Astronomy & Astrophysics, 2001, 379, 936:
Non-LTE line formation for neutral and singly-ionized carbon
Model atom and first results on BA-type stars
N. Przybilla (1,2), K. Butler (1), R.P. Kudritzki (3)
(1) Universitäts-Sternwarte München,
Scheinerstraße 1, D-81679 München, Germany
(2) Max-Planck-Institut für Astrophysik,
Karl-Schwarzschild-Straße 1, D-85740 Garching bei München, Germany
(3) Institute of Astronomy, University of Hawaii, 2680 Woodlawn
Drive, Honolulu, HI 96822, USA
A comprehensive model atom for non-LTE line-formation calculations for
neutral and singly-ionized carbon 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, carbon
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.
The computed non-LTE line profiles fit the observations well for a single carbon
abundance in each object. For two supergiants, eta Leo and HD111613,
lines of both species are simultaneously present in the spectra, giving
consistent CI and CII abundances
(within the error bars).
However, the uncertainties of the abundances are large, on the order of
~0.3 dex (statistical+systematical error), thus the ionization
equilibrium of CI/II is of restricted use for the
determination of stellar parameters.
All supergiants within our sample show a depletion of carbon on the order
of 0.2-0.5 dex, indicating the mixing of CN-cycled material into the
atmospheric layers, with the sum of the CNO abundances remaining close to solar.
This finding is in accordance with recent stellar evolution models
accounting for mass loss and rotation. For Vega, an underabundance of carbon by
0.3 dex is found, in excellent agreement with the similar underabundance of
other light elements.
The dependence of the non-LTE effects on the atmospheric parameters is discussed
and the influence of systematic errors is estimated. Special emphasis is
given to the supergiants where a strong radiation field at low particle
densities favours deviations from LTE .
Non-LTE effects systematically strengthen the CI/II
lines. For the CI lines in the near-IR, a strong
sensitivity to modifications in the photoionization and collisional
excitation data is found. An increasing discrepancy between our model
predictions and the observations for the CII doublet
at 6578-82 A is perceived with rising luminosity, while the other
CII doublet and quartet lines remain consistent.
Furthermore, the influence of
microturbulence on the statistical-equilibrium calculations is
investigated.
