Astronomy & Astrophysics, 2001, 379, 955:
Non-LTE line formation for NI/II: abundances and stellar parameters
Model atom and first results on BA-type stars
N. Przybilla & K. Butler
Universitäts-Sternwarte München,
Scheinerstraße 1, D-81679 München, Germany
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
eta Leo (A0 Ib), HD111613 (A2 Iabe), HD92207 (A0 Iae) and beta Ori (B8 Iae),
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 ~0.3-0.6 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.
