IMPROVEMENTS ON RADIATIVE ACCELERATION CALCULATIONS IN STELLAR ENVELOPES

The various processes that need to be included in calculations of radiative acceleration in stellar envelopes are presented and their importance is evaluated. The general procedure used to compute radiative accelerations and the usual approximations are recalled, together with a summary of the useful equations. Major improvements are obtained by using the newly available atomic data provided by the OPACITY project (OF). They first allow to compute the frequency dependence of the background opacity and to improve the evaluation of the collisional line broadening. The contributions of bound-bound and bound-free transitions to the radiative acceleration are then calculated for each ion of a given element. A method of averaging among ions is developed to obtain the net acceleration on the element, in a simple model taking into account the effects of rapid reactions between ions. Examples of results are given for carbon and iron in a model-envelope with T-eff = 8000 K, log g = 4.2, Z = 0.02. They differ significantly from previous calculations. The effects of the different improvements are discussed.