Radiation-driven Stellar Wind Research Articles

Observations of outflows of massive stars

AbstractMass loss plays a key role in the evolution of massive stars and their environment. High mass-loss events are traced by complex circumstellar ejecta and intricate line profiles across the upper Hertzsprung-Russell diagram for massive stars in different evolutionary stages. The basic physics of radiation-driven stellar wind for hot stars is well understood. However, the driving mechanisms and related instabilities for their enhanced mass-loss episodes and the driving mechanisms for the mass loss of cool stars are still debated. In this review, the mass-loss characteristics and the possible mechanisms will be surveyed for an observational set of prominent massive stellar populations that experience outflows, strong stellar winds, and periods of enhanced and eruptive mass loss; massive young stellar objects, OB-type stars, red supergiants, warm hypergiants, luminous blue variables, and Wolf-Rayet stars.

Coupling hydrodynamics with comoving frame radiative transfer

CONTEXT: For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. AIMS: We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (non-LTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. METHODS: Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. RESULTS: The concepts for obtaining hydrodynamically consistent models using a comoving-frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant Zeta Pup. The obtained stellar and wind parameters are within the current range of literature values. CONCLUSIONS: For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. (...)

Modeling the wind and photosphere of massive stars with the radiative transfer code CMFGEN

Massive stars are extremely luminous and characterized by mass loss through radiation-driven stellar winds. The radiation emitted at the stellar surface may interact with the wind, making the analysis of the emerging spectrum a very challenging task. In addition to the luminosity, effective temperature, and surface gravity, several other stellar parameters impact the spectral morphology of these objects, remarkably the mass-loss rate and the wind terminal velocity. That is generally the case for OB supergiants, Luminous Blue Variables, and WR stars. CMFGEN [1] comprises the state-of-the-art in non-LTE radiative transfer and has been successfully applied to the above classes of objects over the last decade. The code assumes spherical symmetry, stationary outflow, and both photospheric and wind lines can be treated in non-LTE. Full line blanketing due to hundreds of thousands of spectral lines is included, as well as wind clumping. Here I discuss the assumptions behind CMFGEN and present examples of models and spectroscopic analyses.

Nonlocal radiative coupling in non monotonic stellar winds

There is strong observational evidence of shocks and clumping in radiation-driven stellar winds from hot, luminous stars. The resulting non monotonic velocity law allows for radiative coupling between distant locations, which is so far not accounted for in hydrodynamic wind simulations. In the present paper, we determine the Sobolev source function and radiative line force in the presence of radiative coupling in spherically symmetric flows, extending the geometry-free formalism of Rybicki & Hummer (1978, ApJ, 219, 654) to the case of three-point coupling, which can result from, e.g., corotating interaction regions, wind shocks, or mass overloading. For a simple model of an overloaded wind, we find that, surprisingly, the flow decelerates at all radii above a certain height when nonlocal radiative coupling is accounted for. We discuss whether radiation-driven winds might in general not be able to re-accelerate after a non monotonicity has occurred in the velocity law.

Two-dimensional hydrodynamical simulations of wind-compressed disks around rapidly rotating B stars

We use a 2-D 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 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, 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 oblated 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, cases of inner disk inflow not possible in the heuristic approach of assuming a fixed outward velocity law. In addition, the disk opening typically has a half-angle of 2–4 degrees, somewhat larger than the ∼ 0.5° predicted from the analytic model, and there is no evidence for the predicted detachment of the disk that arises in the fixed outflow picture.

Two-dimensional hydrodynamical simulations of wind-compressed disks around rapidly rotating B-stars

We use a 2-D 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 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, 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 oblated 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, cases of inner disk inflow not possible in the heuristic approach of assuming a fixed outward velocity law. In addition, the disk opening typically has a half-angle of 2–4 degrees, somewhat larger than the ~ 0.5° predicted from the analytic model, and there is no evidence for the predicted detachment of the disk that arises in the fixed outflow picture.

