A Computer Program for Solving Multilevel Non-Lte Radiative Transfer Problems in Spherical Geometry in Moving or Static Atmospheres
The interpretation of spectra of the optically thick atmospheres of late-type stars requires that the details of radiative transfer be taken into account. For single mainsequence stars, the solution of the radiative transfer problem can, in most cases, be made with the assumption of plane-parallel geometry. For evolved late-type stars, the assumption of spherical symmetry is more appropriate. We present modifications to an existing plane-parallel, non-LTE radiative transfer code - MULTI which include spherical symmetry in static and moving atmospheres. The code is suitable for application to stars where the wind velocities are up to several times the microturbulent velocity (i.e. where the Sobolev approximation is not valid) and where the flows are not necessarily monotonic. We compare the accuracy of the adopted scheme with previous independent techniques and find good agreement. A global approximate operator suitable for spherical geometry and complete redistribution is also presented.