Abstract:
The partial frequency redistribution function for zero natural line width and dipole scattering is used to obtain a simultaneous solution of the statistical equilibrium and line transfer equations in the comoving frame of an expanding gas. A non-LTE, two-level atom in an expanding spherical medium is considered, and the population ratio of the two levels, N2/N1, is calculated. The upper level population (N2), initially set equal to zero, is found to be enhanced significantly from its initial value. Variation in the velocity gradient had little effect on the N2/N1 ratio when the geometrical thickness of the medium was three or ten times the stellar radius; when the thickness was increased to twenty times the central radius, however, the velocity gradients changed this ratio considerably in the region where log T is less than or equal to two, T being the total optical depth.
