THE TRANSFER OF RESONANCE-LINE RADIATION IN STATIC ASTROPHYSICAL MEDIA

An analytic solution is derived for the mean intensity of resonance-line radiation within an absorbing medium of large scattering optical depth but low density. The transfer of such radiation is described by a second-order, linear partial differential equation, in an extension of the method of Harrington. The absorption considered may arise either from a source of continuum opacity or from narrow absorption lines lying close to the resonance line. I derive the line profile, the mean number of scatterings before absorption or escape, the mean path length traveled, and the fraction of the photons which escape the medium. The results obtained agree well with available numerical results. Approximate methods are also developed which permit the treatment of media of intermediate optical depth, where scattering takes place in the Doppler core. Astrophysical applications are briefly discussed. They include the absorption of H Lyα radiation by interstellar dust, the pumping of H2 Lyman band lines by H Lyα, and the Bowen fluorescence.