The effects of coherence and redistribution on radiative transfer in large velocity gradients.

Radiative trasfer in an atmosphere with a velocity gradient is generally treated under the assumption of complete redistribution. Using Monte Carlo techniques the author explores the consequences of relaxing this condition within the Sobolev approximation that the velocity gradient is sufficiently large. Two cases of coherent scattering are considered, namely coherence in the atomic frame and coherence in the fluid frame. It is found by computer simulation that coherence in the atomic frame yields escape probabilities almost identical to complete redistribution, whereas coherence in the fluid frame doubles the rate at large optical depths. It is shown that the results are independent of the phase factor for scattering.