Chemical Abundances in the Ultra-faint Dwarf Galaxies Grus I and Triangulum II: Neutron-capture Elements as a Defining Feature of the Faintest Dwarfs
Chemical Abundances in the Ultra-faint Dwarf Galaxies Grus I and Triangulum II: Neutron-capture Elements as a Defining Feature of the Faintest Dwarfs
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DOI：
10.3847/1538-4357/aaf3bb
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发表时间：
2018-09
期刊：
影响因子：
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通讯作者：
A. Ji;J. Simon;A. Frebel;K. Venn;T. Hansen
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文献类型：
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作者：
A. Ji;J. Simon;A. Frebel;K. Venn;T. Hansen
我们展示了两个候选超微弱矮星系Grus I （Gru I）和Triangulum II （Tri II）中的四颗恒星的高分辨率光谱。这两个天体目前都没有明确确定的速度色散，将它们置于矮星系和球状星团（GCs）之间参数空间的模糊区域。两颗Gru I星的金属丰度没有明显的差异，但两颗星都缺乏中子俘获元素。我们验证了先前的结果，即三氧化二氮在金属丰度和[α/Fe]方面表现出显著的扩散。在我们的Tri II数据中没有检测到中子捕获元素，但我们在银河晕恒星的下包络层设置了上限，与之前非常低的探测一致。具有类似的低中子捕获元素丰度的恒星在ufd中很常见，但在其他环境中很少见。这种低中子捕获元素丰度的特征可以追溯到质量最小的恒星形成暗物质晕中的化学富集，并进一步表明，在金属贫乏的恒星中，中子捕获元素的主要来源是罕见的。相比之下，所有已知的GCs与晕星的中子捕获元素比例相似，这表明GCs不是在它们自己的暗物质晕的中心形成的。低中子捕获元素丰度可能是Gru I和Tri II是（或曾经是）星系而不是gc的最有力证据，我们预计未来对这些系统的观测将有力地发现非零速度色散或潮汐破坏的迹象。然而，这种低中子捕获元素的核合成起源仍然未知。
We present high-resolution spectroscopy of four stars in two candidate ultra-faint dwarf galaxies (UFDs), Grus I (Gru I) and Triangulum II (Tri II). Neither object currently has a clearly determined velocity dispersion, placing them in an ambiguous region of parameter space between dwarf galaxies and globular clusters (GCs). No significant metallicity difference is found for the two Gru I stars, but both stars are deficient in neutron-capture elements. We verify previous results that Tri II displays significant spreads in metallicity and [α/Fe]. Neutron-capture elements are not detected in our Tri II data, but we place upper limits at the lower envelope of Galactic halo stars, consistent with previous very low detections. Stars with similarly low neutron-capture element abundances are common in UFDs but rare in other environments. This signature of low neutron-capture element abundances traces chemical enrichment in the least massive star-forming dark matter halos and further shows that the dominant sources of neutron-capture elements in metal-poor stars are rare. In contrast, all known GCs have similar ratios of neutron-capture elements to those of halo stars, suggesting that GCs do not form at the centers of their own dark matter halos. The low neutron-capture element abundances may be the strongest evidence that Gru I and Tri II are (or once were) galaxies rather than GCs, and we expect future observations of these systems to robustly find nonzero velocity dispersions or signs of tidal disruption. However, the nucleosynthetic origin of this low neutron-capture element floor remains unknown.