Author name code: owocki ADS astronomy entries on 2022-09-14 author:"Owocki, Stanley P." ------------------------------------------------------------------------ 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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). 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. Bibcode: 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 $>4$ 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 $>100$ 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. 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. Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. Title: On the Dynamical Requirements for Slow, Extended Acceleration in Massive-Star Winds Authors: Hilligoss, D.; Owocki, S. Bibcode: 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, & 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. 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 Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. Title: High Angular Resolution Astrophysics: Evolutionary Impact of Stellar Mass Loss Authors: Gies, Douglas; Owocki, S. P.; Ridgway, S.; ten Brummelaar, T. Bibcode: 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. 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. Bibcode: 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 (>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. 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. Bibcode: 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. 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 Bibcode: 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. Title: Disruption of circumstellar discs by large-scale stellar magnetic fields Authors: ud-Doula, Asif; Owocki, Stanley P.; Kee, Nathaniel Dylan Bibcode: 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. Title: Radiation Transport Through Super-Eddington Stellar Winds Authors: Guzik, Joyce A.; Fryer, Chris; Urbatsch, Todd J.; Owocki, Stanley P. Bibcode: 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. Title: Hot-Star Winds: CIRs, DACs & BRITE Spots Authors: Owocki, Stanley P. Bibcode: 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 & 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. 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. Bibcode: 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. Title: Characterizing the turbulent porosity of stellar wind structure generated by the line-deshadowing instability Authors: Owocki, Stanley P.; Sundqvist, Jon O. Bibcode: 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. Title: Diffusion-plus-drift models for the mass leakage from centrifugal magnetospheres of magnetic hot-stars Authors: Owocki, Stanley P.; Cranmer, Steven R. Bibcode: 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. Title: Line-driven ablation of circumstellar discs - II. Analysing the role of multiple resonances Authors: Kee, Nathaniel Dylan; Owocki, Stanley; Kuiper, Rolf Bibcode: 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. Title: Accretion, Outflows, and Winds of Magnetized Stars Authors: Romanova, Marina M.; Owocki, Stanley P. Bibcode: 2018smfu.book..347R Altcode: No abstract at ADS Title: On the optically thick winds of Wolf-Rayet stars Authors: Gräfener, G.; Owocki, S. P.; Grassitelli, L.; Langer, N. Bibcode: 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. Title: Super-Eddington stellar winds: unifying radiative-enthalpy versus flux-driven models Authors: Owocki, Stanley P.; Townsend, Richard H. D.; Quataert, Eliot Bibcode: 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 > 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 < 1 - 1/Γo exceeds the Eddington luminosity, Lobs > 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. Title: Destruction of Be star disk by large scale magnetic fields Authors: Ud-Doula, Asif; Owocki, Stanley; Kee, Nathaniel Dylan; Vanyo, Michael Bibcode: 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. Title: New Insights into the Puzzling P-Cygni Profiles of Magnetic Massive Stars Authors: Erba, Christiana; David-Uraz, Alexandre; Petit, Véronique; Owocki, Stanley P. Bibcode: 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. 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. Bibcode: 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. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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 >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 >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. Bibcode: 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. Title: Destruction of Be star disk by large scale magnetic fields Authors: Ud-Doula, Asif; Owocki, Stanley P.; Kee, Nathaniel; Vanyo, Michael Bibcode: 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. Title: Line-driven ablation of circumstellar disks . Authors: Kee, N. Dylan; Owocki, S.; Kuiper, R.; Sundqvist, J. Bibcode: 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. Title: Pulsational Mass Ejection in Be Star Disks Authors: Kee, N. D.; Owocki, S.; Townsend, R.; Müller, H. -R. Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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 >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 < 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. 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. Bibcode: 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). 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. Bibcode: 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. 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 Bibcode: 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. 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. Bibcode: 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. 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. Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. 