Astronomy and Astrophysics, volume 379, 955-975 (2001/12-1)
Non-LTE line formation for N I/II: Abundances and stellar parameters. Model atom and first results on BA-type stars.
PRZYBILLA N. and BUTLER K.
Abstract (from CDS):
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 (A0V) and the supergiants eta Leo (A0Ib), HD 111613 (A2Iabe), HD 92207 (A0Iae) and beta Ori (B8Iae), using high S/N 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 ∼0.3-0.6dex, 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 0.25dex 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.
atomic data - line: formation - stars: abundances, early-type, fundamental parameters, supergiants