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Astron. Astrophys. 359, 780-787 (2000)

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The diffusion of radiation in moving media

I. Basic assumptions and formulae

R. Wehrse 1,3, B. Baschek 1,3 and W. von Waldenfels 2,3

1 Institut für Theoretische Astrophysik, Tiergartenstrasse 15, 69121 Heidelberg, Germany
2 Institut für Angewandte Mathematik, Im Neuenheimer Feld 294, 69120 Heidelberg, Germany
3 Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, Im Neuenheimer Feld 368, 69120 Heidelberg, Germany

Received 18 January 2000 / Accepted 27 April 2000


The 3D radiative transfer equation for differentially moving media is derived upon the assumption that the motions are sufficiently slow. Its solution is then applied to the limiting case of large optical depths, i.e. to the diffusion approximation. Although the effective extinction for static 1D media has been derived by Rosseland already in 1924, it is for the first time in this paper that for moving 3D media with many spectral lines general expressions for radiative quantities are derived in a rigorous way. Given are simple to use monochromatic as well as wavelength-integrated expressions for the flux and the radiative acceleration, and a generalized version of the Rosseland mean opacity. The essential effects of the motions upon the radiative flux are discussed for the simple case of a single spectral line on a continuum.

Key words: diffusion – radiative transfer – stars: interiors – stars: novae, cataclysmic variables – stars: supernovae: general

Send offprint requests to: R. Wehrse (wehrse@ita.uni-heidelberg.de)

© European Southern Observatory (ESO) 2000

Online publication: July 7, 2000