HR-GO II: 低轨道能逆行动态标记组的化学丰度：揭示Thamnos为一个非常金属贫的子结构
HR-GO II: chemical abundances of low-$E$ retrograde dynamically-tagged-groups: Revealing Thamnos as a very metal-poor substructure
AI总结：
HR-GO II研究了低轨道能逆行动态标记组的化学丰度，揭示Thamnos为一个非常金属贫的子结构，证明其外源起源。
摘要：
银河系晕子结构在动力学空间中被识别出来后，会受到银河系本地恒星的污染，这使得它们的吞噬起源变得不确定。我们给出了属于两个动态标记组（Rg8和Rg9）的35颗恒星的详细化学丰度，以研究它们的吞噬性质。这两个组都由具有低轨道能和非常逆行轨道的恒星组成。我们发现Rg8和Rg9在所有元素（从C到Eu）上化学上无法区分，强烈表明它们属于同一结构。该低轨道能逆行组的铁丰度分布在[Fe/H]≈-2.1处有显著峰值，表明其主要人口非常金属贫，而在[Fe/H]≈-1.5处的次峰值很可能由于银河系本地恒星的污染。这些组在动力学空间中也与Thamnos子结构有显著重叠，因此我们利用它们来研究Thamnos的化学性质。主导的低金属licity人口为Thamnos的外源起源提供了有力证据，以及其非常金属贫的性质。我们没有在样本中发现任何α膝的证据，这与之前的研究所一致。与Cetus-Palca流在化学空间中的比较显示相似的丰度分布，因此这表明Thamnos的前身矮星系由于早期与银河系的合并而具有截断的恒星形成历史。
英文摘要：
Milky Way halo substructures identified in dynamical space are known to suffer from contamination from the Milky Way in-situ stars, which makes their accreted origins uncertain. We present detailed chemical abundances of 35 stars belonging to two sets of dynamically tagged groups, Rg8 and Rg9, to investigate their accreted nature. Both groups are composed of stars with low orbital energy and very retrograde orbits. We find that Rg8 and Rg9 are chemically indistinguishable across all elements, from C to Eu, strongly indicating that they belong to the same structure. The iron-abundance distribution of this low-$E$ retrograde group has a prominent peak at [Fe/H] $\approx-2.1$, revealing that its main population is very metal-poor, and a secondary peak at [Fe/H] $\approx-1.5$, very likely due to contamination from Milky Way in-situ stars. These groups also heavily overlap with the Thamnos substructure in dynamical space, and we thus use them to investigate the chemical properties of Thamnos. The dominant, low-metallicity population provides strong evidence for the ex-situ origin of Thamnos, as well as its very metal-poor nature. We do not see any evidence of an $α$ knee in our sample, which is consistent with previous studies. Comparison with the Cetus-Palca stream in the chemical space shows similar abundance distributions, and thus it suggests that the Thamnos progenitor dwarf galaxy had a truncated star formation history due to its early merger with the Milky Way.
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