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 energy distribution and line profiles from stellar atmospheres with extended outfiowing envelopes. The code is fast, which makes it suited for empirical modelling. The statistical equilibrium equations and the radiative transfer in the continuum are solved with an efficient approximate lambda iteration method. The line transfer is treated using the Sobolev approximation, including the effects of the diffuse radiation field, and the continuous opacity inside the line resonance zone.

We investigate the validity of the Sobolev approximation by comparing our code with one that solves the line transfer using the co-moving frame method. We find good agreement in the calculated emission lines for Wolf-Rayet stars, O-type stars and for Luminous Blue Variables (LBVs). The modelling of photospheric hydrogen lines of O-stars with our method is less reliable. For the LBVs, we conclude that the continuum spectrum and the relatively strong wind tines, computed using the Sobolev method, agree with those computed using the comoving frame method. The mass-loss rate of the latter stars, derived by fitting individual emission lines, yields a maximum difference of 30% between the two codes.