The opacity of an expanding medium

Details The opacity of an expanding medium The opacity of an expanding medium

Jan 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996astl...22...79b&link_type=abstract

Astronomy Letters, Volume 22, Issue 1, January 1996, pp.79-84; Pis'ma v Astronomicheskii Zhurnal, Vol. 22, p. 92

Astronomy and Astrophysics

Astronomy

13

Radiative Transfer, Expansion Opacity

Scientific paper

Based on heuristic arguments, Karp et al. (1977) derived their well-known expression for the line opacity that allows for the effect of expansion. We use the general solution and Hilbert-type asymptotic expansions of the Boltzmann equation for photons to derive an expression for this "expansion opacity" rigorously. The difference between our result and the published results on this subject is discussed, as is the applicability of the expression for the expansion opacity to calculations of radiative transfer in a comoving frame in the moment approximation. For the zeroth moment (i.e., the energy equation), there is no need to take the expansion effect into account, and the absorption coefficient can be averaged just as in the case of a static medium. We also show how to properly average the expansion opacity in the flux equation.

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