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Photon loss from the helium Ly line - the key to the acceleration of Wolf-Rayet winds
Received 15 February 1995 / Accepted 1 October 1996
It is demonstrated that the ionization equilibrium of helium in non-LTE atmospheres for Wolf-Rayet stars is very sensitive to photon loss from the He II Ly line. A removal of 0.001% of the photons is sufficient to initiate an abruptly recombining ionization equilibrium. The assumption of photon loss allows to address the wind momentum problem of Wolf-Rayet stars. It is possible for the first time to construct a line blanketed non-LTE model of a Wolf-Rayet star that reproduces the observed spectrum and simultaneously, provides the radiation force to drive its outer velocity structure.
A method is developed to determine the free model parameters L, , , , , C (clumping factor), and f (photon loss factor), by an analysis of an observed Wolf-Rayet spectrum. The method is applied to the spectrum of the WN5 star HD 50896 resulting in good fits in shape and strength to the observed helium emission lines. In particular the profile of the He I line, which is a tracer of the outer velocity structure, is reproduced remarkably well. The hydrodynamically calculated velocity law differs significantly from the commonly adopted -law with . The outer part can be approximated by a -law with if the core radius of the atmosphere model is used, or by , if the velocity law is calculated referring to the hydrostatic radius of a stellar evolution model in the Wolf-Rayet phase. Close to the photosphere the velocity structure is flat with an expansion velocity of km s-1. The resulting luminosity and terminal wind velocity km s-1 are found to be considerably larger than the values from previous determinations. On the other hand, the mass loss rate is lower yr-1 due to an inhomogeneous wind with a clumping factor . There is evidence for a decrease of the clumping factor with distance from the star.
The photon loss factor is determined empirically to have a value of . It is proposed that a Bowen resonance-fluorescence mechanism removes a small fraction of photons from the radiation field of the helium Ly resonance line. Photon loss calculated theoretically from the interaction of metal lines close in wavelength to the He II Ly line yields a depth dependent factor in the range . In the recombination zone, where the photon loss influences the ionization structure, its value is in excellent agreement with the empirically determined value. The lines Ca V , Fe VI , and O III are roughly of equal importance.
The wind momentum calculated by the present model exceeds the single scattering limit by a factor of 6 in contrast to previous estimates that yielded factors 50 - 100. With a momentum ratio of 6 the Wolf-Rayet winds are no longer distinct from other radiation driven winds but they fit as more extreme versions to the winds of O stars.
Key words: hydrodynamics radiative transfer methods: numerical stars: HD 50896 stars: mass-loss stars: Wolf-Rayet
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998