标题： 年轻星团的形成与动力学演化 显示英文标题

标题： The formation and dynamical evolution of young star clusters

摘要： 近期的观测揭示了各种年轻的星团，包括嵌入系统、年轻的大质量星团和星协。 我们通过模拟与理论模型的结合来研究这些星团的形成及其动力学演化。 我们的模拟始于一个在自身引力作用下坍缩的湍流分子云。 假定恒星在分子云的初始自由落体时间之后，在坍缩云中最稠密的区域形成。 这些恒星分布的动力学演化通过直接的$N$-体模拟继续进行。 银河系典型的分子云倾向于形成嵌入式星团，这些星团随后演化为开放星团。 星协最初相当不规则，但由于松弛过程，在大约一个动力学时间尺度内失去了它们的不规则性。 在银河系中不存在但在星暴星系中典型的密集分子云会形成大质量的年轻恒星团。 它们在银河系中确实很少见。 我们的模型表明从分子云到开放星团和星协，或者到大质量恒星团存在一条明显的演化路径。 两种演化路径的质量-半径关系与观测结果非常吻合。 根据我们的计算，开放星团和星协的半质量半径的时间演化遵循$r_{\rm h}/{\rm pc}=2.7(t_{\rm age}/{\rm pc})^{2/3}$，而大质量恒星团则遵循$r_{\rm h}/{\rm pc}=0.34(t_{\rm age}/{\rm Myr})^{2/3}$。 这两种趋势与银河系中星团的观测年龄-质量-半径关系一致。 显示英文摘要

摘要： Recent observations have revealed a variety of young star clusters, including embedded systems, young massive clusters, and associations. We study the formation and dynamical evolution of these clusters using a combination of simulations and theoretical models. Our simulations start with a turbulent molecular cloud that collapses under its own gravity. The stars are assumed to form in the densest regions in the collapsing cloud after an initial free-fall times of the molecular cloud. The dynamical evolution of these stellar distributions are continued by means of direct $N$-body simulations. The molecular clouds typical for the Milky Way Galaxy tend to form embedded clusters which evolve to resemble open clusters. The associations were initially considerably more clumpy, but lost their irregularity in about a dynamical time scale due to the relaxation process. The densest molecular clouds, which are absent in the Milky Way but are typical in starburst galaxies, form massive young star clusters. They indeed are rare in the Milky Way. Our models indicate a distinct evolutionary path from molecular clouds to open clusters and associations or to massive star clusters. The mass-radius relation for both types of evolutionary tracks excellently matches the observations. According to our calculations the time evolution of the half-mass radius for open clusters and associations follows $r_{\rm h}/{\rm pc}=2.7(t_{\rm age}/{\rm pc})^{2/3}$, whereas for massive star clusters $r_{\rm h}/{\rm pc}=0.34(t_{\rm age}/{\rm Myr})^{2/3}$. Both trends are consistent with the observed age-mass-radius relation for clusters in the Milky Way.