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arXiv 0911.0796astro-ph.GAastro-ph.COastro-ph.SR
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年轻和中等年龄的大质量星团

Young and intermediate-age massive star clusters

Soeren S. Larsen

备注：
21 pages, 3 figures. To appear as invited review article in a special issue of the Phil. Trans. Royal Soc. A: Ch. 9 "Star clusters as tracers of galactic star-formation histories" (ed. R. de Grijs). Fully peer reviewed. PDFLaTeX, requires rspublic.cls style file
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摘要：

概述了我们目前对星团形成和演化的认识，重点是大质量星团。团簇深深地嵌入在稠密的分子气体云中。剩下的气体在几百万年内被清除，根据恒星形成的效率，星团可能会几乎立即分散或保持引力束缚。目前的证据表明，有百分之几的恒星形成发生在仍然束缚的星团中，尽管还不清楚这一部分是否真的是普遍存在的。对于银河系中的低质量疏散星团来说，内部的两体弛豫和外部的冲击将导致其在数亿年的时间尺度上进一步逐渐溶解，而质量最大的星团（> 10^5 Msun）的寿命与宇宙的年龄相当或超过宇宙的年龄。初始星系团质量函数的低质量端可以很好地近似为幂律分布dN/dM ~ M^{-2}，但越来越多的证据表明，静止的螺旋盘形成相对较少的质量M > 2 x 10^5 Msun的星系团。在星暴星系和古老的球状星团系统中，这个极限似乎更高，至少是几个× 10 ^6 Msun。这种差异可能与星暴星系中更高的气体密度和压力有关，这使得更密集，更大质量的巨型分子云形成。因此，低质量的星团可能会相当普遍地追踪恒星的形成，而寿命较长的大质量星团似乎更倾向于在剧烈的恒星形成过程中形成。

英文摘要：

An overview of our current understanding of the formation and evolution of star clusters is given, with main emphasis on high-mass clusters. Clusters form deeply embedded within dense clouds of molecular gas. Left-over gas is cleared within a few million years and, depending on the efficiency of star formation, the clusters may disperse almost immediately or remain gravitationally bound. Current evidence suggests that a few percent of star formation occurs in clusters that remain bound, although it is not yet clear if this fraction is truly universal. Internal two-body relaxation and external shocks will lead to further, gradual dissolution on timescales of up to a few hundred million years for low-mass open clusters in the Milky Way, while the most massive clusters (> 10^5 Msun) have lifetimes comparable to or exceeding the age of the Universe. The low-mass end of the initial cluster mass function is well approximated by a power-law distribution, dN/dM ~ M^{-2}, but there is mounting evidence that quiescent spiral discs form relatively few clusters with masses M > 2 x 10^5 Msun. In starburst galaxies and old globular cluster systems, this limit appears to be higher, at least several x 10^6 Msun. The difference is likely related to the higher gas densities and pressures in starburst galaxies, which allow denser, more massive giant molecular clouds to form. Low-mass clusters may thus trace star formation quite universally, while the more long-lived, massive clusters appear to form preferentially in the context of violent star formation.

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