超微弱矮星系鹤群I和三角座星系II的化学丰度：中子俘获元素是最微弱矮星系的一个决定性特征
Chemical Abundances in the Ultra-Faint Dwarf Galaxies Grus I and Triangulum II: Neutron-Capture Elements as a Defining Feature of the Faintest Dwarfs
摘要：
本文介绍了两个候选超暗矮星系Grus I和Triangulum II中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之间的参数空间的模糊区域。没有显着的金属丰度差异被发现的两个格鲁I星，但两颗恒星都缺乏中子捕获元素。我们验证了以前的结果，三II显示显着的传播金属丰度和[ $α$ /Fe]。在我们的Tri II数据中没有检测到中子捕获元素，但我们将上限放在星系晕星的下包络线上，与以前非常低的检测结果一致。中子捕获元素丰度同样低的恒星在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，表明了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，贫金属恒星中中子捕获元素的主要来源是罕见的。相反，所有已知的球状星团都有类似的中子捕获元素比率，这表明球状星团是作为相对大质量星系的一部分形成的，而不是在它们自己的暗物质晕中形成的。低中子捕获元素丰度可能是最有力的证据，表明格鲁I和三II是（或曾经是）星系，而不是球状星团，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，这种低中子俘获元素层的核合成起源仍然未知。
英文摘要：
We present high-resolution of 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. 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 globular clusters have similar ratios of neutron-capture elements to those of halo stars, suggesting that globular clusters form as part of relatively massive galaxies rather than in 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 globular clusters, and we expect future observations of these systems to robustly find non-zero velocity dispersions or signs of tidal disruption. However, the nucleosynthetic origin of this low neutron-capture element floor remains unknown.
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