Hydrodynamic Line Blanketed Atmospheres of Wolf-Rayet Stars

Abstract

In this paper I examine the acceleration of Wolf-Rayet winds in the optically thin part of the atmosphere. First, I investigate the radiative force using atmosphere models with specified velocity structures. It is found that “standard” model atmospheres that reproduce the observed spectra of Wolf-Rayet stars do not provide by a large factor (≈ 10) the force needed to accelerate the adopted velocity structure. Second, using clumping in order to have a favorable luminosity to mass loss ratio, it was attempted to find a hydrodynamic solution of a WR wind. If large clumping factors of the order of 30 are assumed then solutions are possible. However, such models are very unlikely to be correct since they predict much stronger emission lines than observed in any WR star. Third, a modification of the non-LTE rate equations is investigated. It is assumed that a Bowen resonance-fluorescence mechanism removes a small fraction of photons from the radiation field of the helium Lyα resonance line. It turns out that the ionization equilibrium of winds where helium is recombining from He⁺⁺ to He⁺ is extremely sensitive to a modification of the radiation field of this line. A removal of 1% of the photons is sufficient to initiate an abruptly changing ionization equilibrium. It is proposed that the key to understand the acceleration of WR winds is the correct calculation of the ionization structure. If this hypothesis is true then today’s non-LTE atmospheres yield systematically wrong results. This would not only affect the radiative acceleration but also the observable helium lines that are used for the diagnostic of the stellar temperature. It would imply that the luminosities of WR stars are underestimated by the present atmosphere models.