The massive winds of luminous peculiar B-type stars (original) (raw)
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Radiation driven winds of hot luminous stars
Astronomy and Astrophysics Supplement Series, 2000
Radiation-driven winds of hot luminous stars XVIII. The unreliability of stellar and wind parameter determinations from optical vs. UV spectral analysis of selected central stars of planetary nebulae and the possibility of some CSPNs as single-star supernova Ia progenitors
Radiation induced coronal wind in late B stars
New Astronomy, 2004
Multicomponent radiatively driven winds in the late B stars cause significant heating of the layers just above the photosphere. These heated layers form a hot region-a corona, which may provide a significant fraction of energy necessary for the formation of a coronal wind driven by gas pressure. Fast rotation of this radiation induced coronal wind may cause the formation of the disk. Disk inhibition by nonradial line force would have only a little effect in this case, since the main driving mechanism is pressure gradient and not absorption in spectral lines. Finally, we show that this coronal region may be a site of significant X-ray emission which is able to explain observed X-ray activity of early B stars.
Winds of M- and S-type AGB stars: an unorthodox suggestion for the driving mechanism
Astronomy & Astrophysics, 2007
Context. Current knowledge suggests that the dust-driven wind scenario provides a realistic framework for understanding mass loss from C-rich AGB stars. For M-type objects, however, recent detailed models demonstrate that radiation pressure on silicate grains is not sufficient to drive the observed winds, contrary to previous expectations. Aims. In this paper, we suggest an alternative mechanism for the mass loss of M-type AGB stars, involving the formation of both carbon and silicate grains due to non-equilibrium effects, and we study the viability of this scenario. Methods. We model the dynamical atmospheres and winds of AGB stars by solving the coupled system of frequency-dependent radiation hydrodynamics and time-dependent dust formation, using a parameterized description of non-equilibrium effects in the gas phase. This approach allows us to assess under which circumstances it is possible to drive winds with small amounts of carbon dust and to get silicate grains forming in these outflows at the same time. Results. The properties of the resulting wind models, such as mass-loss rates and outflow velocities, are well within the observed limits for M-type AGB stars. Furthermore, according to our results, it is quite unlikely that significant amounts of silicate grains will condense in a wind driven by a force totally unrelated to dust formation, as the conditions in the upper atmosphere and wind acceleration region put strong constraints on grain growth. Conclusions. The proposed scenario provides a natural explanation for the observed similarities in wind properties of M-type and C-type AGB stars and implies a smooth transition for stars with increasing carbon abundance, from solar-composition to C-rich AGB stars, possibly solving the longstanding problem of the driving mechanism for stars with a C/O close to one.
Wind modelling of very massive stars up to 300 solar masses
Astronomy & Astrophysics, 2011
The stellar upper-mass limit is highly uncertain. Some studies have claimed there is a universal upper limit of ∼150M ⊙ . A factor that is often overlooked is that there might be a significant difference between the present-day and the initial masses of the most massive stars -as a result of mass loss. The upper-mass limit may easily supersede ∼200M ⊙ . For these reasons, we present new mass-loss predictions from Monte Carlo radiative transfer models for very massive stars (VMS) in the mass range 40-300 M ⊙ , and with very high luminosities 6.0 ≤ log(L ⋆ /L ⊙ ) ≤ 7.03, corresponding to large Eddington factors Γ. Using our new dynamical approach, we find an upturn or "kink" in the mass-loss versus Γ dependence, at the point where the model winds become optically thick. This coincides with the location where our wind efficiency numbers surpass the single-scattering limit of η = 1, reaching values up to η ≃ 2.5. In all, our modelling suggests a transition from common O-type winds to Wolf-Rayet characteristics at the point where the winds become optically thick. This transitional behaviour is also revealed with respect to the wind acceleration parameter, β, which starts at values below 1 for the optically thin O-stars, and naturally reaches values as high as 1.5-2 for the optically thick Wolf-Rayet models. An additional finding concerns the transition in spectral morphology of the Of and WN characteristic He ii line at 4686Å. When we express our mass-loss predictions as a function of the electron scattering Eddington factor Γ e ∼ L ⋆ /M ⋆ alone, we obtain anṀ vs. Γ e dependence that is consistent with a previously reported power lawṀ ∝ Γ 5 e (Vink 2006) that was based on our previous semi-empirical modelling approach. When we expressṀ in terms of both Γ e and stellar mass, we find optically thin winds andṀ ∝ M ⋆ 0.68 Γ 2.2 e for the Γ e range 0.4 < ∼ Γ e < ∼ 0.7, and mass-loss rates that agree with the standard Vink et al. recipe for normal O stars. For higher Γ e values, the winds are optically thick and, as pointed out, the dependence is much steeper,Ṁ ∝ M ⋆ 0.78 Γ 4.77 e . Finally, we confirm that the effect of Γ on the predicted mass-loss rates is much stronger than for the increased helium abundance (cf. Vink & de Koter 2002 for Luminous Blue Variables), calling for a fundamental revision in the way stellar mass loss is incorporated in evolutionary models for the most massive stars.
