A Census of Blue Stragglers in Gaia DR2 Open Clusters as a Test of Population Synthesis and Mass Transfer Physics
A Census of Blue Stragglers in Gaia DR2 Open Clusters as a Test of Population Synthesis and Mass Transfer Physics
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DOI：
10.3847/1538-4357/abd7e9
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发表时间：
2021-01
期刊：
影响因子：
--
通讯作者：
E. Leiner;A. Geller
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作者：
E. Leiner;A. Geller
我们使用光度测量和自行从盖亚DR 2确定的蓝离散星星（BSS）人口的16岁（1-10 Gyr），附近（d < 3500 pc）疏散星团。我们发现，与红巨星分支星相比，BSS的分数随着年龄的增加而增加，从1 Gyr的零附近开始，到4 Gyr的0.35。拟合恒星的演化轨迹，这些BSS，我们发现它们的质量分布峰值在太阳质量的十分之几以上的主序星关闭。高于关闭处0.5 M水柱以上的BSS仅占样本的1.25%，而高于关闭处1.0 M水柱以上的BSS很少见。我们比较紧凑的对象合成和蒙特卡罗调查代码人口合成模型的BSS通过质量转移形成的。我们发现，标准的人口合成的假设显着不足，产生旧的开放集群的BSS的数量。我们还发现，这些模型过度生产高质量的BSS相对于低质量的BSS。通过动力学形成的BSS的预期数量并不能完全解释这种差异。我们的结论是，为了解释所观察到的BSS人口从罗氏瓣溢出，从巨大的捐助者的传质必须更稳定，比假设在典型的传质处方，包括非保守传质是很重要的，在生产现实的BSS群众。即使有了这些修改，也很难实现在最古老的疏散星团中观察到的大量BSS。我们讨论了一些额外的物理，可以解释大量的老恒星种群中观察到的蓝离散。
We use photometry and proper motions from Gaia DR2 to determine the blue straggler star (BSS) populations of 16 old (1–10 Gyr), nearby (d < 3500 pc) open clusters. We find that the fractional number of BSS compared to red giant branch stars increases with age, starting near zero at 1 Gyr and flattening to ∼0.35 by 4 Gyr. Fitting stellar evolutionary tracks to these BSSs, we find that their mass distribution peaks at a few tenths of a solar mass above the main-sequence turnoff. BSSs more than 0.5 M ⊙ above the turnoff make up only ∼25% of the sample, and BSSs more than 1.0 M ⊙ above the turnoff are rare. We compare this to Compact Object Synthesis and Monte Carlo Investigation Code population synthesis models of BSSs formed via mass transfer. We find that standard population synthesis assumptions dramatically under-produce the number of BSS in old open clusters. We also find that these models overproduce high-mass BSSs relative to lower-mass BSSs. The expected number of BSSs formed through dynamics do not fully account for this discrepancy. We conclude that in order to explain the observed BSS populations from Roche lobe overflow, mass transfer from giant donors must be more stable than assumed in canonical mass-transfer prescriptions, and including nonconservative mass transfer is important in producing realistic BSS masses. Even with these modifications, it is difficult to achieve the large number of BSSs observed in the oldest open clusters. We discuss some additional physics that may explain the large number of observed blue stragglers among old stellar populations.