宇宙中的致密星团——动力学、多星族与引力波研究

Dense star clusters are spectacular and distinguished self-gravitating stellar systems in our Galaxy and across the Universe - in many respects. They populate disks and spheroids of galaxies (often there are many more than the just 150 globular clusters which belong to our own Galaxy) as well as the empty space between galaxies in clusters. The evolution of dense star clusters is not only governed by the aging of their stellar populations and simple Newtonian dynamics. The stellar densities become so high that stars can interact and collide, stellar evolution and binary stars change the dynamical evolution, black holes can accumulate in their centers and merge with relativistic effects becoming important. Recent high-resolution imaging has revealed even more complex structural properties with respect to multiple stellar populations, binary fractions and compact objects as well as - the still controversial - existence of intermediate mass black holes in clusters of intermediate mass. From simulations it has become clear that they harbor a few hundred or more stellar mass black holes - some of them give rise to relativistic mergers and gravitational wave emission just like the one recently observed by LIGO. Dense star clusters therefore are the ideal laboratory for the study of stellar evolution and Newtonian as well as relativistic dynamics. .The purpose of this project is the detailed investigation of dynamics and evolution of globular clusters - as most important new feature we will study the coupled evolution of first generation (FG) and second generation (SG) stars in globular clusters (GCs) in the framework of an in situ formation of both populations. The initial concentration of SG stars at the centre of the GC, suggested by recent formation theories and supported by observations, will have a significant impact in the evolution and general properties of the GC and in the mixing of the stellar populations. For this study we will perform a large set of direct summation N-body simulations using different parameters for the initial distribution and fraction of FG and SG stars, for the fraction of binaries among FG and SG stars and for the initial rotation of the FG and SG components. In this context we will study the early as well as the long-term evolution and spatial mixing of FG and SG stars and the distribution and evolution of binary systems (including stellar mass binary black holes – potential gravitational wave (GW) sources) in the GC. These detailed models will be calibrated by and compared with star-by-star observations of globular clusters in the Milky Way and the Local Group; radio astronomy will help to constrain the number of neutron stars remaining in the cluster. In the following a larger set of models for extragalactic globular clusters will be simulated to predict integrated light and gravitational wave emission from binary black holes.

致密星团是我们星系以至整个宇宙中普遍存在的一种壮观的自引力恒星系统。其演化并非简单地只依赖于星族年龄和单纯的牛顿动力学。极高的恒星密度使得恒星之间会相互作用甚至碰撞。恒星的演化与双星系统也会改变其动力学演化。而黑洞也会在其中心区域聚集合并同时伴有很多相对论效应产生。当然，还有仍然存在争议的中等质量黑洞。数值模拟清楚地表明星团中普遍藏有几百个甚至更多的恒星质量黑洞。它们中的一些会产生相对论性合并及引力波辐射，正如最近LIGO所发现的一样。因此致密星团是研究恒星演化、牛顿力学以及相对论的理想实验室。..本项目计划深入研究球状星团的动力学和演化。其中最重要的创新在于研究第一代（FG）与第二代（SG）恒星在球状星团（GCs）内的耦合演化。申请人及其合作者准备进行大量采用不同参数的直接叠加N体数值模拟，以便研究GC中FG与SG恒星的早期和长期演化、空间混合以及双星系统的分布和演化，并与观测结果进行比

稠密星团的演化是一个横跨多个尺度，涉及多种物理过程的问题。它并非简单地只受恒星之间的引力相互作用及其星族年龄演化的支配。在稠密星团中，恒星的密度可以高至令其相互作用与碰撞，恒星的演化与双星的存在也能改变星团的动力学演化，黑洞还可以在其中心积累增长并在与其它天体并合的过程中产生重要的相对论效应。 .最近的高分辨率成像揭开了稠密星团更多更复杂的结构特征，如多个星族的存在、双星比例与致密天体、以及（仍然有争议的）中等质量星团中存在的中等质量黑洞。从模拟中可以清楚地看到，在这类星团中隐藏了数百个甚至更多的恒星质量黑洞——其中一些导致了相对论并合与引力波辐射，就像LIGO最近观测到的那样。.在本项目中，我们通过引力直接N体模拟对稠密星团进行了建模。其中考虑了存在两个星族所带来的影响、大量原初近距双星、单星和双星的恒星演化、恒星或致密天体（黑洞）的相互碰撞等。我们已经着手建立一个公共数据库，其中包含通过模拟获得的所有数据，如恒星的位置、速度、质量、恒星类型以及双星参数等。 .本项目的主要结果是（i）通过第一代和第二代恒星在星团中的长期模拟，研究了两代恒星的单星与双星是如何在星团中分布的，以及如何与麦哲伦星系那样的可观测系统进行比较；（ii）将我们在模拟中使用的恒星演化模型全面升级到最高的水平，例如包括不稳定对超新星的形成、电子捕获超新星形成时新的反冲速度、更新后的质量损失和恒星碰撞模型，以及其它很多类似的改进；（iii）得到了一组非常密集的星团模型，该模型首次在完全自洽的模拟中演示了中等质量黑洞的形成。 .正如在申请书和报告中所述，本项目很大程度上依赖于国际合作；所得的代码和数据均由我们国家天文台的数据库存储和发布。