[Paper Review] 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

M. F. Nieva, N. Przybilla|ArXiv.org|Nov 23, 2007

Stellar, planetary, and galactic studies62 references48 citations

TL;DR

This study presents a self-consistent non-LTE analysis of carbon lines in early B-type stars using high-S/N spectra of six solar-neighborhood stars, employing carefully calibrated atomic data and iterative atmospheric parameter determination. It achieves a highly homogeneous carbon abundance of log(C/H) + 12 = 8.32 ± 0.04, resolving long-standing discrepancies in previous analyses and indicating minimal carbon depletion from rotational mixing, with implications for galactic chemical evolution and stellar abundance modeling.

ABSTRACT

Precise determinations of the chemical composition in early B-type stars consitute fundamental observational constraints on stellar and galactochemical evolution. Carbon is one of the most abundant metals in the Universe but analyses in early-type stars show inconclusive results, like large discrepancies between analyses of different lines in C II, a failure to establish the C II/III ionization balance and the derivation of systematically lower abundances than from other objects. We present a comprehensive and robust C II/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 the derivation of chemical abundances. We establish the carbon ionization balance for all sample stars based on a unique set of input atomic data, achieving consistency for all modelled lines. Highly accurate atmospheric parameters and a homogeneous carbon abundance with reduced systematic errors are derived. 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 H II region. The homogeneous present-day carbon abundance also conforms with predictions of chemical-evolution models for the Galaxy. The present approach allows us to constrain the effects of systematic errors on fundamental parameters and abundances. (abridged)

Motivation & Objective

• To resolve long-standing inconsistencies in carbon abundance determinations from early B-type stars, particularly the discrepancy between strong and weak C ii lines.
• To reduce systematic uncertainties in stellar atmospheric parameters and atomic data selection, which have plagued previous non-LTE analyses.
• To establish a self-consistent, robust non-LTE line-formation model for C ii, C iii, and C iv using empirical calibration.
• To derive a precise, homogeneous carbon abundance in the solar vicinity that aligns with current galactic chemical evolution models.
• To assess the impact of rotational mixing on carbon abundance and validate the reliability of stellar abundance determinations using high-quality spectroscopy.

Proposed method

• Constructed a comprehensive non-LTE model atom for C ii, C iii, and C iv using carefully selected atomic data, including radiative and collisional transition rates.
• Performed an iterative spectral analysis to simultaneously determine atmospheric parameters (Teff, log g, microturbulent velocity) and select optimal atomic data for each star.
• Calibrated the model using high signal-to-noise, high-resolution spectra of six slowly rotating early B-type dwarfs and giants in the solar neighborhood.
• Used ionization equilibrium of carbon between C ii, C iii, and C iv as a key constraint to validate the consistency of the derived abundances.
• Applied a statistical and systematic error analysis to quantify uncertainties, distinguishing between random (1σ ≈ 0.05–0.10 dex) and systematic (1σ ≈ 0.10–0.15 dex) components.
• Validated results against independent determinations from H and He lines, near-IR spectra, and spectral energy distributions to ensure parameter consistency.

Experimental results

Research questions

• RQ1 Why do previous non-LTE analyses of carbon in early B-type stars yield inconsistent abundances from different C ii lines?
• RQ2 To what extent do systematic errors in atmospheric parameters and atomic data selection affect carbon abundance determinations?
• RQ3 What is the true, homogeneous carbon abundance in the solar vicinity when non-LTE effects and self-consistent parameter determination are properly accounted for?
• RQ4 How significant is carbon depletion due to rotational mixing in massive stars, and does it affect the derived abundance in the solar neighborhood?
• RQ5 How do the derived stellar carbon abundances compare with the solar value and gas-phase abundances in H ii regions like Orion?

Key findings

• A highly homogeneous carbon abundance of log(C/H) + 12 = 8.32 ± 0.04 was derived across six early B-type stars, with minimal scatter.
• The carbon ionization balance between C ii, C iii, and C iv is consistently established for all stars, confirming the reliability of the atomic data and model.
• Systematic uncertainties in atmospheric parameters and atomic data were identified as the primary source of previous inconsistencies in carbon abundance studies.
• Rotational mixing causes less than +0.05 dex depletion in carbon abundance per star, indicating minimal impact on the derived solar neighborhood value.
• The derived stellar carbon abundance is in excellent agreement with the revised solar carbon abundance and the gas-phase abundance in the Orion H ii region.
• The study demonstrates that accurate abundance determinations require not only high-quality data but also rigorous empirical calibration of atomic data and iterative parameter determination.