Issue |
A&A
Volume 481, Number 1, April I 2008
Science with Hinode
|
|
---|---|---|
Page(s) | 199 - 216 | |
Section | Stellar atmospheres | |
DOI | https://doi.org/10.1051/0004-6361:20078203 | |
Published online | 17 January 2008 |
Carbon abundances of early B-type stars in the solar vicinity *,**
Non-LTE line-formation for C II/III/IV and self-consistent atmospheric parameters
1
Dr. Remeis Sternwarte Bamberg. Sternwartstr. 7, 96049 Bamberg, Germany e-mail: nieva@sternwarte.uni-erlangen.de
2
Observatório Nacional, Rua General José Cristino 77 CEP 20921-400, Rio de Janeiro, Brazil
Received:
2
July
2007
Accepted:
18
November
2007
Context.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 , a failure to establish the
ionization balance, and
the derivation of systematically lower abundances than from other indicators
like
regions and young FG-type stars all pose long-standing
problems.
Aims.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.
Methods.We present a comprehensive and robust 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.
Results.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
(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 <+0.05 dex 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
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.
Key words: line: formation / stars: early type / stars: fundamental parameters / stars: abundances / Galaxy: abundances / solar neighbourhood
© ESO, 2008
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