J0815+4729中的碳：对最原始恒星中CH吸收的解释投下质疑的阴影
Carbon in J0815+4729: Casting A Shadow of Doubt on the Interpretation of CH Absorption in the Most Primitive Stars
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中文总结 AI 辅助
本研究针对极贫铁恒星J0815+4729，通过NLTE条件下的中性原子跃迁推导其碳氧丰度，发现CH线的碳丰度结果与C I、C₂线存在显著差异，对CH线作为碳丰度常用指示器的可靠性提出了质疑。
中文摘要 AI 辅助
我们重新研究了贫铁原始恒星J0815+4729（[Fe/H] = -5.49）中的碳丰度。J0815+4729的碳丰度极高且氧含量丰富，除了CH和C₂分子线外，其光谱中还可观测到较弱的中性原子跃迁，这为探索这类极贫铁恒星中C I和O I跃迁的线形成提供了独特机会。基于高分辨率光谱以及新构建的碳和氧原子模型，我们首次在非局部热力学平衡（NLTE）条件下，通过中性原子跃迁推导了这颗超贫铁恒星的碳和氧丰度。由C I线推导得到的碳丰度为A(C)NLTE = 8.08 ± 0.07，与从C₂线推导的结果（A(C)LTE = 7.96 ± 0.08）一致，但比从CH线得到的值（A(C)LTE = 7.47 ± 0.17）高约0.6 dex。由于CH线是极贫金属恒星最常用的碳丰度指示器，该结果对最原始恒星中CH吸收的定量解释投下了质疑的阴影。我们确认J0815+4729是目前已知[Fe/H] ≤ -4.5的碳增强程度最高的恒星之一。我们还确定A(O)NLTE为7.25 ± 0.11，NLTE校正约为-0.11 dex，该恒星的碳氧比为[C/O] = 1.06 ± 0.11。
英文摘要
We revisit the carbon abundance in the primitive iron-poor star J0815+4729 ([Fe/H] $=$ $-$5.49). J0815+4729 is so carbon-enhanced and oxygen-rich that, in addition to CH and C$_{2}$ molecular lines, weaker neutral atomic transitions are measurable in its spectrum, providing a unique opportunity to explore the line formation of C I and O I transitions in such an extremely iron-deficient star. Based on high-resolution spectroscopy and newly constructed atomic models for carbon and oxygen, we present the first investigation to derive carbon and oxygen abundances of this hyper iron-poor star from neutral atomic transitions under non-local thermodynamic equilibrium (NLTE) conditions. The carbon abundance derived from C I lines is A(C)$_{\rm NLTE} = 8.08 \pm 0.07$, consistent with that inferred from C$_{2}$ lines (A(C)$_{\rm LTE} = 7.96 \pm 0.08$), but around 0.6 dex higher than the value from CH lines (A(C)$_{\rm LTE} = 7.47 \pm 0.17$). Since CH lines are the most commonly-used carbon abundance indicator for extremely metal-poor stars, this result casts doubt on the quantitative interpretation of CH absorption in the most primitive stars. We confirm that J0815+4729 is one of the most carbon-enhanced stars with [Fe/H] $\le$ $-$4.5 known to date. We have also determined A(O)$_{\rm NLTE}$ as 7.25 $\pm$ 0.11, with NLTE corrections of around $-$0.11\,dex. The carbon-to-oxygen ratio of the star is [C/O] $=$ 1.06 $\pm$ 0.11.
