A FAST NON-LTE CODE FOR EXPANDING ATMOSPHERES - A TEST OF THE VALIDITY OF THE SOBOLEV APPROXIMATION

We present a new non-LTE atmosphere code to calculate the continuum en ergy distribution and line profiles from stellar atmospheres with exte nded outflowing envelopes. The code is fast, which makes it suited for empirical modelling. The statistical equilibrium equations and the ra diative transfer in the continuum are solved with an efficient approxi mate lambda iteration method. The line transfer is treated using the S obolev approximation, including the effects of the diffuse radiation f ield, and the continuous opacity inside the line resonance zone. We in vestigate the validity of the Sobolev approximation by comparing our c ode with one that solves the line transfer using the co-moving frame m ethod. We find good agreement in the calculated emission lines for Wol f-Rayet stars, O-type stars and for Luminous Blue Variables (LBVs). Th e modelling of photospheric hydrogen lines of O-stars with our method is less reliable. For the LBVs, we conclude that the continuum spectru m and the relatively strong wind lines, computed using the Sobolev met hod, agree with those computed using the co-moving frame method. The m ass-loss rate of the latter stars, derived by fitting individual emiss ion lines, yields a maximum difference of 30% between the two codes.