The FN method for spectral-line formation by completely noncoherent scattering

The F-N method is used to develop a solution to a class of nongrey problems in the theory of radiative transfer. The model considered allows for scattering with complete frequency redistribution (completely noncoherent scattering) and continuum absorption. In addition to a general formulation, a specific solution is developed for an inhomogeneous source term (Planck function) that varies linearly with optical depth in a semi-infinite medium. Test problems based on Doppler and Lorentz profiles of the line-scattering coefficient are considered, and numerical results (thought to be correct to five significant figures) are given for the frequency-dependent intensity exiting the medium and for the source function within the medium. For comparison purposes, a previously reported solution that is expressed in terms of Chandrasekhar's H function is evaluated numerically. (C) 1998 Elsevier Science Ltd. All rights reserved.