AEGIS巡天的星系考古学：星系晕中碳和铁的演化
Galactic Archeology with the AEGIS Survey: The Evolution of Carbon and Iron in the Galactic Halo
摘要：
了解银河系晕中碳和铁的演化非常重要，因为这两种元素在早期宇宙中约束恒星形成、银河系组装和化学演化方面发挥着至关重要的作用。在这里，我们探索碳的空间分布，[C/Fe]和金属，[Fe/H]，晕系统的基础上中分辨率（$R \sim$ 1，300）光谱的$\sim$ 58，000恒星在南半球的AAOmega银河系结构演化（AEGIS）调查。AEGIS的碳丰度图显示出正的梯度与距离，同样发现的斯隆数字巡天（SDSS）的碳丰度图的李等人。金属丰度图证实，[Fe/H]随着距离的减少，从内晕到外晕。我们还探讨了光环的形成和化学演化的历史，考虑人口的碳增强贫金属（CEMP）恒星中存在的AEGIS样本。CEMP-no恒星（根据其特征性的较低的绝对碳丰度水平进行分类，次巨星和巨星的$A$（C）$\leq$ 7.1）的累积和微分频率随着金属丰度的降低而增加，并且对于CEMP恒星整体来说是\textit{substantially higher than previous determinations}。相反，$s$型恒星（具有较高的$A$（C））几乎保持平坦，$\sim$值为10%，在$-\,4.0 \lesssim$ [Fe/H] $\lesssim-$ 2.0范围内. CEMP-no和CEMP- $s$星的明显不同的行为缓解了假设双星质量转移起源的人口合成模型的紧张局势，以前很难解释CEMP星的更高频率，作为一个整体，在低金属丰度。
英文摘要：
Understanding the evolution of carbon and iron in the Milky Way's halo is of importance because these two elements play crucial roles constraining star formation, Galactic assembly, and chemical evolution in the early Universe. Here, we explore the spatial distributions of carbonicity, [C/Fe], and metallicity, [Fe/H], of the halo system based on medium-resolution ($R \sim$ 1,300) spectroscopy of $\sim$58,000 stars in the Southern Hemisphere from the AAOmega Evolution of Galactic Structure (AEGIS) survey. The AEGIS carbonicity map exhibits a positive gradient with distance, as similarly found for the Sloan Digital Sky Survey (SDSS) carbonicity map of Lee et al. The metallicity map confirms that [Fe/H] decreases with distance, from the inner halo to the outer halo. We also explore the formation and chemical-evolution history of the halo by considering the populations of carbon-enhanced metal-poor (CEMP) stars present in the AEGIS sample. The cumulative and differential frequencies of CEMP-no stars (as classified by their characteristically lower levels of absolute carbon abundance, $A$(C) $\leq$ 7.1 for sub-giants and giants) increases with decreasing metallicity, and is \textit{substantially higher than previous determinations} for CEMP stars as a whole. In contrast, that of CEMP-$s$ stars (with higher $A$(C)), remains almost flat, at a value $\sim$10\%, in the range $-\,4.0 \lesssim$ [Fe/H] $\lesssim-$2.0. The distinctly different behaviors of the CEMP-no and CEMP-$s$ stars relieve the tension with population-synthesis models assuming a binary mass-transfer origin, which previously struggled to account for the higher reported frequencies of CEMP stars, taken as a whole, at low metallicity.
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