STATISTICAL EQUILIBRIUM AND PHOTOSPHERIC ABUNDANCE OF NITROGEN IN THESUN AND IN VEGA

Non-LTE calculations for nitrogen in the Sun and in the prototype AO V star Vega are presented. The model atom comprises 96 levels and 82 li nes of neutral and singly ionized nitrogen. The influence of atomic da ta and atmospheric parameters on the resulting non-LTE abundance corre ctions is investigated in detail. In the Sun non-LTE effects are small ; abundance corrections (with respect to standard LTE calculations) ar e typically - 0.05 dex. The solar abundance of nitrogen derived by non -LTE analysis becomes log epsilon(N) = 8.05 +/- 0.09. In Vega, non-LTE effects are found to be large and to depend on the adopted abundance of carbon via the UV radiation field. Collision cross-sections, which are still poorly known, play an important role in this analysis. For a standard set of N I lines in the visible and near infrared, a non-LTE abundance correction of -0.32 dex is derived. The corresponding abund ance of nitrogen in Vega is log epsilon(N) = 7.76 +/- 0.14, implying a deficiency of -0.29 dex with respect to the Sun.