In Disguise or Out of Reach: First Clues about In Situ and Accreted Stars in the Stellar Halo of the Milky Way from Gaia DR2
In Disguise or Out of Reach: First Clues about In Situ and Accreted Stars in the Stellar Halo of the Milky Way from Gaia DR2
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DOI：
10.3847/1538-4357/aad235
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发表时间：
2018-05
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影响因子：
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通讯作者：
M. Haywood;P. Di Matteo;M. Lehnert;O. Snaith;S. Khoperskov;A. Gomez
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作者：
M. Haywood;P. Di Matteo;M. Lehnert;O. Snaith;S. Khoperskov;A. Gomez
我们研究了在高横向速度恒星的Gaia DR2 HR图中观察到的双色等序列的性质。最红的颜色序列中的恒星很可能是由厚盘种群的动态高温尾巴主导的。来自Nissen&Schuster和远地点调查的信息表明，蓝色序列中的恒星具有元素丰度模式，这可以用这一群体在其形成过程中恒星形成效率相对较低来解释。在动力学和轨道空间中，如图姆图，这两个序列显示出显著的重叠，但蓝色序列上的恒星有占据无自转或逆行旋转和总轨道能量高的区域的趋势。在由轨道相对于其远点的最大垂直偏移所定义的平面内，这两个序列的恒星重新分布成离散的楔形。我们的结论是，通常被指定为太阳附近晕的恒星实际上既是沿着HR图中蓝色序列的吸积恒星，也是旧银盘的低自转速度尾巴，可能是过去吸积事件动态加热的。我们的结果表明，在厚盘形成之前，一个在原地形成并对早期化学富集物负责的晕群尚未得到确凿的鉴定，而被定义为银河系原地恒星晕的恒星实际上可能是它最后一次重大合并的化石记录。
We investigate the nature of the double color–magnitude sequence observed in the Gaia DR2 HR diagram of stars with high transverse velocities. The stars in the reddest-color sequence are likely dominated by the dynamically hot tail of the thick disk population. Information from Nissen & Schuster and from the APOGEE survey suggests that stars in the blue-color sequence have elemental abundance patterns that can be explained by this population having a relatively low star formation efficiency during its formation. In dynamical and orbital spaces, such as the “Toomre diagram,” the two sequences show a significant overlap, but with a tendency for stars on the blue-color sequence to dominate regions with no or retrograde rotation and high total orbital energy. In the plane defined by the maximal vertical excursion of the orbits versus their apocenters, stars of both sequences redistribute into discrete wedges. We conclude that stars that are typically assigned to the halo in the solar vicinity are actually both accreted stars lying along the blue sequence in the HR diagram, and the low rotational velocity tail of the old Galactic disk, possibly dynamically heated by past accretion events. Our results imply that a halo population formed in situ and responsible for the early chemical enrichment prior to the formation of the thick disk has yet to be robustly identified, and that what has been defined as the stars of the in situ stellar halo of the Galaxy may in fact be fossil records of its last significant merger.