Black Hole Mergers from Star Clusters with Top-heavy Initial Mass Functions
Black Hole Mergers from Star Clusters with Top-heavy Initial Mass Functions
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DOI：
10.3847/2041-8213/abd79c
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发表时间：
2021-01
期刊：
影响因子：
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通讯作者：
Newlin C. Weatherford;G. Fragione;K. Kremer;S. Chatterjee;Claire S. Ye;C. Rodriguez;F. Rasio
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文献类型：
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作者：
Newlin C. Weatherford;G. Fragione;K. Kremer;S. Chatterjee;Claire S. Ye;C. Rodriguez;F. Rasio
最近对球状星团(GC)的观测提供了证据，表明恒星初始质量函数(IMF)可能不是通用的，特别是表明IMF随着金属丰度的降低和气体密度的增加而变得越来越头重脚轻。非正则IMF可以极大地影响GC的演化，主要是因为高端决定了有多少黑洞(BHS)形成。在这里，我们计算了一组新的GC模型，在观测不确定的情况下改变IMF。我们发现，具有顶部重IMF的GC通过恒星风和潮汐剥离，在几吉年内失去了大部分质量。通过BH质量偏析对团簇的加热大大增强了这一过程。我们发现，当它们接近完全溶解时，具有顶部重IMF的GC可以演化成“暗星团”，其中大部分由质量较大的BHS组成。除了产生更多的BH外，具有头重脚轻IMF的GC还产生更多的二进制BH(BBH)合并。尽管这些星团是短暂的，但被逐出的BBH的合并速度与具有规范IMF的长期GC的合并速度相当或更高。因此，尽管这些星系团今天不再可见，但它们仍然可以对LIGO/Virgo探测到的局部BBH合并率做出重大贡献，特别是对于成分质量更高的源，特别是进入BH质量间隙的源。我们还报告说，我们的一个GC模型与顶重的IMF产生了数十个中等质量的黑洞(IMBH)的质量，其中包括一个。最终，额外的引力波观测将对旧GC中的恒星IMF和高红移IMBH的形成提供强有力的约束。
Recent observations of globular clusters (GCs) provide evidence that the stellar initial mass function (IMF) may not be universal, suggesting specifically that the IMF grows increasingly top-heavy with decreasing metallicity and increasing gas density. Noncanonical IMFs can greatly affect the evolution of GCs, mainly because the high end determines how many black holes (BHs) form. Here we compute a new set of GC models, varying the IMF within observational uncertainties. We find that GCs with top-heavy IMFs lose most of their mass within a few gigayears through stellar winds and tidal stripping. Heating of the cluster through BH mass segregation greatly enhances this process. We show that, as they approach complete dissolution, GCs with top-heavy IMFs can evolve into “dark clusters” consisting of mostly BHs by mass. In addition to producing more BHs, GCs with top-heavy IMFs also produce many more binary BH (BBH) mergers. Even though these clusters are short-lived, mergers of ejected BBHs continue at a rate comparable to, or greater than, what is found for long-lived GCs with canonical IMFs. Therefore, these clusters, though they are no longer visible today, could still contribute significantly to the local BBH merger rate detectable by LIGO/Virgo, especially for sources with higher component masses well into the BH mass gap. We also report that one of our GC models with a top-heavy IMF produces dozens of intermediate-mass black holes (IMBHs) with masses , including one with . Ultimately, additional gravitational wave observations will provide strong constraints on the stellar IMF in old GCs and the formation of IMBHs at high redshift.