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arXiv 1410.2248astro-ph.SR
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球状星团诞生时的状态II：原始双星

The state of globular clusters at birth II: primordial binaries

Nathan W. C. Leigh, Mirek Giersz, Michael Marks, Jeremy J. Webb, Arkadiusz Hypki, Craig O. Heinke, Pavel Kroupa, Alison Sills

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17 pages; 5 figures; 1 table; accepted for publication in MNRAS
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摘要：

本文利用MOCCA蒙特卡罗程序对银河系球状星团中的原始双星布居进行了约束。我们的结果与Milone等人（2012）的观测结果进行了比较，使用光度双星作为真实潜在分布的代理，以检验数据与Kroupa（1995）的双星轨道参数分布和接近1的普遍初始双星分数一致的假设。除了少数可能的异常值，我们发现，数据是一阶一致的普适性假设。具体地说，现在在半质量半径r $_{\rm h}$内的双星分数可以被再现，假设高初始双星分数接近于1，并且具有像在Kroupa分布中那样的主导软双星分量，结合高初始密度（10 $^4$-10 $^6$ M $_{\odot}$ pc $^{-3}$），或低初始二元组分（$\sim$ 5-10%），其主要硬二元组分与接近其现今值的中等初始密度（10 $^2$-10 $^3$ M $_{\odot}$ pc $^{-3}$）结合。这种明显的简并可以使用r $_{\rm h}$之外的二元分数来打破-只有具有显著软组分的高初始二元分数与高初始密度相结合才能有助于再现所观察到的r $_{\rm h}$之外的二元分数与总簇质量之间的反相关。我们进一步使用模拟的现今双星轨道参数分布和Leigh et al.（2012）中介绍的技术来说明，硬和软双星的相对分数可以用来进一步限制初始集群密度和质量密度关系。我们的结果有利于初始质量密度关系的形式r $_{\rm h} \propto$ M $_{\rm clus}^α$与$α<$ 1/3。

英文摘要：

(abridged) In this paper, we constrain the properties of primordial binary populations in Galactic globular clusters using the MOCCA Monte Carlo code for cluster evolution. Our results are compared to the observations of Milone et al. (2012) using the photometric binary populations as proxies for the true underlying distributions, in order to test the hypothesis that the data are consistent with an universal initial binary fraction near unity and the binary orbital parameter distributions of Kroupa (1995). With the exception of a few possible outliers, we find that the data are to first-order consistent with the universality hypothesis. Specifically, the present-day binary fractions inside the half-mass radius r$_{\rm h}$ can be reproduced assuming either high initial binary fractions near unity with a dominant soft binary component as in the Kroupa distribution combined with high initial densities (10$^4$-10$^6$ M$_{\odot}$ pc$^{-3}$), or low initial binary fractions ($\sim$ 5-10%) with a dominant hard binary component combined with moderate initial densities near their present-day values (10$^2$-10$^3$ M$_{\odot}$ pc$^{-3}$). This apparent degeneracy can be broken using the binary fractions outside r$_{\rm h}$- only high initial binary fractions with a significant soft component combined with high initial densities can contribute to reproducing the observed anti-correlation between the binary fractions outside r$_{\rm h}$ and the total cluster mass. We further illustrate using the simulated present-day binary orbital parameter distributions and the technique introduced in Leigh et al. (2012) that the relative fractions of hard and soft binaries can be used to further constrain the initial cluster density and mass-density relation. Our results favour an initial mass-density relation of the form r$_{\rm h} \propto$ M$_{\rm clus}^α$ with $α<$ 1/3.

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