R136-type星爆星团中超正则星的出现
The emergence of super-canonical stars in R136-type star-burst clusters
摘要：
R136是位于大麦云星云剑鱼座30复合体中心的年轻大质量星团，其恒星星族中最显著的特征之一是其质量大大超过标准恒星质量上限150 M_sun的单星。我们最近的一项研究，即，Banerjee，Kroupa & Oh（2012; Paper I）的研究涉及模拟R136的星团的真实N体计算，表明这种“超正则”（SC）恒星可以由具有正则初始质量函数（IMF）的致密恒星星族通过最大质量双星的动态诱导合并形成。在这里，我们详细研究了论文I的R136模型中SC星的形成。为了避免形成外来的SC星从最初的高偏心原始二进制文件I，我们计算额外的模型，只有最初的圆形原始二进制文件。我们还考虑到质量演化的SC恒星使用详细的恒星演化模型，将更新的治疗恒星风。我们发现，SC星开始形成通过动态合并的大规模双星约。100万年星团年龄。我们获得SC星的初始质量高达约。250 M_sun从这些计算。多个SC星被发现保持绑定到集群同时在SC寿命。动力学形成的SC星的这些性质与在R136中观察到的一致。事实上，SC恒星的恒星演化模型暗示，如果它们最初与R136星团的其他部分一起形成，违反规范上限，他们将在规范的150 M太阳极限以下进化约。300万年前，可能是R136的年龄，在目前的R136中还不能作为SC星被观测到。这有力地支持了在R136中观测到的SC星的动力学形成情景。
英文摘要：
[abridged] Among the most remarkable features of the stellar population of R136, the central, young, massive star cluster in the 30 Doradus complex of the Large Magellanic Cloud, are the single stars whose masses substantially exceed the canonical stellar upper mass limit of 150 M_sun. A recent study by us, viz., that of Banerjee, Kroupa & Oh (2012; Paper I), which involves realistic N-body computations of star clusters mimicking R136, indicates that such "super-canonical" (SC) stars can be formed out of a dense stellar population with a canonical initial mass function (IMF) through dynamically induced mergers of the most massive binaries. Here we study the formation of SC stars in the R136 models of Paper I in detail. To avoid forming extraneous SC stars from initially highly eccentric primordial binaries as in Paper I, we compute additional models with only initially circular primordial binaries. We also take into account the mass-evolution of the SC stars using detailed stellar evolutionary models that incorporate updated treatments of stellar winds. We find that SC stars begin to form via dynamical mergers of massive binaries from approx. 1 Myr cluster age. We obtain SC stars with initial masses up to approx. 250 M_sun from these computations. Multiple SC stars are found to remain bound to the cluster simultaneously within a SC-lifetime. These properties of the dynamically formed SC stars are consistent with those observed in R136. In fact, the stellar evolutionary models of SC stars imply that had they formed primordially along with the rest of the R136 cluster, i.e., violating the canonical upper limit, they would have evolved below the canonical 150 M_sun limit by approx. 3 Myr, the likely age of R136, and would not have been observable as SC stars at the present time in R136. This strongly supports the dynamical formation scenario of the observed SC stars in R136.
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