SEGUE 1: AN UNEVOLVED FOSSIL GALAXY FROM THE EARLY UNIVERSE
SEGUE 1: AN UNEVOLVED FOSSIL GALAXY FROM THE EARLY UNIVERSE
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DOI：
10.1088/0004-637x/786/1/74
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发表时间：
2014-03
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影响因子：
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通讯作者：
A. Frebel;J. Simon;E. Kirby
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作者：
A. Frebel;J. Simon;E. Kirby
我们给出了矮星系Segue 1中6颗红巨星的高分辨率光谱。包括Norris等人观察到的另外一颗Segue 1恒星，现在已经获得了Segue 1中每一颗红巨星的高分辨率光谱。值得注意的是，这7颗恒星中有3颗的金属度低于[Fe/H]=−3.5，这表明Segue 1是已知的化学演化程度最低的星系。我们证实了之前的中分辨率分析，表明1号段恒星的金属丰度范围超过2dex，范围从[Fe/H]=−1.4到[Fe/H]=−3.8。所有的SEGUE 1明星都是α增强的，具有[α/Fe]∼0.5。高α元素丰度对于缺乏金属的恒星来说是典型的，但在之前研究的每个星系中，金属丰富的恒星的[α/Fe]下降，这通常被解释为来自超新星Ia的铁的富集度。在Segue 1中没有这个特征表明它完全是由大质量恒星富集而成。SEGUE 1中的其他轻元素丰度比，包括三颗最缺乏金属的恒星中的碳增强，与缺乏金属的晕星非常相似。最后，考虑到其碳元素和S过程元素的大量过剩，我们将最富金属的恒星归类为CH星。其他六颗恒星的中子俘获元素丰度非常低，分别为[Sr/H]<；−4.9和[Ba/H]<；−4.2，这与在晕星中检测到的最低水平相当。这意味着最小的中子俘获浓缩，可能仅限于单个r过程或弱的S过程合成事件。总而言之，Segue 1的化学丰度表明它没有实质性的化学演化，这支持了它可能是一个仅经历了一次恒星形成爆发的幸存的第一个星系的想法。
We present Magellan/MIKE and Keck/HIRES high-resolution spectra of six red giant stars in the dwarf galaxy Segue 1. Including one additional Segue 1 star observed by Norris et al., high-resolution spectra have now been obtained for every red giant in Segue 1. Remarkably, three of these seven stars have metallicities below [Fe/H] = −3.5, suggesting that Segue 1 is the least chemically evolved galaxy known. We confirm previous medium-resolution analyses demonstrating that Segue 1 stars span a metallicity range of more than 2 dex, from [Fe/H] = −1.4 to [Fe/H] = −3.8. All of the Segue 1 stars are α-enhanced, with [α/Fe] ∼ 0.5. High α-element abundances are typical for metal-poor stars, but in every previously studied galaxy [α/Fe] declines for more metal-rich stars, which is typically interpreted as iron enrichment from supernova Ia. The absence of this signature in Segue 1 indicates that it was enriched exclusively by massive stars. Other light element abundance ratios in Segue 1, including carbon enhancement in the three most metal-poor stars, closely resemble those of metal-poor halo stars. Finally, we classify the most metal-rich star as a CH star given its large overabundances of carbon and s-process elements. The other six stars show remarkably low neutron-capture element abundances of [Sr/H] < −4.9 and [Ba/H] < −4.2, which are comparable to the lowest levels ever detected in halo stars. This suggests minimal neutron-capture enrichment, perhaps limited to a single r-process or weak s-process synthesizing event. Altogether, the chemical abundances of Segue 1 indicate no substantial chemical evolution, supporting the idea that it may be a surviving first galaxy that experienced only one burst of star formation.