Where are the most ancient stars in the Milky Way?
Where are the most ancient stars in the Milky Way?
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DOI：
10.1093/mnras/sty1864
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发表时间：
2018-04
影响因子：
4.8
通讯作者：
K. El-Badry;J. Bland-Hawthorn;A. Wetzel;E. Quataert;D. Weisz;M. Boylan-Kolchin;P. Hopkins;C. Faucher-Giguère;D. Keres̆;S. Garrison-Kimmel
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作者：
K. El-Badry;J. Bland-Hawthorn;A. Wetzel;E. Quataert;D. Weisz;M. Boylan-Kolchin;P. Hopkins;C. Faucher-Giguère;D. Keres̆;S. Garrison-Kimmel
银河系中最古老的恒星（MW）具有银河系早期聚集历史的印记。我们使用FIRE宇宙学放大模拟了三个mw质量的盘状星系，研究了mw类星系中最古老的幸存恒星（z形≥5）的空间分布、化学和运动学。我们预测，最古老的恒星在z = 0的中心集中程度要低于后来形成的恒星，这是两个过程的结果。首先，大多数最古老的恒星不是在原地形成的，而是在分层组装过程中被吸积的。这些离地恒星沉积在色散支持的光环状轨道上，但在太阳附近形成的旧恒星中占主导地位，在一些模拟中，甚至在银河系中心。其次，在原地形成的老恒星被爆发恒星形成和能量反馈过程向外驱动，这些过程在z > 2处产生随时间变化的引力势，类似于在爆发矮星系中产生暗物质核心和扩大恒星轨道的过程。在远离凸起和内晕的视线中，古老恒星的总比例比在向内的视线中高出一个数量级。尽管确定特定恒星是古老恒星的任务仍然具有挑战性，但我们预计，针对金属贫乏恒星的百万恒星光谱调查和光度调查已经包括数百颗在z = 5之前形成的恒星。我们预测这些目标的金属丰度（-3 < [Fe/H] < -2）比最极端的金属贫乏恒星高。
The oldest stars in the Milky Way (MW) bear imprints of the Galaxy's early assembly history. We use FIRE cosmological zoom-in simulations of three MW-mass disc galaxies to study the spatial distribution, chemistry, and kinematics of the oldest surviving stars (z form ≳ 5) in MW-like galaxies. We predict the oldest stars to be less centrally concentrated at z = 0 than stars formed at later times as a result of two processes. First, the majority of the oldest stars are not formed in situ but are accreted during hierarchical assembly. These ex situ stars are deposited on dispersion-supported, halo-like orbits but dominate over old stars formed in situ in the solar neighbourhood, and in some simulations, even in the galactic centre. Secondly, old stars formed in situ are driven outwards by bursty star formation and energetic feedback processes that create a time-varying gravitational potential at z ≳ 2, similar to the process that creates dark matter cores and expands stellar orbits in bursty dwarf galaxies. The total fraction of stars that are ancient is more than an order of magnitude higher for sight lines away from the bulge and inner halo than for inward-looking sight lines. Although the task of identifying specific stars as ancient remains challenging, we anticipate that million-star spectral surveys and photometric surveys targeting metal-poor stars already include hundreds of stars formed before z = 5. We predict most of these targets to have higher metallicity (-3 < [Fe/H] < -2) than the most extreme metal-poor stars.