SIMBAD references

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 Heii Lyα 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, R_phot, {dot}(M), v_∞, v_phot, 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 Hei λ10830 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 β-law with β=1. The outer part can be approximated by a β-law with β=3 if the core radius of the atmosphere model is used, or by β=8, 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 v_phot ≃1100 km/s. The resulting luminosity L= 5.5x10⁵ L_☉ and terminal wind velocity v_∞=2060 km/s are found to be considerably larger than the values from previous determinations. On the other hand, the mass loss rate is lower {dot}(M)=3.2x10^–5 M_☉yr^–1 due to an inhomogeneous wind with a clumping factor C≃4. 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 f≃10^–4. It is proposed that a Bowen resonance-fluorescence mechanism removes a small fraction of photons from the radiation field of the helium Lyα resonance line. Photon loss calculated theoretically from the interaction of metal lines close in wavelength to the Heii Lyα line yields a depth dependent factor in the range 10^–4...10^–3. In the recombination zone, where the photon loss influences the ionization structure, its value is f^*≃10^–4 in excellent agreement with the empirically determined value. The lines CaV λ303.74, FeVI λλ303.70,303.80, 303.83, and OIII λ303.80 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.