The emergence of super-canonical stars in R136-type starburst clusters
The emergence of super-canonical stars in R136-type starburst clusters
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DOI：
10.1111/j.1365-2966.2012.21672.x
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发表时间：
2012-08
影响因子：
4.8
通讯作者：
S. Banerjee;P. Kroupa;Seungkyung Oh
中科院分区：
文献类型：
--
作者：
S. Banerjee;P. Kroupa;Seungkyung Oh
在R136的恒星群中，最显著的特征之一是，位于大麦哲伦星云的30 Doradus复合体中的中心、年轻的大质量星团是质量大大超过150m⊙的典型恒星质量上限的单星。我们最近的一项研究，即Banerjee，Kroupa&oh(2012；此后的论文I)的研究表明，这种“超正则”(以下简称SC)恒星可以通过最大质量双星的动态合并，从具有正则初始质量函数(IMF)的密集恒星群中形成。上面的研究包括模拟R136的完全质量分离星团的真实N体计算，其中所有的大质量恒星都是原始双星。在目前的工作中，我们详细地研究了文献I中计算的R136模型中SC星的形成。考虑到文献I的计算模型中由于原始双星的初始偏心率而形成的外来SC星，我们计算了额外的模型，其中所有原始双星最初都是圆形的。我们还使用SC质量范围内的详细恒星演化模型考虑了SC恒星的质量演化及其在SC阶段的结果寿命，该模型包含了对恒星风的最新处理。在所有这些计算中，我们发现从�1Myr星团年龄开始，SC恒星就开始通过大质量双星的动力学合并而形成。通过这些计算，我们得到了初始质量高达�250m⊙的SC星。在Sc寿命内，多颗Sc星被发现同时与星系团结合。然而，我们也注意到，SC恒星可以以逃逸的速度形成，这些恒星在诞生时逃离了星团。从我们的计算中得到的动态形成的Sc星的这些性质，与在R136观测到的Sc恒星群是一致的。事实上，SC星的演化模型暗示，如果它们与R136星系团的其余部分一起形成，即违反了正则上限，它们将在R136的可能年龄--�3-Myr之前演化到低于正则的150m Mar极限。因此，根据新的恒星演化模型，目前在R136中应该看不到原始形成的SC星。这有力地支持了R136中观测到的SC星的动力学形成情景。
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 150M⊙. A recent study by us, viz., that of Banerjee, Kroupa & Oh (2012; hereafter Paper I) indicates that such "super-canonical" (hereafter SC) stars can be formed out of a dense stellar population with a canonical initial mass func- tion (IMF) through dynamically induced mergers of the most massive binaries. The above study consists of realistic N-body computations of fully mass-segregated star clusters mimicking R136 in which all the massive stars are in primordial binaries. In the present work, we study the formation of SC stars in the computed R136 models of Paper I in detail. Taking into consideration that extraneous SC stars form in the computed models of Paper I due to the primordial binaries' initial eccentricities, we compute additional models where all primordial binaries are initially circular. We also take into account the evolution of the mass of the SC stars and the resulting lifetime in their SC phase using detailed stellar evolutionary models over the SC mass range that incorporate updated treatments of the stellar winds. In all these computations, we find that SC stars begin to form via dynamical mergers of massive binaries from � 1 Myr cluster age. We obtain SC stars with initial masses up to � 250M⊙ from these computations. Multiple SC stars are found to remain bound to the cluster si- multaneously within a SC-lifetime. However, we also note that SC stars can be formed at runaway velocities which escape the cluster at birth. These properties of the dy- namically formed SC stars, as obtained from our computations, are consistent with the observed SC stellar population in R136. In fact, the 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 150M⊙ limit by � 3 Myr, the likely age of R136. Thus according to the new stellar evolutionary models, primordially-formed SC stars should not be observable at the present time in R136. This strongly supports the dynamical formation scenario of the observed SC stars in R136.