Equatorial disk formation around rotating stars due to Ram pressure confinement by the stellar wind

We present a simple approximation that permits us to obtain the axisymmetric two-dimensional supersonic solution of a rotating radiation-driven stellar wind from the Friend & Abbott one-dimensional model of the equatorial flow. Our solution predicts the formation of a dense equatorial disk if the rotation rate of the star is above a threshold value, which depends on the ratio of the terminal speed of the wind to the escape speed of the star. Along the upper main sequence (earlier than B2), both the observed and theoretical values for this ratio decrease monotonically toward later spectral types. For early O stars, the disk can only form if the rotation speed is in excess of 90% of the critical (breakup) speed

Non-LTE analysis of the Ofpe/WN9 star HDE 269227 (R84)

The paper presents the results of a spectral analysis of the Ofpe/WN9 star HD 269227 (R84), which assumes a spherically expanding atmosphere to find solutions for equations of radiative transfer. The spectra of hydrogen and helium were predicted with a non-LTE model. Six stellar parameters were determined for R84. The shape of the velocity law is empirically found, since it can be probed from the terminal velocity of the wind. The six stellar parameters are further employed in a hydrodynamic model where stellar wind is assumed to be directed by radiation pressure, duplicating the mass-loss rate and the terminal wind velocity. The velocity laws found by computation and analysis are found to agree, supporting the theory of radiation-driven stellar wind. R84 is surmised to be a post-red supergiant which lost half of its initial mass, possibly during the red-supergiant phase. This mass loss is also suggested by its spectroscopic similarity to S Doradus.

Continuum polarization by electron scattering in rotationally distorted, radiation-driven stellar winds - B(e) supergiants

view Abstract Citations (18) References (30) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Continuum Polarization by Electron Scattering in Rotationally Distorted, Radiation-driven Stellar Winds: B[e] Supergiants Boyd, Cameron J. ; Marlborough, J. M. Abstract We have constructed a simple, axisymmetric, radiation-driven wind model for a rotating B[e] supergiant in order to determine the expected continuum linear polarization arising from electron scattering in the rotationally distorted wind driven by strong lines. We solve for the structure of the radiation driven wind along the rotation axis and in the equatorial plane with rotations at various fractions of the critical rotation speed. A simple combination of the polar and equatorial solutions is then used to obtain the structure of the wind in a meridional plane. We then compute the ionization-excitation equilibrium at a discrete grid of points in the meridional plane and use these results to evaluate the continuum linear polarization produced by single scattering of the photospheric radiation field. The rotationally distorted wind for a star rotating at 80% of the critical speed produces a polarization of ∼1.5% on the red side of the Balmer limit, similar to the observational data for the B[e] star MWC 939. By choosing extreme values for some model parameters, it may also be possible to reproduce the polarization data for MWC 645. However, the model wind is completely ionized to 3000 stellar radii so that dust and molecules cannot exist closer to the star than this distance. Some directions for possible improvement of the model are briefly discussed. Publication: The Astrophysical Journal Pub Date: March 1991 DOI: 10.1086/169750 Bibcode: 1991ApJ...369..191B Keywords: B Stars; Electron Scattering; Linear Polarization; Stellar Rotation; Stellar Winds; Supergiant Stars; Balmer Series; Hertzsprung-Russell Diagram; Stellar Luminosity; Stellar Models; polarization; stars: rotation stars: supergiants stars: winds; Astrophysics; POLARIZATION; STARS: ROTATION; STARS: SUPERGIANTS; STARS: WINDS full text sources ADS | data products SIMBAD (4)

A radiation-driven stellar wind model with a line force cutoff

This paper presents a model for a radiation-driven stellar wind in which the driving force is abruptly cut off at an adjustable distance from the star. The model is intended to give a first approximation of the effects of ionizing shocks in a stellar wind on the terminal velocity and mass-loss rate. As expected, the wind velocity is found to decrease after the line force is cut off. The terminal velocity depends directly on the velocity of the wind at the point where the driving force is cut off. The mass-loss rate is found to be unaffected as long as the cutoff is outside the critical point of the flow. The model is applied to the star Tau Sco, a strong X-ray source with an anomalously low terminal velocity. It is shown that this low terminal velocity can be caused by a cutoff of the line force at a distance which is consistent with the idea that the observed X-rate emission is produced by shocks in the wind.

