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arXiv 1907.05652astro-ph.GA
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在盖亚DR2中高速赫兹-罗素图中看到的两个序列的特征

Characteristics of the two sequences seen in the high-velocity Hertzsprung-Russell diagram in Gaia DR2

Christian L. Sahlholdt, Luca Casagrande, Sofia Feltzing

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9 pages, 9 figures, published in ApJ
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摘要：

在这项研究中，我们使用了大约900万个SkyMapper恒星的样本，金属丰度调查的两个恒星星族的属性中看到的高速（$V_{\rm T} > 200$ km/s）盖亚DR 2赫兹-罗素图。基于10，000颗红巨星分支（RGB）恒星（75，000颗高速），我们发现这两个序列具有不同的金属丰度分布函数;一个峰值在$-1.4$ dex（蓝色序列），另一个峰值在$-0.7$ dex（红色序列）。年龄在$11$ - $13.5$ Gyr范围内的等时线，以及为匹配每个序列的观测结果而选择的金属丰度，都很好地拟合了转折点和广泛的RGB，表明这两个群体在不确定性范围内的可比时间形成。我们发现，盘星的平均切向速度随着金属丰度的降低而稳步增加，红色序列是由厚盘人口中金属丰度最低的高速星组成的。使用相对数密度，我们进一步发现，红序星更集中在银河系的中心，我们估计这个人口的径向尺度长度为$2$ - $3$ kpc的顺序。另一方面，蓝序星的径向密度分布几乎是平的。这些发现加强了红序恒星和化学定义的厚盘之间的联系。

英文摘要：

In this study we use a sample of about 9 million SkyMapper stars with metallicities to investigate the properties of the two stellar populations seen in the high-velocity ($V_{\rm T} > 200$ km/s) Gaia DR2 Hertzsprung-Russell diagram. Based on 10,000 red giant branch (RGB) stars (out of 75,000 with high velocity), we find that the two sequences have different metallicity distribution functions; one peaks at $-1.4$ dex (blue sequence) and the other at $-0.7$ dex (red sequence). Isochrones with ages in the range $11$-$13.5$ Gyr, and metallicities chosen to match the observations for each sequence, fit the turnoffs and broad RGBs well, indicating that the two populations formed at comparable times within the uncertainties. We find that the mean tangential velocity of disk stars increases steadily with decreasing metallicity, and that the red sequence is made up of the high-velocity stars at the lowest metallicities of the thick-disk population. Using relative number densities, we further find that the red-sequence stars are more centrally concentrated in the Galaxy, and we estimate the radial scale length of this population to be on the order of $2$-$3$ kpc. The blue-sequence stars, on the other hand, follow a nearly flat radial density profile. These findings tighten the link between the red-sequence stars and the chemically defined thick disk.

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