The Escape of Resonance-Line Radiation from an Optically Thick Nebula.

The number of scatterings an average Lyman-a photon undergoes before it either escapes from a gaseous nebula with large optical depth in the center of this line or else is absorbed by conversion to the two-photon continuum is estimated. The treatments according to the hypothesis of complete redistribution of frequency after scattering and according to the exact redistribution function are shown by numerical examples to give equivalent results in the Doppler core, but it is necessary that the calculations extend to frequencies far enough from the center of the line that, at the greatest shift considered, the nebula is optically thin The treatment is extended by an approximate diffusion theory (in frequency space) to the case in which the nebula is so thick that it becomes transparent only in the damping wings. For an isolated nebula the probability that a photon is absorbed by conversion to the two-photon continuum before it escapes is small for optical depths of about ro < 1060, but in a real nebula the reflection by any surrounding H i region may increase the absorption within the nebula.