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 Bibcode: 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. Title: Accretion, Outflows, and Winds of Magnetized Stars Authors: Romanova, Marina M.; Owocki, Stanley P. Bibcode: 2016smfu.book..347R Altcode: No abstract at ADS 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 Bibcode: 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. Title: Effect of scattering on the transonic solution topology and intrinsic variability of line-driven stellar winds Authors: Sundqvist, Jon O.; Owocki, Stanley P. Bibcode: 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. Title: Hydrodynamic and radiative transfer modeling of X-ray emission from colliding WR winds: WR 140 & 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. Bibcode: 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$10", leading to a smorgasbord of wind-wind collisions. To model the X-ray emission of WR 140 and the Galactic center, we perform 3D hydrodynamic simulations to trace the complex gaseous flows, and then carry out 3D radiative transfer calculations to compute the variable X-ray spectra. The model WR 140 RXTE light curve matches the data well for all phases except the X-ray minimum associated with periastron, while the model spectra agree with the RXTE hardness ratio and the shape of the Suzaku observations throughout the orbit. The Galactic center model of the Chandra flux and spectral shape match well in the region r$<$3", but the model flux falls off too rapidly beyond this radius. 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. Bibcode: 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. Title: Accretion, Outflows, and Winds of Magnetized Stars Authors: Romanova, Marina M.; Owocki, Stanley P. Bibcode: 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. Bibcode: 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 <Bz> ≈ -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. Bibcode: 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. 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. Bibcode: 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 & 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. 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. Bibcode: 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. Bibcode: 2015AAS...22520303R Altcode: 2015AAS...22520303P The Galactic center is a hotbed of astrophysical phenomena. The ~30 evolved massive stars orbiting the SMBH on scales <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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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 Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 < 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. Bibcode: 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" (2013, ApJ, 770, 80) Authors: Leutenegger, Maurice A.; Cohen, David H.; Sundqvist, Jon O.; Owocki, Stanley P. Bibcode: 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 Bibcode: 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 > 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 Bibcode: 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. Bibcode: 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 ∞ <~ 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 > RK, versus dynamical magnetospheres (DM), with RK > 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 2013AAS...22114816M Altcode: An extremely luminous (>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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 > 32, kT > 1 keV and log LX/Lbol > -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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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 HD 96446 have been obtained. HD 96446 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 HD 96446 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 HD 96446.
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, HD 96446 does not present signatures of the presence of such confined material, such as Hα emission.
Conclusions: We conclude that, even though HD 96446 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. <title type="main">Footnotes<label>1</label>Low- and high-ionization refer here to atomic species with ionizations potentials (IPs) below and above the IP of hydrogen, 13.6 eV.<label>2</label>Measured in degrees from north to east.<label>3</label>θ is the same as the angle φ defined in fig. 3 of O08.<label>4</label>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. Bibcode: 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. Bibcode: 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 > 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. Bibcode: 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 & Owocki, applied to the specific case of σ Ori E by Townsend, Owocki & 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. <title type="main">Footnotes<label>1</label> Title: The Many Facets of Massive Star Mass Loss Authors: Owocki, S. Bibcode: 2012iac..talk..363O Altcode: 2012iac..talk..284O No abstract at ADS Title: Instability & Mass Loss near the Eddington Limit Authors: Owocki, S. P.; Shaviv, N. J. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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 Bibcode: 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 & WR 140 from SPH simulations Authors: Russell, Christopher M. P.; Corcoran, Michael F.; Okazaki, Atsuo T.; Madura, Thomas I.; Owocki, Stanley P. Bibcode: 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. Bibcode: 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 >14,000 X-ray point sources; >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. Bibcode: 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. Bibcode: 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. Bibcode: 2011BSRSL..80..694M Altcode: The extremely massive (> 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 & WR140 from SPH Simulations Authors: Russell, Christopher M. P.; Corcoran, Michael F.; Okazaki, Atsuo T.; Madura, Thomas I.; Owocki, Stanley P. Bibcode: 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. Bibcode: 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. Bibcode: 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 >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, <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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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 (< 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 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 < 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 > 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 < RK, but can be substantial and even dominant for models with RA >~ 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 >> 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 2007ApJ...660..687M Altcode: 2007astro.ph..2007M One-dimensional (1D) stellar wind models for hot stars rotating at >~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 >~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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 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. Bibcode: 2007ASPC..367..165A Altcode: We present a practical, efficient semi-analytic formalism for computing steady-state X-ray emission from radiative shocks from 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. Title: A Simple Nozzle Analysis of Slow-Acceleration Solutions in 1-D Models of Rotating Line-Driven Stellar Winds Authors: Owocki, S. