CARBON ABUNDANCES OF THREE CARBON-ENHANCED METAL-POOR STARS FROM HIGH-RESOLUTION GEMINI-S/bHROS SPECTRA OF THE λ8727 [C I] LINE

Abstract

We present the results from an analysis of the λ8727 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 (HE 0507-1653: [Fe/H] = –1.42 and HE 0054-2542: [Fe/H] = –2.66) to be in good agreement with previously determined values. For the most Fe-deficient star in our sample (HE 1005-1439: [Fe/H] = –3.08), 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. We have also derived the abundances of nitrogen, potassium, and iron for each star. The Fe abundances agree well with previously derived values, and the K abundances are similar to those of C-normal metal-poor stars. Nitrogen abundances have been derived from resolved lines of the CN red system assuming the C abundances derived from the [C I] feature. The abundances are found to be approximately 0.44 dex larger than literature values, which have been derived from CN blue bands near 3880 and 4215 A. We discuss evidence that suggests that analyses of the CN blue system bands underestimate the N abundances of metal-poor giants.