SIMPLIFIED TREATMENT OF THE RADIATIVE-TRANSFER PROBLEM IN EXPANDING ENVELOPES

We study the application of a simplified method to solve the dynamic r adiative transfer problem in expanding envelopes. The method, which re quires a computational effort similar to that of the diffusion approxi mation, is based on the use of a generalization of the Eddington closu re relationship that allows the inclusion of scattering and relativist ic corrections up to O(v/c). We apply this method to the calculation o f light curves of Type Ia supernovae, showing that it yields much more accurate results than the diffusion approximation and that the latter is seriously in error when applied to determine emergent flux and its spectral distribution.