Astronomy and Astrophysics, volume 379, 936-954 (2001/12-1)
Non-LTE line-formation for neutral and singly-ionized carbon. Model atom and first results on BA-type stars.
PRZYBILLA N., BUTLER K. and KUDRITZKI R.P.
Abstract (from CDS):
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 (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 well for a single carbon abundance in each object. For two supergiants, η Leo and HD 111613, 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 ∼0.3dex (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 0.2-0.5dex, 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 0.3dex 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 infrared, 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 λλ6578-82 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.