GALAH巡天：通过RR Lyrae恒星追溯银河系的形成与演化
The GALAH survey: Tracing the Milky Way's formation and evolution through RR Lyrae stars
AI总结：
本研究基于GALAH DR3及文献共535颗场RR Lyrae星进行化学、运动学与动力学分析，发现富金属古老薄盘成分，并识别与多个恒星流的关联。
摘要：
恒星并合与吸积事件在塑造银河系（MW）演化过程中一直至关重要。此前，人们利用红巨星和主序星，从动力学上识别这些事件并刻画其化学特征。RR Lyrae（RRL）变星能够在追溯银河系早期形成方面发挥关键作用，因为它们普遍存在、年龄古老（t≥10 Gyr）、属于低质量恒星，并且是精确的距离指示器。我们利用GALAH巡天第三次数据发布，识别出78颗适合化学分析的场RRL星。通过合成光谱计算，我们测定了大气参数以及Fe、Mg、Ca、Y和Ba的丰度。我们的大多数恒星表现出类似晕的化学组成，铁丰度峰值位于[Fe/H]≈−1.40附近，并具有增强的Ca和Mg含量。值得注意的是，我们发现了一个富金属尾，其[Fe/H]值从−1延伸至接近太阳金属丰度。该子群约占样本的1/4，具有薄盘运动学特征，并显示出低于太阳的α元素丰度，勉强与大多数银河系恒星相符。令人惊讶的是，就s过程元素Y和Ba而言，它们与典型的银河系盘星明显不同。我们利用文献中可得的类似数据，构建了一个包含535颗场RRL星的总样本，并为其估算了运动学和动力学性质。我们发现富金属RRL星（占样本的1/3）很可能代表银河系薄盘的一个古老组成部分。我们还探测到具有逆行轨道的RRL星，并给出了它们与Gaia-Sausage-Enceladus、Helmi、Sequoia、Sagittarius和Thamnos恒星流的初步关联。
英文摘要：
Stellar mergers and accretion events have been crucial in shaping the evolution of the Milky Way (MW). These events have been dynamically identified and chemically characterised using red giants and main-sequence stars. RR Lyrae (RRL) variables can play a crucial role in tracing the early formation of the MW since they are ubiquitous, old (t$\ge$10 Gyr) low-mass stars and accurate distance indicators. We exploited Data Release 3 of the GALAH survey to identify 78 field RRLs suitable for chemical analysis. Using synthetic spectra calculations, we determined atmospheric parameters and abundances of Fe, Mg, Ca, Y, and Ba. Most of our stars exhibit halo-like chemical compositions, with an iron peak around [Fe/H]$\approx -$1.40, and enhanced Ca and Mg content. Notably, we discovered a metal-rich tail, with [Fe/H] values ranging from $-$1 to approximately solar metallicity. This sub-group includes almost ~1/4 of the sample, it is characterised by thin disc kinematics and displays sub-solar $α$-element abundances, marginally consistent with the majority of the MW stars. Surprisingly, they differ distinctly from typical MW disc stars in terms of the s-process elements Y and Ba. We took advantage of similar data available in the literature and built a total sample of 535 field RRLs for which we estimated kinematical and dynamical properties. We found that metal-rich RRLs (1/3 of the sample) likely represent an old component of the MW thin disc. We also detected RRLs with retrograde orbits and provided preliminary associations with the Gaia-Sausage-Enceladus, Helmi, Sequoia, Sagittarius, and Thamnos stellar streams.
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