Opacity of an expanding medium

Shortly after the breakout of the initial shock wave generated by the collapsing core of a supernova, the expansion of the envelope becomes nearly isotropic. Hydrodynamc models of this phase can be quite simple. One complication is that standard opacity tables do not properly account for the way the radiation interacts with the expanding gas. Since a photon is continually Doppler sifted relative to the gas it is passing through, the probability that it interacts with a spectral line is greatly enhanced. It has been shown that this enhancement can be as large as an order of magnitude in some circumstances. This paper is divided into four parts. After the expansion opacity for a homogeneous, isotropically expanding medium is reviewed the assumptions are relaxed. First, the expansion opacity in a homogeneous, non-isotropically expanding gas is considered. Next, the assumption of homogeneity is dropped, but attention is restricted to the case of an optically thick gas. Finally, the general problem in which the photons are allowed to have an aribtrarily large mean free path is considered