A Walk on the Retrograde Side (WRS) 项目. II. 解开 Thamnos 中原位与吸积成分的化学分析
A Walk on the Retrograde Side (WRS) project. II. Chemistry to disentangle in situ and accreted components in Thamnos
AI总结：
本研究基于 WRS 项目框架，利用 VLT/UVES 高分辨率光谱分析了银河系 Thamnos 亚结构中 212 颗红巨星的 16 种化学丰度，发现其化学特征更符合原位银河系晕而非吸积矮星系，证实了该结构存在高度原位人群混入。
摘要：
我们展示了针对 Thamnos 亚结构首次系统性、专门的高分辨率化学分析结果，该亚结构是银河系等级合并过程的候选遗迹。该分析是在与 Gaia-Sausage-Enceladus (GSE) 遗迹的对比中进行的，处于 'A Walk on the Retrograde Side' (WRS) 项目建立的完全自洽且同质的框架内。基于运动积分空间中的选择，我们分析了使用 VLT 上的 UVES 获取的 212 颗红巨星支恒星的高分辨率和高信噪比光谱，这些恒星被归类为 Thamnos 和 GSE 的候选成员。我们导出了 16 种原子物种的精确丰度。与 GSE 相比，被归属于 Thamnos 亚结构的恒星平均更贫金属，但其中大多数在多种元素中显示出更高的 [X/Fe] 丰度比，如 Na、Mg、Al、Ca、Cu、Zn，以及更低的 [Eu/Fe]。大多数 Thamnos 候选恒星显示出的化学特征更符合原位银河系晕，而不是典型的低质量吸积矮星系。我们的发现进一步得到了与定制星系化学演化模型对比的支持，这些模型在重现观测到的 α 元素增强方面有所不足，但能够拟合 Thamnos 亚结构中存在的更贫金属成分。这些结果证实了 Thamnos 亚结构中原位人群存在高度混入，以及较小程度上来自 GSE 的混入，同时仍为一个来自被瓦解小矮星系的真实吸积人群留出了空间。
英文摘要：
We present the results of the first systematic and dedicated high-resolution chemical analysis of the Thamnos substructure, a candidate relic of the process of hierarchical merger of the Milky Way. The analysis was perfomed in comparison with the Gaia-Sausage-Enceladus (GSE) remnant, within the fully self-consistent and homogeneous framework established by the 'A Walk on the Retrograde Side' (WRS) project. We analysed high-resolution and high signal-to-noise ratio spectra obtained with UVES at VLT for 212 red giant branch stars classified as candidate members of Thamnos and GSE, based on selections in the space of the integrals of motion. We derived precise abundances for 16 atomic species. Compared to GSE, stars attributed to the Thamnos substructure are, on average, more metal-poor, yet most of them show higher [X/Fe] abundance ratios in several elements, such as Na, Mg, Al, Ca, Cu, Zn, as well as lower [Eu/Fe]. The majority of candidate Thamnos stars show chemical signatures more consistent with the in situ Milky Way halo rather than a typical low-mass accreted dwarf galaxy. Our findings are further supported by comparisons with tailored galactic chemical evolution models, which fall short in reproducing the observed enhancement in the $α$-elements, but are able to fit the more metal-poor component present in the Thamnos substructure. These results confirm a high level of contamination in the Thamnos substructure from the in situ population and to a lesser degree from GSE, while still leaving room for a genuine accreted population from a small disrupted dwarf galaxy.
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