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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'>~1, implying large porosity lengths h>~r. The associated wind structure must thus have either a relatively large scale l<~r, or a small volume filling factor f~l/r<<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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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'' (ApJ, 628, 986 [2005]) 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. Bibcode: 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-->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. (ApJ, 628, 986 [2005]) and F. Martins et al. (ApJ, 628, 986 [2005]) 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. (ApJ, 628, 986 [2005]). For Mg XI and Si XIII, we further show the predictions based on the R0 values of D. Porquet et al. (ApJ, 628, 986 [2005]). 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<R/R*<2.1, that is consistent with all three measured f/i ratios. Using G. R. Blumenthal et al. (ApJ, 628, 986 [2005]) (Fig. 13, solid lines), the range of radii is 1.6<R/R*<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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 < 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 < 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 > 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. Bibcode: 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<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 (>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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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&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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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* <= 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* > 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 (< 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. Bibcode: 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. Bibcode: 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, $\eta_\ast > 1$, 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. Bibcode: 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. Bibcode: 2003IAUS..209..467D Altcode: No abstract at ADS Title: Magnetic Spin-Up of Line-Driven Stellar Winds Authors: Owocki, S.; Ud-Doula, A. Bibcode: 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. 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. Bibcode: 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. Title: Radiatively Driven Winds and the Shaping of Bipolar Luminous Blue Variable Nebulae Authors: Dwarkadas, Vikram V.; Owocki, Stanley P. Bibcode: 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. Title: Analysis of Doppler-Broadened X-ray Emission Line Profiles from Hot Stars Authors: Cohen, D. H.; Kramer, R. H.; Owocki, S. P. Bibcode: 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. 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. Bibcode: 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. Title: Wavelet analysis of instability-generated line profile variations in hot-star winds Authors: Dessart, L.; Owocki, S. P. Bibcode: 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. Title: Dynamical Simulations of Magnetically Channeled Line-driven Stellar Winds. I. Isothermal, Nonrotating, Radially Driven Flow Authors: ud-Doula, Asif; Owocki, Stanley P. Bibcode: 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*<=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*>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. Title: Hydrodynamic Models of QSO Disk Winds Authors: Hillier, D. J.; Pereyra, N. A.; Turnshek, D. A.; Owocki, S. P. Bibcode: 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. Title: Ion Runaway Instability in Low-Density, Line-driven Stellar Winds Authors: Owocki, Stanley P.; Puls, Joachim Bibcode: 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. Title: Emission profile variability in hot star winds. A pseudo-3D method based on radiation hydrodynamics simulations Authors: Dessart, L.; Owocki, S. P. Bibcode: 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. Title: Line Forces in Keplerian Circumstellar Disks and Precession of Nearly Circular Orbits Authors: Gayley, K.; Ignace, R.; Owocki, S. Bibcode: 2002BeSN...35...22G Altcode: No abstract at ADS Title: Mass Loss and Magnetospheres: X-rays from Hot Stars and Young Stellar Objects Authors: Gagné, M.; Cohen, D.; Owocki, S.; Ud-Doula, A. Bibcode: 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 >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. Title: The Link between Radiation-Driven Winds and Pulsation in Massive Stars (invited paper) Authors: Owocki, S. P.; Cranmer, S. R. Bibcode: 2002ASPC..259..512O Altcode: 2002rnpp.conf..512O; 2002IAUCo.185..512O No abstract at ADS Title: The outer evolution of instability-generated structure in radiatively driven stellar winds Authors: Runacres, M. C.; Owocki, S. P. Bibcode: 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. Title: Numerical Simulations of Magnetically Confined Line Driven Winds Authors: ud-Doula, A.; Owocki, S. Bibcode: 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. Title: Radiatively Driven Winds and the Shaping of Bipolar LBV Nebulae Authors: Dwarkadas, V. V.; Owocki, S. P. Bibcode: 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. Title: X-ray Emission Line Profile Modeling of Hot Stars Authors: Kramer, R. H.; Reed, J. E.; Cohen, D. H.; Owocki, S. P. Bibcode: 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. 