2008A&A...481..199N

Precise determinations of the chemical composition in early B-type stars constitute fundamental observational constraints on stellar and galactochemical evolution. Carbon, in particular, is one of the most abundant metals in the Universe but analyses in early-type stars are known to show inconclusive results. Large discrepancies between analyses of different lines in CII, a failure to establish the CII/III ionization balance, and the derivation of systematically lower abundances than from other indicators like HII regions and young FG-type stars all pose long-standing problems. We discuss improvements to the non-LTE modelling of the visual line spectrum and to the spectral analysis of early B-type stars, as well as their consequences for stellar parameter and abundance derivations. The most relevant sources of systematic uncertainies and their effects on the analysis are investigated. Consequences for the present-day carbon abundance in the solar vicinity are discussed. We present a comprehensive and robust CII/III/IV model for non-LTE line-formation calculations based on carefully selected atomic data. The model is calibrated with high-S/N spectra of six apparently slow-rotating early B-type dwarfs and giants, which cover a wide parameter range and are randomly distributed in the solar neighbourhood. A self-consistent quantitative spectrum analysis is performed using an extensive iteration scheme to determine stellar atmospheric parameters and to select the appropriate atomic data used for deriving chemical abundances. We establish the carbon ionization balance for all sample stars based on a unique set of input atomic data. Consistency is achieved for all modelled carbon lines of the sample stars. Highly accurate atmospheric parameters and a homogeneous carbon abundance of log(C/H)+12=8.32±0.04 are derived with reduced systematic errors. Present evolution models for massive stars indicate that this value may require only a small adjustment because of the effects of rotational mixing, by <+0.05dex per sample star. This results in a present-day stellar carbon abundance in the solar neighbourhood, which is in good agreement with recent determinations of the solar value and with the gas-phase abundance of the Orion HII region. Our finding of a homogeneous present-day carbon abundance also conforms to predictions of chemical-evolution models for the Galaxy. Moreover, the present approach allows us to constrain the effects of systematic errors on fundamental parameters and abundances. This suggests that most of the difficulties found in previous work may be related to large systematic effects in the atmospheric parameter determination and/or inaccuracies in the atomic data.