SIMBAD references

The possibility of driving the dense winds of Wolf-Rayet stars via multiple scattering of photons in resonance lines is examined. To answer the question as to whether this process can in principle account for the observed high performance numbers η={dot}(M)v_∞/(L*/c) a simple model with a logarithmic frequency distribution of spectral lines is treated by both analytical and numerical (Monte-Carlo) methods. For the case of many overlapping lines it is found that the diffuse radiation arising from the multiple scattering process leads to an even larger radiative acceleration g_rad than the previously used unattenuated flux models where each line is assumed to interact separately with the unattenuated radiative flux coming directly from the stellar photosphere. The reason for the larger multiple scattering acceleration is shown to result from (1) the diffusive behaviour of the photons which undergo many scatterings before escaping to infinity or being backscattered onto the stellar core and (2) the dependence of the Sobolev length, over which photons interact with a resonance line, on direction. The so-called hemisphere scattering in which a photon bounces back and forth between opposite hemispheres is thereby ruled out as a source of the effectivity of multiple scattering. The radial dependence of g_rad in the outer parts of the wind is now weaker than r^–2, in the limiting case of only optically thick lines being proportional to r^–1, which leads to a steeper rise in the velocity field. A dynamically consistent model for a WN5 model star wind indicates that performance numbers considerably higher than unity are possible provided that a large number of overlapping lines in the spectral regions where most of the flux is emitted is present throughout the wind. The performance number is shown to be roughly equal to the number of effectively thick lines per overlap interval {DELTA}ν_∞.