Title:
CARBON ABUNDANCES OF THREE CARBON-ENHANCED METAL-POOR STARS FROM HIGH-RESOLUTION GEMINI-S/bHROS SPECTRA OF THE λ 8727 [C I] LINE

Author(s):
Steven J. Margheim, Simon C. Schuler, Thirupathi Sivarani, Martin Asplund, Verne V. Smith, Katia Cunha, and Timothy C. Beers

Abstract:

We present the results from an analysis of the λ 8727 forbidden [C I] line in high-resolution Gemini-S/bHROS spectra of three Carbon-Enhanced Metal-Poor (CEMP) stars. Previous derivations of C abundances in CEMP stars have primarily used the blue bands of CH and the C2 Swan system, features which are suspected to be sensitive to photospheric temperature inhomogeneities (the so-called three-dimensional effects). We find the [C/Fe] ratios based on the [C I] abundances of the two most Fe-rich stars in our sample to be in good agreement with previously determined values. For the most Fe-deficient star in our sample, however, the [C/Fe] ratio is found to be 0.34 dex lower than the published molecular-based value. We have carried out three-dimensional local thermodynamic equilibrium (LTE) calculations for [C I], and the resulting corrections are found to be modest for all three stars, suggesting that the discrepancy between the [C I] and molecular-based C abundances of HE 1005-1439 is due to more severe three-dimensional effects on the molecular lines. Carbon abundances are also derived from C I high-excitation lines and are found to be 0.45-0.64 dex higher than the [C I]-based abundances. Previously published non-LTE (NLTE) C I abundance corrections bring the [C I] and C I abundances into better agreement; however, targeted NLTE calculations for CEMP stars are clearly needed.