Precise determinations of the chemical composition in OB-type stars constitute fundamental observational constraints to stellar and galactochemical evolution. Among the light elements, carbon is one of the most abundant metals in the Universe but analyses in early-type stars showed inconclusive results in the past decades. In the solar vicinity, carbon abundances derived from mostly unevolved OB stars indicate a large scatter - by more than one order of magnitude - and the mean value is systematically lower than those derived from FG-type stars (including the Sun) and H II regions. Both results cannot be explained in terms of stellar evolution and chemical evolution of the Galaxy. Spectral analyses of this kind of star also give largely discrepant abundances from different C II lines and fail to establish the C II/III ionization balance.
On the other hand, hydrogen and helium line spectra are crucial diagnostic features for the quantitative analysis of OB stars as they are primary indicators for deriving the fundamental atmospheric parameters, i.e. the effective temperature and the surface gravity. Their careful analysis provides the basis for any further study of metal abundances. Hybrid non-LTE line-formation calculations for these elements have not been discussed thoroughly so far, despite a wide use of this approach for analyses of metal line spectra.
In a first step, synthetic spectra of hydrogen and helium are computed on the basis of a hybrid non-LTE approach in order to test the ability of these models to reproduce high-resolution and high-S/N spectra of dwarf and giant OB stars. State-of-the-art model atoms and line-broadening theories are employed to model the H and H I/II spectra. The present hybrid non-LTE synthetic spectra match simultaneously almost all measurable hydrogen and helium lines observed in six test stars over a wide spectral range from the Balmer limit to the near-infrared.
A comparison of atmospheric structures and synthetic spectra of H and He with published line-blanketed non-LTE models validates the suitability of the LTE approximation for modelling the atmospheric structure of dwarf and giant OB stars at metallicities down to (at least) 1/5 times solar. The present approach avoids inconsistencies in the modelling of the He I singlets found in other published non-LTE calculations. It improves on pure LTE models - widely applied for OB star analyses - in many aspects: non-LTE strengthening and the use of detailed line-broadening data result in significant differences in the line profiles and equivalent widths of the Balmer and helium lines. Systematic effects on the stellar parameter determination are quantified. A reliable starting point for studies of the metal spectra is established.
In a second step, a comprehensive and robust C II-IV model for non-LTE line-formation calculations is presented. The model is based on atomic data carefully selected in an empirical calibration. A self-consistent quantitative spectrum analysis is performed using an extensive iteration scheme to determine stellar atmospheric parameters with improved accuracy and to select the appropriate atomic data to be used for the derivation of abundances. The carbon ionization balance is successfully established with a unique set of input atomic data for all sample stars, which cover a wide parameter range. Consistency is achieved for a large number of carbon lines - in total 40. This includes in particular the strongest features that are of highest importance for extragalactic applications. The long-standing problem of inconsistencies in the determination of carbon abundances from different lines and ionization stages is solved.
The self-consistent analysis provides atmospheric parameters and carbon abundances with unprecedented accuracy, with uncertainties as low as ~1% in effective temperature, ~10% in surface gravity and ~20% in carbon abundance, with reduced systematic error. This improves significantly on results from previous studies, which typically give uncertainties of 5-10%, ~25% and a factor ~2-3, respectively.
Moreover, an extremely homogeneous abundance of log(C/H) + 12 = 8.32 ± 0.04 is derived from the star sample. This result constrains the present-day stellar carbon abundance in the solar neighbourhood to log(C/H) + 12 ~ 8.35 ± 0.05, after small adjustments by <+0.05 dex per star for evolutionary effects. This is in agreement with the recently revised solar value and with the gas-phase abundance of the Orion H II region.
The approach presented here allows the effects of systematic errors on fundamental parameters and abundances to be constrained. This suggests that most of the difficulties found in previous work may be related to large systematic effects in the analysis caused by inaccuracies in the atomic data and/or the atmospheric parameter determination.
The finding of a homogeneous present-day carbon abundance in the solar vicinity conforms with analyses of the interstellar medium and also with predictions of chemical-evolution models for the Galaxy. The high accuracy achieved here is a prerequisite for the determination of the Galactic abundance gradient, which is of the order of the present uncertainties (in contrast to an overall abundance scatter of one order of magnitude found in previous studies).
Stellar and galactochemical evolution models can from now on be constrained more tightly with reliable carbon abundances. This can be done for environments of different metallicities (i.e. galaxies), with the only remaining limitation being the quality of the observed stellar spectra.
2 Model Atmospheres
2.1 Radiative Transfer
2.2 Classical Stellar Atmospheres
2.3 Thermodynamic State: LTE vs. non-LTE
2.4 Metal Line Blanketing
2.5 Spectral Line Formation
2.5.1 Line Strength & Broadenning Mechanisms
2.5.2 Non-LTE Line Formation
3 Atomic Data for Spectral Modelling
3.1 Basic Concepts
3.2 Atomic Structure Calculations
3.3 Scattering Calculations
3.4 Construction of Model Atoms
4 Spectroscopic Analysis
4.1 Atmospheric Parameters
4.2 Chemical Abundances
5 Hybrid non-LTE Approach for H and He Line Formation
5.1 Model Calculations
5.2 Observational Data
5.3 Applications to Observations
5.4 Comparison to Other Model Predictions
5.4.1 Atmospheric Structures, SEDS: LTE vs. non-LTE
5.4.2 Spectra: Hybrid non-LTE vs. non-LTE and LTE
5.4.3 Line Formation: Hybrid non-LTE vs. Full non-LTE
6. Non-LTE Line Formation for Carbon: Self-Consistent Analysis
6.1 The C II/III/IV Model Atom
6.2 Model Atom Calibration
6.2.1 Extensive Iteration on Fundamental Variables
6.2.2 Sensitivity of C Lines to Atomic Data
6.2.3 Line-Formation Details
6.2.4 Sensitivity of ε(C) to Parameter Variations
6.4 Comparison with Previous Work
6.4.1 Predictions from Different non-LTE Model Atoms
6.4.2 Effective Temperatures
6.5 The Stellar Present-Day C Abundance in the Solar Neighbourhood
A Basis of Echelle Data Reduction
B Atomic Data for H and He
C Linefits to H and He
D Linefits to C Lines