The opacity of expanding media - The effect of spectral lines

Simple kinetic-theory arguments about the mean free path of a typical photon are employed to show that the presence of a velocity gradient in a medium greatly enhances the probability that a photon will be absorbed in a spectral line, thus decreasing the typical mean free path and increasing the effective opacity of the gas. For optically thick supernova shells, a monochromatic expansion opacity is defined which is inversely proportional to the photon mean free path and which depends on the velocity gradient through an expansion parameter. It is found that while strong lines are obviously important, a high density of weak lines can also contribute to the expansion opacity. An expression is derived for the monochromatic expansion opacity in terms of the expansion parameters as well as the strengths and frequencies of spectral lines, and this expression is integrated to obtain the Rosseland mean expansion opacity. A line list of 260,000 atomic transitions is used to evaluate the expansion opacity for a wide range of temperatures, densities, and velocity gradients, including those expected in a supernova shell.

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