Star Cluster Evolution: Near and Far, Large and Small, Old and Young

星团是具有相同距离、年龄和化学成分的恒星的自引力群。恒星是在星团环境中自然形成的，但其中只有几个星团会从它们诞生的分子云中成为一个自给自足的、受引力约束的实体。然而，恒星形成过程中近邻的存在对可能形成的恒星的质量和多样性以及行星系统的生存能力有着重要的影响。在这些星系团中，确实出现了明显可识别的物体，其中一些将成为巨大的老球状星团。它们在整个星系形成过程中幸存了下来，包括两个星系合并的时候，或者一个矮小星系落入更大的宿主星系的时候。我们可以利用星团的今天的性质(大小、质量、位置、速度、恒星含量)来绘制过去100亿年的星系结构和组装图。 但为了利用星系团来了解恒星形成、行星形成或星系演化，我们需要了解星系团本身是如何形成和随时间变化的。恒星动力学(恒星如何在相互引力下运动)的成熟领域提供了许多答案。然而，仅有恒星动力学是不够的。还必须考虑恒星和星团与其外部环境之间的相互作用，如恒星形成的气体，或星团所在星系的潮汐磁场。该提案描述了一个将解决这些问题的研究计划。未来五年，我们将集中抓好三个方面的具体工作： 1)同时和自洽地模拟星团的形成和演化，它们的气体含量，以及它们的恒星群，从分子云中深度嵌入的阶段开始，产生我们今天在银河系中看到的球形、无气体、开放和球状的星团。 2)追踪球状星团和核星团在其所在星系中的动力学演化，包括星系合并事件。特别是，我们将量化在单个星系团和星系团群体上作为一个整体的任何观测到的增长和合并的特征。 3)预测在宇宙非常早期形成的今天巨大的古老星系团的预期观测特征。 通过结合恒星演化、恒星动力学和流体力学的模拟，我们将在多个时间、大小和距离尺度上绘制星系团形成和演化的综合图景。

Star clusters are self-gravitating groups of stars with the same distance, age, and chemical composition. Stars are naturally formed in clustered environments, but only a few of those clusters will emerge as a self-contained, gravitatationally-bound entity from their natal molecular cloud. Nevertheless, the presence of close neighbours during the star formation process has important consequences for the masses and multiplicity of the stars that can form, and the survivability of planetary systems. Of those clusters that do emerge as clearly-identifiable objects, some will become massive old globular clusters. They have survived the entire galaxy formation process, including those times when two galaxies merged, or when a dwarf galaxy falls into its larger host galaxy. We can use the present-day properties of star clusters (size, mass, position, velocities, stellar content) to map galaxy structure and assembly over the last 10 billion years. But in order to use clusters to learn about star formation, planet formation or galaxy evolution, we need to understand how the clusters themselves form and change with time. The mature field of stellar dynamics (how stars move under their mutual gravity) provides many of the answers. However, stellar dynamics alone is not sufficient. It is important to also consider the interaction between the stars and star cluster with their external environment, such as the gas from which the stars formed, or the tidal field of the galaxy in which the cluster resides. The proposal describes a research program that will address these issues. In the next five years, we will concentrate on three specific projects: 1) to simultaneously and self-consistently model the formation and evolution of star clusters, their gas content, and their stellar populations, starting from the deeply embedded phase in molecular clouds, to produce the spherical, gas-free, open and globular clusters we see in the Milky Way today. 2) to trace the dynamical evolution of globular clusters and nuclear star clusters in their host galaxy, including episodes of galaxy mergers. In particular, we will quantify any observational signatures of accretion and merging on the individual clusters and the cluster populations as an ensemble. 3) to predict the expected observational signatures of the present-day massive old clusters as they are forming in the very early universe. Through simulations that combine stellar evolution, stellar dynamics, and hydrodynamics, we will produce a comprehensive picture of cluster formation and evolution on many scales of time, size, and distance.