Space Science Reviews, 2006
Cool giant and supergiant stars generally present low velocity winds with high mass loss rates. Several models have been proposed to explain the acceleration process of these winds. Although dust is known to be present in these objects, the radiation pressure on these particles is uneffective in reproducing the observed physical parameters of the wind. The most promising acceleration mechanism cited in the literature is the transference of momentum and energy from Alfvén waves to the gas. Usually, these models consider the wind to be isothermal. We present a stellar wind model in which the Alfvén waves are used as the main acceleration mechanism, and determine the temperature profile by solving the energy equation taking into account both the radiative losses and the wave heating. We also determine self-consistently the magnetic field geometry as the result of the competition between the magnetic field and the thermal pressures gradient. As main result, we show that the magnetic geometry present a super-radial index in the region where the gas pressure is increasing. However, this super-radial index is greater than that observed for the solar corona.
Evolution of Line-Force Multiplier Parameters in Radiation Driven Winds of Massive Stars
arXiv: Solar and Stellar Astrophysics, 2020
We present two self-consistent procedures that couple the hydrodynamics with calculations of the line-force in the frame of radiation wind theory. These procedures give us the line-force parameters, the velocity field, and the mass-loss rate. The first one is based on the so-called m-CAK theory. A full set of line-force parameters for Ttexteffge32,000T_\text{eff}\ge 32,000Ttexteffge32,000 K and surface gravities higher than 3.4 dex for two different metallicities are presented, along with their corresponding wind parameters. We find that the dependence of line-force parameters on effective temperature is enhanced by the dependence on logg\log glogg. For the case of homogeneous winds (without clumping) comparison of self-consistent mass-loss rates shows a good agreement with empirical values. We also consider self-consistent wind solutions that are used as input in FASTWIND to calculate synthetic spectra. By comparison with the observed spectra for three stars with clumped winds, we found that varying the clumping fact...
Stellar Winds in Herbig Ae/Be Stars
The Astrophysical Journal, 1998
The winds observed in the preÈmain-sequence phase of intermediate-mass young stars are discussed in light of a model originally developed by Mestel for describing stellar winds ejected by rotating stars. In addition to the acceleration mechanisms of thermal expansion and stellar radiation pressure, which typically play an important role in driving the winds of low-and high-mass stars, respectively, centrifugal acceleration by a corotating magnetic Ðeld and the presence of hydromagnetic waves are taken into account. The relevance of these mechanisms for the acceleration of stellar winds from the surface of young stars is discussed in the context of growing evidence for the presence of magnetic Ðelds and surface convection in preÈmain-sequence evolutionary phases. In the particular case of Herbig Ae/Be stars, the deposition of a fraction of the convection energy in Alfve n waves seems to be required in order to explain the winds observed in these preÈmain-sequence objects. Under some simplifying assumptions, the velocity and density Ðelds around the central stellar object are derived and the emerging line proÐles are computed. A test case is discussed in which the observed line proÐles of the prototypical object AB Aur are used to constrain the model parameters, showing that the magnetic rotator model o †ers a physically consistent description of this system.
Subsonic structure and optically thick winds from Wolf-Rayet stars
Astronomy and Astrophysics, 2018
Wolf-Rayet star's winds can be so dense and so optically thick that the photosphere appears in the highly supersonic part of the outflow, veiling the underlying subsonic part of the star, and leaving the initial acceleration of the wind inaccessible to observations. We investigate the conditions and the structure of the subsonic part of the outflow of Galactic WR stars, in particular of the WNE subclass; our focus is on the conditions at the sonic point. We compute 1D hydrodynamic stellar structure models for massive helium stars adopting outer boundaries at the sonic point. We find that the outflows of our models are accelerated to supersonic velocities by the radiative force from opacity bumps either at temperatures of the order of 200kK by the Fe opacity bump or of the order of 50kK by the HeII opacity bump. For a given mass-loss rate, the conditions in the subsonic part of the outflow are independent from the detailed physical conditions in the supersonic part. The close pro...
Rapidly rotating winds of hot stars
2010
The CAK theory is used for a description of a line driven wind of hot stars. We have developed a code using the Newton-Raphson method to obtain a solution of a 1D isothermal line driven wind with rotation. Our calculations confirmed that there exists a "break" value of stellar rotation velocity, for which the wind solution switches to a new one, which yields much denser and slower wind than in the non-rotating case. For this new solution we found a new critical point, which is located far from the stellar photosphere. Close to the star the outflow is 100 times denser at the equator than at the pole. The wind velocity profile is shallower and reaches a terminal velocity of only several hundred km s-1.