scholarly journals Atmospheric parameters and accelerations in the outer parts of luminous hot stars

1989 ◽  
Vol 113 ◽  
pp. 287-288
Author(s):  
Hans Nieuwenhuijzen ◽  
Cornells de Jager
Keyword(s):  
Radiation Pressure ◽  
Dynamic Pressure ◽  
Great Part ◽  
Hydrostatic Equilibrium ◽  
Stellar Winds ◽  
Gravitational Acceleration ◽  
Atmospheric Parameters ◽  
Hot Stars ◽  
Plane Parallel ◽  
Turbulent Pressure

In the atmospheres of the most extreme luminous stars, close to the Humphreys-Davidson limit, the inward gravitational acceleration is for a great part compensated by outward accelerations due to radiation pressure, turbulent pressure and dynamic pressure of the stellar winds. As a result the effective acceleration is very small, resulting in blown-up atmospheres that can no longer be considered plane-parallel or in hydrostatic equilibrium.

scholarly journals Non-LTE line blanketed atmospheres for hot stars

1997 ◽  
Vol 189 ◽  
pp. 209-216
Author(s):  
D. J. Hillier
Keyword(s):  
Atomic Data ◽  
Computational Techniques ◽  
Stellar Winds ◽  
Hot Stars ◽  
Computing Power ◽  
Plane Parallel ◽  
Spectroscopic Analyses ◽  
Recent Advances ◽  
Parallel Approximation

The modeling of hot star atmospheres falls into two broad classes: those where the plane parallel approximation can be used, and those where the effects of spherical extension and stellar winds are important. In both cases non-LTE modeling is a necessity for reliable spectroscopic analyses.While simple ions (e.g., H, He I, and He II) have been treated routinely in non-LTE for many years it is only recently that advances in computing power, computational techniques, and the availability of atomic data have made it feasible to perform non-LTE line blanketing calculations. Present models, with varying degrees of approximation and sophistication, are now capable of treating the effects of tens of thousands of lines. We review the latest efforts in incorporating non-LTE line blanketing, highlighting recent advances in the modeling of 0 stars, hot sub-dwarfs, Wolf-Rayet stars, novae, and supernovae.

Stellar mass loss and atmospheric Instability

1988 ◽  
Vol 108 ◽  
pp. 102-113
Author(s):  
Cornelis de Jager ◽  
Hans Nieuwenhuijzen
Keyword(s):  
Mass Loss ◽  
Radiation Pressure ◽  
Stellar Mass ◽  
Stellar Winds ◽  
Evolved Stars ◽  
Upper Limit ◽  
Atmospheric Instability ◽  
Turbulent Pressure ◽  
Loss Component ◽  
Break Up

AbstractA review is given of rate of mass-loss values in the upper part of the Hertzsprung-Russell diagram. Near the luminosity limit of stellar existance = −10−4 M⊙ yr−1. Episodical mass loss in bright variable super- and hypergiants does not significantly increase this value. For Wolf-Rayet stars the rate of mass loss is larger by a factor 140 than for non-evolved stars with the same Teff and L; for C stars this factor is ten. This can be explained qualitatively. Rotation appears hardly to influence the rate of mass loss except for vrot-values close to the break-up velocity. This is in accordance with theory. We suggest the existence of a Red Supergiant Branch; along that branch mass loss is virtually independent of luminosity. Stellar winds along the upper limit of stellar existence are mainly due: to radiation pressure for hot supergiants (≳ 10 000 K); to turbulent pressure for cool supergiants (3000-10 000 K), and to dust-driven and pulsation-driven winds for cooler stars. The turbulent pressure may originate in largescale stochastic motions as observed in Alpha Cyg. Episodical mass loss, as observed in P Cyg, HR 8752 and other Very Luminous Variables may be due to occasional violent stochastic motions, resulting in a shock-driven episodical mass-loss component.