Stellar winds driven by multiline scattering

view Abstract Citations (83) References (22) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stellar winds driven by multiline scattering Friend, D. B. ; Castor, J. I. Abstract A model of a radiation-driven stellar wind with overlapping spectral lines is presented. It is based on the Castor, Abbott, and Klein (CAK) theory (1975). The presence of overlapping lines allows a photon to be scattered many times in different lines. A random separation is assumed between strong lines, which makes it possible to find the angular distribution of the wavelength-averaged intensity of radiation from the star. The properties of the wind at any point depend on this intensity, which in turn depends on the structure of the wind. A self-consistent wind model is found. The mass loss rate does not saturate as line overlap becomes more pronounced, but continues to increase. The terminal velocity is much larger than in the CAK model, while the velocity law is shallower. This wind is much more efficient in ejecting matter from the star than the CAK wind. Publication: The Astrophysical Journal Pub Date: September 1983 DOI: 10.1086/161289 Bibcode: 1983ApJ...272..259F Keywords: Electromagnetic Scattering; Radiative Transfer; Stellar Mass Ejection; Stellar Radiation; Stellar Spectra; Stellar Winds; Angular Distribution; O Stars; Radiation Pressure; Stellar Atmospheres; Stellar Models; Wind Velocity; Astrophysics full text sources ADS |

Radiation-driven winds in X-ray binaries

view Abstract Citations (119) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Radiation driven winds in X-ray binaries. Friend, D. B. ; Castor, J. I. Abstract The properties of a radiation-driven stellar wind in an X-ray binary system are discussed. The Castor, Abbott, and Klein line-driven wind model is used, but the effects of the compact companion (gravity and continuum radiation pressure) and the centrifugal force due to orbital motion are included. These forces destroy the spherical symmetry of the wind and can make the mass loss and accretion strong functions of the size of the primary relative to its critical potential lobe. The accretion rate and X-ray luminosity of the known massive X-ray binaries are estimated. It is found that in most systems the wind alone could power the X-ray emission. It also appears that, in the evolution of these systems, there would be a continuous transition from wind accretion to critical potential lobe overflow. The model is also used to make a prediction about the nature of a suspected binary system which is not known to be an X-ray emitter. Publication: The Astrophysical Journal Pub Date: October 1982 DOI: 10.1086/160340 Bibcode: 1982ApJ...261..293F Keywords: Binary Stars; Hot Stars; Stellar Mass; Stellar Models; Stellar Winds; X Ray Binaries; X Ray Sources; B Stars; O Stars; Stellar Luminosity; Stellar Mass Accretion; Stellar Mass Ejection; Astrophysics full text sources ADS | data products SIMBAD (6)

The instability of radiation-driven stellar winds

view Abstract Citations (94) References (50) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The instability of radiation-driven stellar winds Carlberg, R. G. Abstract The stability of a radiation-driven stellar wind is examined by applying a completely linearized stability analysis to a physically realistic wind model. Three types of instability are found. Both the line-shape and the radiation-driven sound wave instabilities are purely vertical motions which have been previously discovered. A new 'gradient' instability has motion in both the vertical and horizontal directions. Typical growth times of the three instabilities are 150, 1000, and 10,000 seconds, respectively, although the growth time of the line-shape instability is a lower limit. The radiation-driven sound wave and gradient instabilities have density and velocity perturbations suggesting that the wind will tend to break up into clouds or slabs of typical size one-billion to 100-billion cm. The existence of such clumps would alter the dynamics of a wind and the interpretation of observations of winds. Publication: The Astrophysical Journal Pub Date: November 1980 DOI: 10.1086/158428 Bibcode: 1980ApJ...241.1131C Keywords: Astrophysics; Stellar Radiation; Stellar Winds; Flow Stability; Stellar Models; Astrophysics full text sources ADS |

Radiative amplification of sound waves in the winds of O and B stars

The velocity perturbation associated with an outwardly propagating sound wave in a radiation-driven stellar wind gives rise to a periodic Doppler shifting of absorption lines formed in the flow. A linearized theory applicable to optically thin waves is used to show that the resulting fluctuation in the absorption-line force can cause the wave amplitude to grow. Detailed calculations of the acceleration due to a large number of lines indicate that significant amplification can occur throughout the high-velocity portion of winds in which the dominant force-producing lines have appreciable optical depths. In the particular case of the wind of Zeta Pup (O4f), it is found that the e-folding distance for wave growth is considerably shorter than the scale lengths over which the physical properties of the flow vary. A qualitative estimate of the rate at which mechanical energy due to nonlinear waves can be dissipated suggests that this mechanism may be important in heating the supersonic portion of winds of early-type stars.