Origin of the Excess of High-energy Retrograde Stars in the Galactic Halo
Origin of the Excess of High-energy Retrograde Stars in the Galactic Halo
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DOI：
10.3847/2041-8213/ab0ec0
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发表时间：
2019-03
期刊：
影响因子：
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通讯作者：
T. Matsuno;W. Aoki;T. Suda
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文献类型：
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作者：
T. Matsuno;W. Aoki;T. Suda
我们报道了一组具有高轨道能量和在银河系晕中有大的逆行运动的贫金属星的非常低的α元素丰度，最近从金属丰度和运动学角度报道了它们的过剩。我们构建了一个具有测量丰度和精确运动学的晕星样本，其中包括[Fe/H] <-0.7的10880颗恒星，通过交叉匹配银河系考古学恒星丰度数据库和盖亚的第二次数据发布。在能量-角动量空间中有三个区域被选中：最里面的晕，盖亚-恩克拉多斯/香肠，以及高能逆行晕。虽然最内层的晕和盖亚土卫二区域的化学丰度分别与晕中的高α和低α族群一致，但高能逆行晕中恒星的化学丰度与这两个族群不同;它们的[X/Fe]（X代表Na、Mg和Ca）甚至低于盖亚土卫二。这些丰度，以及它们的低平均金属丰度，提供了一个新的支持的想法，逆行的组成部分是由吸积矮星系，具有较长的星星形成的时间尺度和质量小于盖亚土卫二/香肠为主。
We report on the very low α-element abundances of a group of metal-poor stars with high orbital energy and with large retrograde motion in the Milky Way halo, whose excess has been reported recently from metallicity and kinematics. We constructed a sample of halo stars with measured abundances and precise kinematics, including ∼880 stars with [Fe/H] < −0.7, by crossmatching the Stellar Abundances for Galactic Archaeology database to the second data release of Gaia. Three regions in the energy-angular momentum space have been selected: the innermost halo, Gaia Enceladus/Sausage, and the high-energy retrograde halo. While the innermost halo and Gaia Enceladus regions have chemical abundances consistent with high- and low-α populations in the halo, respectively, chemical abundances of stars in the high-energy retrograde halo are different from the two populations; their [X/Fe], where X represents Na, Mg, and Ca, are even lower than those in Gaia Enceladus. These abundances, as well as their low mean metallicity, provide a new support for the idea that the retrograde component is dominated by an accreted dwarf galaxy which has a longer star formation timescale and is less massive than Gaia Enceladus/Sausage.