Physics
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...289..505s&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 289, no. 2, p. 505-523
Physics
62
Momentum Transfer, Radiative Transfer, Resonance Lines, Resonance Scattering, Stellar Winds, Wolf-Rayet Stars, Acceleration (Physics), Diffusion, Flux (Rate), Monte Carlo Method, Opacity, Photosphere, Stellar Models
Scientific paper
The possibility of driving the dense winds of Wolf-Rayett 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 eta = M upsiloninfinity/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 grad 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 behavior of the photons which undergo many scatterings before 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 grad 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 peformance 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 nuinfinity.
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