标题： 年轻星团NGC 6231的结构。 II。 结构、形成与命运 显示英文标题

标题： The Structure of the Young Star Cluster NGC 6231. II. Structure, Formation, and Fate

摘要： 年轻的星团NGC 6231（恒星年龄~2-7 Myr）在恒星形成活动停止后不久被观测到。 使用从钱德拉、VVV和光学巡天获得的2148个可能星团成员的目录（论文I），我们研究了星团的空间结构和动力状态。 恒星的空间分布很好地由一个具有适度拉长的等温球体拟合，而其他常用的模型如普尔默球体、多变量正态分布或幂律模型则拟合效果较差。 该星团的核心半径为$1.2\pm0.1$pc，中心密度约为200颗恒星pc$^{-3}$。 恒星的分布表现出轻微的质量分层。 然而，按年龄来看，恒星没有出现径向分层。 尽管大多数恒星属于一个单一星团，但发现一个小的子星团叠加在主星团上，并且在约4个核心半径以外存在团状的非各向同性的恒星分布。 当将NGC 6231的大小、质量和年龄与其他附近活跃恒星形成区域中的年轻星团和子星团进行比较时，它位于分布的高质量端，但沿着相同的趋势线。 这可能是由于相似的形成过程，可能是分层的星团组装。 我们认为NGC 6231已经从初始大小扩展了，但它仍然处于引力束缚状态。 显示英文摘要

摘要： The young cluster NGC 6231 (stellar ages ~2-7 Myr) is observed shortly after star-formation activity has ceased. Using the catalog of 2148 probable cluster members obtained from Chandra, VVV, and optical surveys (Paper I), we examine the cluster's spatial structure and dynamical state. The spatial distribution of stars is remarkably well fit by an isothermal sphere with moderate elongation, while other commonly used models like Plummer spheres, multivariate normal distributions, or power-law models are poor fits. The cluster has a core radius of $1.2\pm0.1$ pc and a central density of ~200 stars pc$^{-3}$. The distribution of stars is mildly mass segregated. However, there is no radial stratification of the stars by age. Although most of the stars belong to a single cluster, a small subcluster of stars is found superimposed on the main cluster, and there are clumpy non-isotropic distributions of stars outside ~4 core radii. When the size, mass, and age of NGC 6231 are compared to other young star clusters and subclusters in nearby active star-forming regions, it lies at the high-mass end of the distribution but along the same trend line. This could result from similar formation processes, possibly hierarchical cluster assembly. We argue that NGC 6231 has expanded from its initial size but that it remains gravitationally bound.