Non-LTE line formation for Ni/ii: Abundances and stellar parameters

A comprehensive model atom for non-LTE line formation calculations for neutral and singly-ionized nitrogen is presented. Highly accurate radiative and collisional atomic data are incorporated, recently determined for astrophysical and fusion research using the R-matrix method in the close-coupling approximation. As a test and first application of the model, nitrogen abundances are determined on the basis of line-blanketed LTE model atmospheres for five stars, the main sequence object Vega (A0 V) and the supergiants η Leo (A0 Ib), HD 111613 (A2 Iabe), HD 92207 (A0 Iae) and β Ori (B8 Iae), using high and high-resolution spectra at visual and near-IR wavelengths. The computed non-LTE line profiles fit the observations excellently for a given nitrogen abundance in each object. Moreover, the ionization equilibrium of Ni/ii proves to be a sensitive temperature indicator for late B-type and early A-type supergiants -even at low metallicities -due to the apparent nitrogen overabundance in these objects. All supergiants within our sample show an enrichment of nitrogen on the order of ~0.3-0.6 dex, indicating the mixing of CN-cycled material into atmospheric layers, with the sum of the CNO abundances staying close to solar. This finding is in accordance with recent stellar evolution models accounting for mass-loss and rotation. For Vega, an underabundance of nitrogen by 0.25 dex is found, in good agreement with the similar underabundance of other light elements. The dependence of the non-LTE effects on the atmospheric parameters is discussed with special emphasis on the supergiants where a strong radiation field at low particle densities favours deviations from LTE. Non-LTE effects systematically strengthen the Ni/ii lines. For some Ni lines in supergiants non-LTE abundance corrections in excess of 1 dex are found and they react sensitively to modifications of the collisional excitation data. The influence of microturbulence on the statistical-equilibrium calculations is also investigated: the line-strengths of the strong Ni features show some sensitivity due to modifications of the line-formation depths and the departure coefficients, while the -in this parameter range -weak Nii lines remain unaffected.