scholarly journals Stellar-wind theory for O and B stars

1970 ◽  
Vol 36 ◽  
pp. 236-237
Author(s):  
Philip M. Solomon
Keyword(s):  
Mass Loss ◽  
Radiation Pressure ◽  
Stellar Wind ◽  
Conclusive Evidence ◽  
Driving Mechanism ◽  
Gravitational Acceleration ◽  
Hot Stars ◽  
B Stars ◽  
Wind Theory ◽  
Ultraviolet Observations

The rocket-ultraviolet observations of strong Doppler-shifted absorption lines of Siiv, Civ, Nv and other ions in the spectrum of O and B supergiants clearly indicate a high velocity outflow of matter from these stars. The presence of moderate ionisation stages in the stellar wind is conclusive evidence that the flow cannot be due to a high temperature corona as is the case for the solar wind. It is shown that the driving mechanism for the hot-star mass loss is radiation pressure exerted on the gas through absorption in resonance lines occurring at wavelengths near the maximum of the star's continuum flux. In the upper layers of these stars the outward force per gram of matter due to the radiation pressure can greatly exceed the gravitational acceleration making a static atmosphere impossible.The problem of a steady-state moving reversing layer is formulated and the solution leads to predictions of mass-loss rates as a function of effective temperature and gravity for all hot stars. These results are in substantial agreement with the observations.

scholarly journals Radiatively-Driven Stellar Winds

1986 ◽  
Vol 89 ◽  
pp. 75-87 ◽  
Author(s):  
L.B. Lucy
Keyword(s):  
Radiation Pressure ◽  
Stellar Winds ◽  
Radiation Hydrodynamics ◽  
Marked Contrast ◽  
Hot Stars ◽  
Supersonic Zones

In this review, attention will be focussed exclusively on the winds of hot stars, concentrating for the most part on spectral types 0 and B. For these stars, a clear consensus has emerged that it is the gradient of selective radiation pressure - i.e., line-driving - that explains the high terminal velocities, typically ~ 2000 km s−1, of their winds. Accordingly, few would now doubt that the supersonic zones of these winds present us with rather clean examples of line-driven flow, whose investigation therefore properly belongs under the heading “Radiation Hydrodynamics”. Moreover, in marked contrast with other astronomical environments where line-driven winds may exist, the stellar case is geometrically and parametrically well defined and is thus by far the best natural example from which to learn about such flows.

scholarly journals Convectively Generated Turbulent Pressure: A Possible Cause for η Car - Type Shell Ejections

1991 ◽  
Vol 143 ◽  
pp. 549-549
Author(s):  
M. Kiriakidis ◽  
N. Langer ◽  
K.J. Fricke
Keyword(s):  
Massive Star ◽  
Energy Transport ◽  
Convection Zone ◽  
Hydrostatic Equilibrium ◽  
Convective Zone ◽  
Contraction Phase ◽  
Hydrodynamic Calculation ◽  
Turbulent Pressure ◽  
Possible Cause ◽  
Η Car

A selfconsistent hydrodynamic calculation of a very massive star (MZAMS = 2OOM⊙) including turbulent pressure and energy has been performed. In the contraction phase after core hydrogen exhaustion, the star moves towards cool surface temperatures in the HR diagram (cf. Fig. 1). Consequently, (at Teff ⋍ 8000K) an envelope convection zone developes, and its inner boundery moves inwards with time. First, the envelope remains in hydrostatic equilibrium, with radiation pressure correspondingly decreasing as turbulent pressure increases (gas pressure is small). However, due to the fact, that the gradient of the turbulent pressure is directed inwards at the bottom of the convective zone, this part of the star rapidly contracts. Due to the released contraction energy, the luminosity locally exceeds the Eddington-luminosity. It cannot be transported outwards by convection in the upper part of the convection zone, where convective energy transport is inefficient (▽c ⋍ ▽r) . Thus, the local super-Eddington luminosity leads to the ejection of the overlying layers.

Radiation Pressure and Accelerating Stellar Winds

1971 ◽  
Vol 169 ◽  
pp. 441 ◽  
Author(s):  
J. M. Marlborough
Keyword(s):  
Radiation Pressure ◽  
Stellar Winds
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