The opacity of expanding media - The effect of spectral lines

Abstract

Spectral lines are more effective in slowing the transport of radiation in expanding (or contracting) objects than in static ones. The velocity gradient associated with the expansion causes the frequency of the photons to be continuously redshifted relative to the rest frame of the gas through which they travel. Those photons which are redshifted to the frequency of a sufficiently strong line will be absorbed by the corresponding bound-bound transition, and the net effect will be to increase the effective opacity of the gas. In certain cases the effect can be taken into account by using an effective opacity, the expansion opacity, which is a function not only of the temperature and density but also of the velocity gradient.Practical formulae for computing the expansion opacity and its Rosseland mean in terms of sums over spectral lines are derived. It is shown that the cumulative effect of many weak lines can be important, implying that a large list of spectral lines is required to obtain results of even modest accuracy. Numerical computations using the 260,000-entry line list of Kurucz and Peytremann have been completed and some samples of the result are given. The general effect may be important in many astronomicalmore » objects, but only in some of these will be detailed approach of this paper be appropriate. In optically thick supernova shells, the effect is important both in maintaining the radiation in thermal equilibrium as it diffuses out of the shell and in increasing the value of the total opacity. The enhancement of the opacity ranges from less than 1% to more than an order of magnitude, depending on the temperature, density, and velocity gradient.« less