Mapping Milky Way Halo Substructure Using Stars in the Extended Blue Tail of the Horizontal Branch
Mapping Milky Way Halo Substructure Using Stars in the Extended Blue Tail of the Horizontal Branch
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DOI：
10.3847/1538-4357/abe385
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发表时间：
2021-03
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影响因子：
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通讯作者：
Robert Gryncewicz;H. Newberg;Charles Martin;Thomas Donlon II;P. Amy
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作者：
Robert Gryncewicz;H. Newberg;Charles Martin;Thomas Donlon II;P. Amy
虽然蓝色水平分支（BHB）星通常用于追踪光晕的亚结构，但由于难以确定其绝对星等，较蓝(g−r) <−0.3的恒星被忽略。水平分支（HBX）的蓝色延伸包括BHB尾星和极端水平分支（EHB）星。我们提出了一种在野外识别HBX恒星的方法，使用来自斯隆数字巡天数据发布14 （SDSS DR14）的光谱和光度测定。我们将BHB尾星和EHB星的绝对星等估计为颜色的函数，并使用这种关系来计算距离。我们发现了一个高密度的HBX恒星，似乎可以追溯到大力神-天鹰座云的北端。我们确定了三颗恒星可能是潮汐流的一部分，但这还不足以解释观测到的超密度现象。将SDSS数据与盖亚DR2固有运动相结合，我们可以显示出大多数HBX恒星在高偏心率的轨道上，类似于室女座径向合并/盖亚-土卫二/盖亚香肠结构中的轨道，并且高偏心率轨道的过密度一直延伸到室女座的过密度。我们利用恒星运动学将HBX恒星分为盘状恒星和晕状恒星。晕星主要在高度偏心（径向）轨道上运行。HBX恒星中EHBs的比例在盘群中是最高的，在低离心率的晕群中是最低的。
Although blue horizontal branch (BHB) stars are commonly used to trace halo substructure, the stars bluer than (g − r) < −0.3 are ignored owing to the difficulty in determining their absolute magnitudes. The blue extension of the horizontal branch (HBX) includes BHB tail stars and extreme horizontal branch (EHB) stars. We present a method for identifying HBX stars in the field, using spectra and photometry from the Sloan Digital Sky Survey Data Release 14 (SDSS DR14). We derive an estimate for the absolute magnitudes of BHB tail and EHB stars as a function of color and use this relationship to calculate distances. We identify an overdensity of HBX stars that appears to trace the northern end of the Hercules-Aquila Cloud. We identify three stars that are likely part of a tidal stream, but this is not enough stars to explain the observed overdensity. Combining SDSS data with Gaia DR2 proper motions allows us to show that the majority of the HBX stars in the overdensity are on high-eccentricity orbits similar to those in the Virgo Radial Merger/Gaia–Enceladus/Gaia Sausage structure and that the overdensity of high-eccentricity orbits extends all the way to the Virgo Overdensity. We use stellar kinematics to separate the HBX stars into disk stars and halo stars. The halo stars are primarily on highly eccentric (radial) orbits. The fraction of HBX stars that are EHBs is highest in the disk population and lowest in the low-eccentricity halo stars.