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. Bibcode: 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 > 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. Title: X-Ray Line Profiles from Parameterized Emission within an Accelerating Stellar Wind Authors: Owocki, Stanley P.; Cohen, David H. Bibcode: 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. Title: Line Forces in Keplerian Circumstellar Disks and Precession of Nearly Circular Orbits Authors: Gayley, K. G.; Ignace, R.; Owocki, S. P. Bibcode: 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. Title: X-ray Line Spectroscopy of Hot Stars Authors: Cohen, David H.; Owocki, Stanley P. Bibcode: 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. 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. Bibcode: 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. Title: Radiative Forces in Be Disks: Precession of Nearly Circular Orbits Authors: Gayley, K.; Ignace, R.; Owocki, S. Bibcode: 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. Bibcode: 2000AAS...197.7811L Altcode: 2000BAAS...32R1531L We have analyzed the UV and optical spectrum of the O9.5V Star, NGC 346-12, using sophisticated, NLTE line-blanketed model atmospheres calculated with our code TLUSTY. The following stellar parameters were derived: T eff=30,000 K, log g=3.5. An abundance analysis yields a metallicity, [Fe/H]=-1.0. The N/C abundance ratio is 25 times the solar ratio, indicating that material processed through the CNO-cycle has been brought up to the surface. Assuming a distance modulus, (m-M)=18.9, for the SMC, we have derived the luminosity, radius, and mass of the star. We found, similarly to higher luminosity galactic stars, a discrepancy between the mass derived from the spectroscopic analysis and from stellar evolution theory. We conclude that it is very likely that NGC 346-12 is a fast rotator, whose evolution has been affected by rotation. Furthermore, the wind of NGC 346-12 appears abnormal: while the C IV resonance lines do not reveal any indication of a wind, a weak P-Cygni profile is observed in N 5 1240. Various possible explanations for the low inferred ion density in the stellar wind, including an enhanced degree of wind ionization associated with ion frictional heating, or ion runaway due to frictional decoupling from the hydrogen-helium bulk plasma, are examined. This work was supported through a NASA/NRC RA award and STScI grants (GO 7437, AR 7985). Title: Radiatively Driven Stellar Winds from Hot Stars Authors: Owocki, S. Bibcode: 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. Bibcode: 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. Bibcode: 2000BeSN...34...38O Altcode: No abstract at ADS Title: Book Review: Introduction to stellar winds / Cambridge U Press, 1999 Authors: Owocki, S. P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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, & 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 1999IAUS..193...96R Altcode: No abstract at ADS Title: Non-spherical Radiation-Driven Wind Models Authors: Puls, Joachim; Petrenz, Peter; Owocki, Stanley P. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1998ASPC..131..245P Altcode: 1998phls.conf..245P No abstract at ADS Title: Modelling Variability in Hot-Star Winds Authors: Owocki, S. P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 <~ 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 <= 18.6 days for one and <= 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. Bibcode: 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&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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 <= 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. Bibcode: 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 < E < 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1995ApJ...452L..53M Altcode: Wind variability in OB stars may be ubiquitous, and a connection between projected stellar rotation velocity and wind activity is well established. However, the origin of this connection is unknown. To probe the nature of the rotation connection, several of the attendees at the workshop on Instability and Variability of Hot-Star Winds drafted an IUE observing proposal. The goal of this program was to follow three stars for several rotations to determine whether the rotation connection is correlative or causal. The stars selected for monitoring all have rotation periods <=5 days. They were HD 50896 (WN5), HD 64760 (B0.5 Ib), and HD 66811 [ zeta Pup; O4 If(n)]. During 16 days of nearly continuous observations in 1995 January (dubbed the "MEGA" campaign), 444 high-dispersion IUE spectra of these stars were obtained. This Letter presents an overview of the results of the MEGA campaign and provides an introduction to the three following Letters, which discuss the results for each star. Title: Momentum Deposition in Wolf-Rayet Winds: Nonisotropic Diffusion with Effectively Gray Opacity Authors: Gayley, Kenneth G.; Owocki, Stanley P.; Cranmer, Steven R. Bibcode: 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 & 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. Bibcode: 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. Bibcode: 1995IAUS..163..158G Altcode: No abstract at ADS Title: The dynamics of Wolf-Rayet winds (Invited) Authors: Owocki, S. P.; Gayley, K. G. Bibcode: 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 & 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 Bibcode: 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 & 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. Bibcode: 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 Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 1991IAUS..143..318S Altcode: No abstract at ADS Title: Theory of Intrinsic Variability in Hot-Star Winds (review) Authors: Owocki, S. P. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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. Bibcode: 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 Bibcode: 1982PhDT........33O Altcode: No abstract at ADS Title: The Ionization State of the Solar Wind. Authors: Owocki, S. P. Bibcode: 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. Bibcode: 1981BAAS...13..812O Altcode: No abstract at ADS Title: The Ionization State of the Solar Wind Authors: Owocki, S. P. Bibcode: 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. Bibcode: 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. Bibcode: 1980BAAS...12..517O Altcode: No abstract at ADS