Transfer of resonance line radiation in the comoving frame of an expanding cylinder

Abstract

The two-level line transfer problem is solved numerically in the comoving frame of a radially expanding infinite cylinder by the variable Eddington factor method. The specific intensity at each frequency in the line depends upon polar radius and upon both a polar and azimuthal angle coordinate. This necessitates the generalization of the Eddington factor technique to radiation fields with full angular dependence. The method admits partial frequency redistribution and is applicable to both astrophysical modeling and laboratory spectroscopy. Leung's moment equations for the stationary case are recovered in the zero velocity limit. Source functions, emergent profiles, and angular variation of the emergent intensity are presented for several examples of stationary and expanding atmospheres. Finally, the numerical method is briefly discussed with respect to its potential for generalization to the time-coupled transfer equation, for the analysis of time-resolved spectroscopic data, and for multilevel line transfer problems.