## Photon loss from the helium Ly line - the key to the acceleration of Wolf-Rayet winds
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
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.
## Contents- 1. Introduction
- 2. Method
- 2.1. Numerical calculations
- 2.1.1. Clumping
- 2.1.2. Photon loss
- 2.2. Fitting procedure
- 2.2.1. Photospheric and terminal expansion velocity
- 2.2.2. Mass loss rate and clumping
- 2.2.3. Photon loss factor
- 2.2.4. Luminosity and stellar radius
- 2.2.5. Inner velocity law and stellar mass
- 2.1. Numerical calculations
- 3. Results
- 3.1. The terminal velocity
- 3.2. The photospheric velocity
- 3.3. The mass loss rate
- 3.4. The clumping factor and the photon loss factor
- 3.5. The hydrodynamical solution
- 3.5.1. Procedure
- 3.5.2. Convergence properties
- 3.6. The final velocity law
- 3.7. The luminosity
- 4. Calculation of the photon loss factor
- 5. Discussion
- 6. Conclusions
- Acknowledgements
- Appendix A: definition of the photon loss factor
- References
© European Southern Observatory (ESO) 1997 Online publication: June 30, 1998 |