The calculation of line transfer in expanding media

An efficient method for calculating 1-D line transfer that allows bulk-Doppler motion is described. The method uses a ray-based integral formalism combined with complete linearization. The calculations of line transfer in planar, spherical, and cylindrical geometries are compared. If there are steep velocity gradients in the medium, simplification is possible and escape probabilities provide accurate results. Results using local escape probabilities in an expanding cylindrical medium are compared with those from a nonlocal line-transfer calculation. It is shown that the accuracy of the escape-probability treatment for a given transition can be affected by coupling with neighboring lines via bulk-Doppler motions.