DIRECT N-BODY MODELING OF THE OLD OPEN CLUSTER NGC 188: A DETAILED COMPARISON OF THEORETICAL AND OBSERVED BINARY STAR AND BLUE STRAGGLER POPULATIONS
DIRECT N-BODY MODELING OF THE OLD OPEN CLUSTER NGC 188: A DETAILED COMPARISON OF THEORETICAL AND OBSERVED BINARY STAR AND BLUE STRAGGLER POPULATIONS
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DOI：
10.1088/0004-6256/145/1/8
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发表时间：
2012-10
期刊：
影响因子：
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通讯作者：
A. Geller;J. Hurley;R. Mathieu
中科院分区：
文献类型：
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作者：
A. Geller;J. Hurley;R. Mathieu
在最近完成的对老（7 Gyr）疏散星团NGC 188的径向速度观测中，我们详细研究了星团中的太阳型硬双星和蓝离散星，在这里，我们通过一个复杂的N体模型研究了NGC 188的动力学演化。重要的是，我们采用观察到的年轻（1.8亿年）的疏散星团M35的二进制属性，在可能的情况下，指导我们的初始二进制人口的参数的选择。我们应用前主序潮汐循环和大幅增加的主序潮汐循环率，这两个都是必要的，以匹配观测到的潮汐循环期在文献中，包括NGC 188。在7 Gyr的主序太阳型硬二进制人口的模型中匹配的NGC 188的二进制频率和轨道参数的分布。模型和观测结果之间的这种一致性在很大程度上是由于NGC 188和M35太阳型双星之间的相似性。事实上，在模型中的7个Gyr主序星双星中，只有那些P <1000天的双星开始显示出动力学碰撞的潜在可观察证据，即使在星星团内演化了7个Gyr之后。这强调了为星星团模型定义精确初始条件的重要性，我们建议最好通过与像M35这样的年轻疏散星团的观测结果进行比较来完成。此外，这一发现表明，即使是在古老的疏散星团中对现在的双星进行观测，也可以提供有关其原始双星种群的有价值的信息。然而，尽管该模型的成功匹配观测到的太阳型主序人口，该模型生产不足的蓝色离散和生产过剩的长周期圆形主序白矮星双星相比，真正的集群。我们探讨了几个潜在的解决方案，蓝离散的缺乏，并得出结论，该模型显着不足的蓝离散通过传质过程。我们认为，共同包络演化可能被错误地强加在伪长周期圆形主序白矮星双星的祖先上，而这些双星可能应该通过稳定的质量转移来产生蓝色离散星，从而使模型中蓝色离散星的数量和二进制频率与NGC 188中真正的蓝色离散星一致。因此，改进模型中采用的质量传递和共同包络演化的物理学可能实际上解决与观测的两个差异。该项目突出了疏散星团对全面观测调查和全尺度N体模拟的独特可访问性，这两种方法都是最近才成熟到足以使这样一个项目成为可能，并强调了疏散星团对星星星团动力学研究的重要性。
Following on from a recently completed radial-velocity survey of the old (7 Gyr) open cluster NGC 188 in which we studied in detail the solar-type hard binaries and blue stragglers of the cluster, here we investigate the dynamical evolution of NGC 188 through a sophisticated N-body model. Importantly, we employ the observed binary properties of the young (180 Myr) open cluster M35, where possible, to guide our choices for parameters of the initial binary population. We apply pre-main-sequence tidal circularization and a substantial increase to the main-sequence tidal circularization rate, both of which are necessary to match the observed tidal circularization periods in the literature, including that of NGC 188. At 7 Gyr the main-sequence solar-type hard-binary population in the model matches that of NGC 188 in both binary frequency and distributions of orbital parameters. This agreement between the model and observations is in a large part due to the similarities between the NGC 188 and M35 solar-type binaries. Indeed, among the 7 Gyr main-sequence binaries in the model, only those with P ≳ 1000 days begin to show potentially observable evidence for modifications by dynamical encounters, even after 7 Gyr of evolution within the star cluster. This emphasizes the importance of defining accurate initial conditions for star cluster models, which we propose is best accomplished through comparisons with observations of young open clusters like M35. Furthermore, this finding suggests that observations of the present-day binaries in even old open clusters can provide valuable information on their primordial binary populations. However, despite the model's success at matching the observed solar-type main-sequence population, the model underproduces blue stragglers and produces an overabundance of long-period circular main-sequence–white-dwarf binaries as compared with the true cluster. We explore several potential solutions to the paucity of blue stragglers and conclude that the model dramatically underproduces blue stragglers through mass-transfer processes. We suggest that common-envelope evolution may have been incorrectly imposed on the progenitors of the spurious long-period circular main-sequence–white-dwarf binaries, which perhaps instead should have gone through stable mass transfer to create blue stragglers, thereby bringing both the number and binary frequency of the blue straggler population in the model into agreement with the true blue stragglers in NGC 188. Thus, improvements in the physics of mass transfer and common-envelope evolution employed in the model may in fact solve both discrepancies with the observations. This project highlights the unique accessibility of open clusters to both comprehensive observational surveys and full-scale N-body simulations, both of which have only recently matured sufficiently to enable such a project, and underscores the importance of open clusters to the study of star cluster dynamics.