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arXiv 2302.09640astro-ph.GAastro-ph.SR
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银河系老年恒星族群化学增丰的年表

Chronology of the chemical enrichment of the old Galactic stellar populations

Riano E. Giribaldi, Rodolfo Smiljanic

备注：
Paper accepted in A&A. People interested in Galactic evolution may preferably read the Introduction and Discussion sections. Those interested in accuracy of stellar parameters, abundances, and calibration of spectroscopic surveys may be interested in reading sections 3, 4, 5, and 6 in detail
发表信息：
A&A 673, A18 (2023)
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AI总结：

本研究利用Titans与GALAH恒星的年龄、化学丰度及轨道信息，重建银河系原位和吸积贫金属族群的化学演化年表，精确限定Gaia-Enceladus并合时间及其对高低α盘形成的影响。

摘要：

银河系在其历史上吸积了数个较小的卫星星系。这些并合事件为银河系增添了恒星和气体，并影响了先前已存在的恒星族群的性质。要确定发生在此类并合之前、期间和之后的事件的时间顺序，需要恒星的化学丰度和年龄。我们报告了Titans样本的精确年龄（约6.5%）和化学丰度，该样本由具有准确大气参数的老年贫金属矮星和亚巨星组成。我们还从GALAH巡天中选取了一个矮星样本，并获得了平均精度为10%的年龄。我们利用这些位于距太阳约1千秒差距范围内的恒星，分析了原位形成与吸积而来的贫金属恒星族群化学演化的时间顺序。我们通过等年龄线拟合确定年龄。对于Titans，我们使用光谱综合法测定了Mg、Si、Ca、Ti、Ni、Ba和Eu的丰度。GALAH恒星的[Mg/Fe]丰度经过重新标度，以与Titans的丰度保持一致。我们主要利用化学丰度和轨道来区分恒星族群。我们发现，在所谓的Gaia-Enceladus（又称Gaia-Sausage）星系——最后一个与银河系并合的主要系统——中，恒星形成至少持续了30亿年，并在96±2亿年前被截断。这以极高精度标志着其并合过程的最后阶段。我们还识别出一个被加热的贫金属原位族群的恒星，其净旋转几乎为零，可能受到早期银河系数次并合的扰动。我们表明，在任何时期，该族群都比Gaia-Enceladus更富金属。我们分析揭示的事件序列支持这样一个假说：Gaia-Enceladus截断了高α盘的形成，并引发了形成低α盘的气体落入，这与理论预测一致。

英文摘要：

The Milky Way accreted several smaller satellite galaxies in its history. These mergers added stars and gas to the Galaxy and affected the properties of the pre-existing stellar populations. Stellar chemical abundances and ages are needed to establish the chronological order of events that occur before, during, and after such mergers. We report precise ages ($\sim$6.5%) and chemical abundances for the Titans, a sample of old metal-poor dwarfs and subgiants with accurate atmospheric parameters. We also obtain ages with an average precision of 10% for a selected sample of dwarf stars from the GALAH survey. We used these stars, located within $\sim$1 kiloparsec of the Sun, to analyse the chronology of the chemical evolution of in-situ and accreted metal-poor stellar populations. We determined ages by isochrone fitting. For the Titans, we determined abundances of Mg, Si, Ca, Ti, Ni, Ba, and Eu using spectrum synthesis. The [Mg/Fe] abundances of the GALAH stars were re-scaled to be consistent with the abundances of the Titans. We separated stellar populations by primarily employing chemical abundances and orbits. We find that star formation in the so-called Gaia-Enceladus or Gaia-Sausage galaxy, the last major system to merge with the Milky Way, lasted at least 3 billion years and got truncated 9.6 $\pm$ 0.2 billion years ago. This marks with very high precision the last stage of its merging process. We also identified stars of a heated metal-poor in-situ population with virtually null net rotation, probably disturbed by several of the early Milky Way mergers. We show that this population is more metal rich than Gaia-Enceladus at any time. The sequence of events uncovered in our analysis supports the hypothesis that Gaia-Enceladus truncated the formation of the high-$α$ disc and caused the gas infall that forms the low-$α$ disc, in agreement with theoretical predictions.

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