O(+7), could
indeed overestimate the actual coronal electron temperature by nearly
10 to the 6th K if the coronal electron distribution is incorrectly
assumed to be Maxwellian.

---------------------------------------------------------
Title: The Stability of Line-Radiation-Driven Stellar Winds
Authors: Owocki, S. P.; Rybicki, G.
1982BAAS...14..920O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Radar Studies of the Non-Spherically Symmetric Solar Corona
Authors: Owocki, S. P.; Newkirk, G. A.; Sime, D. G.
1982SoPh...78..317O Altcode:
We review the results of radar studies of the Sun made at El Campo,
Texas 1961-69 with particular emphasis on the record of observed solar
radar cross sections. Using ray traces which include the effects of
refraction, absorption, and scattering in non-spherically symmetric
models of the corona, we investigate the role of focusing by large-scale
coronal geometries in enhancing the radar cross section. We find that
certain coronal geometries (e.g. disk-center coronal holes) can, in
principle, significantly increase the radar cross section. However
the observations of large cross sections do not correspond very well
with periods when such large-scale focusing geometries existed in
the corona. We conclude that the present dataset does not support
the hypothesis that radar observations of the Sun will be useful in
determining the properties of large-scale coronal features.

---------------------------------------------------------
Title: The ionization state in a gas with a non-Maxwellian electron
distribution.
Authors: Owocki, S. P.; Scudder, J. D.
1982SAOSR.392A.107O Altcode: 1982csss....2..107O
The inferred degree of ionization of a gas is often used in
astrophysics as a diagnostic of the gas temperature. In the solar
transition region and corona, in the outer atmospheres of cool stars,
and in some portions of the interstellar medium), photoionization
can be neglected, and the ionization state is fixed by the balance
between ion-electron collisional ionization and dielectronic and/or
radiative recombination. Under these conditions, higher degrees of
ionization result from higher energy ion-electron collisions which are
common in a high temperature gas. Actually, ionization occurs through
collisions with electrons that have kinetic energies greater than
the ionization potential of the given ion, and so the ionization rate
depends on to the number of such high-energy electrons in the tail of
the electron velocity distribution. High-velocity electrons move across
large distances between effective coulomb collisions, and, in a strong
temperature or density gradient, the tail can be overpopulated relative
to Maxwell-Boltzmann distribution of equivalent energy density. Thus,
the ionization rate can also be greatly increased. These effects
for a parameterized form of the electron distribution function with
an enhanced high-velocity tail, namely the kappa distribution are
illustrated.

---------------------------------------------------------
Title: The ionization state of the solar wind
Authors: Owocki, Stanley Peter
1982PhDT........33O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Ionization State of the Solar Wind.
Authors: Owocki, S. P.
1982PhDT.........2O Altcode:
The solar wind ionization state is "frozen" within a few solar
radii of the photosphere, and measurements of the ions at 1 a.u. can
therefore potentially yield information about conditions at the base
of the coronal expansion. If ions are assumed to flow at a steady,
common bulk speed in a plasma with a Maxwellian electron distribution,
the interplanetary ionization state is characteristic of the electron
temperature in the coronal "freezing-in" region. In this thesis, we
examine the effect of relaxing one or more of these assumptions. The
effects of intrinsic time variations (e.g. as in a coronal transient) in
a uniform ion flow can be best understood through a Lagrangian approach
of following individual fluid parcels; for example, in coronal outflow
undergoing a strong shock, only fluid parcels shocked at or below the
ambient freezing-in radius have their ionization state modified by
the shock, and time-dependent ionization effects for the transition
between two steady flows are thus likely to be limited to a narrow
range of gas parcels which are shocked in the low corona and pass a
fixed interplanetary observer in a few tens-of-minutes. We also find
that differential flow of the ions in the freezing-in region can exert a
strong influence on the relative abundance of various ionization stages
in the solar wind, and so independent knowledge of ion flow speeds at
the base will be needed if coronal temperatures are to be accurately
inferred from solar wind ionization state measurements. Finally, because
ionization occurs through collisions with electrons with energies above
the ionization threshold, the ionization rate can be greatly increased
in a non-Maxwellian electron distribution function with an enhanced
high-energy tail. The ionization balances of some species are more
sensitive to this high-energy tail than others, and so observations
of the interplanetary ionization state of several species could be
potentially exploited to constrain possible values of both the "core"
and "tail" electron temperatures at the base of the solar wind.

---------------------------------------------------------
Title: Time-Dependent Solar Wind Ionization
Authors: Owocki, S. P.; Hundhausen, A. J.
1981BAAS...13..812O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: The Ionization State of the Solar Wind
Authors: Owocki, S. P.
1981BAAS...13..544O Altcode:
No abstract at ADS

---------------------------------------------------------
Title: Two-dimensional radiative transfer. II. The wings of Ca K
and Mg k.
Authors: Owocki, S. P.; Auer, L. H.
1980ApJ...241..448O Altcode:
The effect of horizontal radiative transfer on the Ca K and Mg k line
wing intensities in two-component models of the solar atmosphere is
investigated. No significant influence on the spatially unresolved
wing profiles of either line was found, even for models in which the
lateral variation was extreme over distances approaching a vertical
scale height. Horizontal contrast as measured by the spatially resolved
Mg k profile was found to be markedly reduced by lateral transfer over
scales at or below the current resolution limit. Contrast as measured
in the Ca K wing was relatively unaffected and was maintained down to
lateral sizes approaching a vertical scale height. The behavior of
Ca K relative to Mg k is attributed to the larger amount of photon
destruction by incoherent scattering in the Ca K wing, which limits
the distance that photons can diffuse laterally and forces the line
to be formed near LTE. It is therefore concluded that the small-scale
photospheric temperature structure predicted by many models can be
detected in Ca K, and that efforts toward reducing seeing limitations
on the resolution of solar observations should be encouraged.

---------------------------------------------------------
Title: Two Dimensional Radiative Transfer in Resonance Line Wings
Authors: Owocki, S. P.
1980BAAS...12..517O Altcode:
No abstract at ADS