基于8727 A [C I]线高分辨Gemini-S/bHROS谱的三颗贫金属星的碳丰度
Carbon Abundances of Three Carbon-Enhanced Metal-Poor Stars from High-Resolution Gemini-S/bHROS Spectra of the 8727A [C I] Line
摘要：
本文给出了对三颗CEMP星的高分辨率Gemini-S/bHROS光谱中8727 ang禁戒[C I]线的分析结果。我们发现，基于我们样本中两颗最富Fe的恒星的[C I]丰度的[C/Fe]比（HIP 0507-1653：[Fe/H] = -1.42和HIP 0054-2542：[Fe/H] = -2.66）与之前确定的CH和C_2线值非常一致。然而，对于我们样品中最缺铁的恒星（HIP 1005-1439：[Fe/H] = -3.08），发现[C/Fe]比比公布的分子基值低0.34 dex。我们对[C I]进行了3D局部热力学平衡（LTE）计算，结果发现所有三颗恒星的修正都是适度的，这表明HIP 1005-1439的[C I]和基于分子的C丰度之间的差异是由于分子线上更严重的3D效应。碳丰度也来自C I高激发线，并被发现是0.45-0.64德克斯高于[C I]为基础的丰度。先前发表的非LTE C I丰度修正使[C I]和C I丰度更好地一致;然而，显然需要针对CEMP星的NLTE计算。我们还推导出了每颗恒星的N、K和Fe的丰度。的Fe丰度同意与以前导出的值，和K丰度是类似的C-正常的贫金属星。氮丰度是从CN红系统的分辨谱线得出的。这些丰度比文献值约大0.44 dex，文献值是由CN蓝带在3880和4215角附近导出的。我们讨论的证据表明，CN蓝系统频带的分析低估了N丰度的贫金属巨星。
英文摘要：
We present the results from an analysis of the 8727ang forbidden [C I] line in high-resolution Gemini-S/bHROS spectra of three CEMP stars. We find the [C/Fe] ratios based on the [C I] abundances of the two most Fe-rich stars in our sample (HIP 0507-1653: [Fe/H] = -1.42 and HIP 0054-2542: [Fe/H] = -2.66) to be in good agreement with previously determined CH and C_2 line-based values. For the most Fe-deficient star in our sample (HIP 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 3D 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 HIP 1005-1439 is due to more severe 3D 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 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 N, K, and Fe 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. 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 ang. We discuss evidence that suggests that analyses of the CN blue system bands underestimate the N abundances of metal-poor giants.
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