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arXiv 1001.3969astro-ph.GA
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在星团溶解阶段中甚宽双星的形成

The formation of very wide binaries during the star cluster dissolution phase

M. B. N. Kouwenhoven, S. P. Goodwin, Richard J. Parker, M. B. Davies, D. Malmberg, P. Kroupa

摘要:

在过去的几十年里,在银河系的场和晕中发现了许多宽(>1000 au)的双星。它们的存在无法用恒星形成过程或磁场中的动力学相互作用来解释,它们的起源长期以来一直是个谜。我们解释了这些广泛的双星的起源形成在溶解阶段的年轻星团:一个最初未绑定的恒星对可能会形成一个二进制时,他们在相空间的距离很小。使用N体模拟,我们发现,所得到的宽二进制分数在半长轴范围1000 AU - 0.1 PC的个别集群是1- 30%,这取决于初始条件。在这个领域中存在着大量的宽双星,这与观测证据相一致,即大多数星团都是从很大程度的子结构开始的。宽双星的比例随着星系团质量的增加而急剧下降,新形成的双星的半长轴由星系团的初始大小决定。由此产生的偏心率分布是热的,和质量比分布是一致的引力聚焦随机配对。由于大部分恒星形成于原始双星,我们预测大量观测到的“宽双星”实际上是三重或四重系统。通过整合的初始集群质量分布,我们预测在半长轴范围1000 au - 0.1 pc的银河系领域,这是小于所观察到的宽二进制分数的百分之几的二进制分数。然而,当我们考虑广泛的簇形态时,这种差异可能会得到解决。

英文摘要:

Over the past few decades, numerous wide (>1000 au) binaries in the Galactic field and halo have been discovered. Their existence cannot be explained by the process of star formation or by dynamical interactions in the field, and their origin has long been a mystery. We explain the origin of these wide binaries by formation during the dissolution phase of young star clusters: an initially unbound pair of stars may form a binary when their distance in phase-space is small. Using N-body simulations, we find that the resulting wide binary fraction in the semi-major axis range 1000 au - 0.1 pc for individual clusters is 1-30%, depending on the initial conditions. The existence of numerous wide binaries in the field is consistent with observational evidence that most clusters start out with a large degree of substructure. The wide binary fraction decreases strongly with increasing cluster mass, and the semi-major axis of the newly formed binaries is determined by the initial cluster size. The resulting eccentricity distribution is thermal, and the mass ratio distribution is consistent with gravitationally-focused random pairing. As a large fraction of the stars form in primordial binaries, we predict that a large number of the observed 'wide binaries' are in fact triple or quadruple systems. By integrating over the initial cluster mass distribution, we predict a binary fraction of a few per cent in the semi-major axis range 1000 au - 0.1 pc in the Galactic field, which is smaller than the observed wide binary fraction. However, this discrepancy may be solved when we consider a broad range of cluster morphologies.

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