分层坍缩云中的团簇组装
Cluster Assembly in Hierarchically Collapsing Clouds
摘要：
我们讨论了分层坍缩分子云中团簇形成的机制。最近的观测和数值证据表明，分子云（MC）可能正在经历全球性的、层次性的引力坍缩。“等级”制度由大规模崩溃中的小规模崩溃组成。后者意味着恒星形成率在演化的早期阶段系统性地增加，并通过向密度、质量和速度分散度更高的“中心”的对流发生，并在小规模坍缩开始形成恒星之后的几百万年达到顶峰。反过来，小规模的坍缩发生在嵌在细丝中的团块中，并且它们自己落入仍在进行的大规模坍缩的更大的势阱中。在早期的小尺度坍缩中形成的恒星与它们的母团一起向更大的势槽方向下落，因此这些暗条将气态和恒星物质输送到中心。这导致了在新的原恒星仍在形成的区域中存在较老的恒星，导致了星团的无标度或分形结构，其中每个单元都由较小尺度的单元组成，并导致了子单元的最终合并，解释了观测到的疏散星团的结构特征。
英文摘要：
We discuss the mechanism of cluster formation in hierarchically collapsing molecular clouds. Recent evidence, both observational and numerical, suggests that molecular clouds (MCs) may be undergoing global, hierarchical gravitational collapse. The "hierarchical" regime consists of small-scale collapses within larger-scale ones. The latter implies that the star formation rate increases systematically during the early stages of evolution, and occurs via filamentary flows onto "hubs" of higher density, mass, and velocity dispersion, and culminates a few Myr after than the small-scale collapses have started to form stars. In turn, the small-scale collapses occur in clumps embedded in the filaments, and are themselves falling into the larger potential well of the still-ongoing large-scale collapse. The stars formed in the early, small-scale collapses share the infall motion of their parent clumps towards the larger potential trough, so that the filaments feed both gaseous and stellar material to the hubs. This leads to the presence of older stars in a region where new protostars are still forming, to a scale-free or fractal structure of the clusters, in which each unit is composed of smaller-scale ones, and to the eventual merging of the subunits, explaining the observed structural features of open clusters.
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