Rethinking the dynamical paradigm of low-mass stellar systems

所有大型星系，包括我们的银河系，都包含数百个“球状星团”。每颗恒星都是一个由大约100万颗恒星组成的密集群体，它们通过相互的引力聚集在一起。从数学家的角度来看，它们对应于一个根深蒂固的问题，即理解N个物体在远程力作用下的运动。通过天文学家的望远镜，它们似乎是古代宇宙的局部遗迹，因为它们是星系形成初期出现的首批恒星结构之一。观测天文学家已经研究星团几十年了，但近年来，这些观测的质量和范围已经超出了现有理论的范围。特别是，欧洲空间天文台盖亚正在以前所未有的精度测量银河系球状星团中数千颗恒星的位置和速度。这新一代的数据，加上哈勃太空望远镜和其他最先进的和即将到来的天文设施的测量，使理论家能够通过研究单个恒星的位置和速度，第一次探索星团的完整“相空间”。理论学家的模型是基于简化的假设，目前几乎总是包括这样的观点：星团是球形的，不旋转，只由恒星组成——所有的恒星都是同时诞生的。最近的三项进展正在破坏这些假设，并要求我们如实对待星团，而不是我们希望它们成为的样子：(1)认识到恒星的运动比我们预期的要复杂得多，它们的内部旋转是规律而不是例外；（ii）经验证据表明，星团中的恒星并不都是在一个单一的种群中同时诞生的（这一假设曾经是星团定义的一部分）；（三）最近发现了许多“混合”恒星系统，这使得现在很难追踪“星团”和“矮星系”之间的区别。此外，星团最近被认为是小型“恒星质量”黑洞的丰富摇篮，长期以来，人们一直认为它们是大型“中等质量”黑洞的理想诞生地。这类天体目前尚未被发现，是宇宙黑洞群中缺失的关键一环。但是像激光干涉仪引力波天文台和神冈引力波探测器这样的新设备将很快帮助我们从一个新的角度解决这个开放的问题。通过数学技术和数值模拟的结合，我的目标是提出一个更真实的星团动力学描述，来解释这些新一代的天文数据。因此，这个研究计划将使我能够解决现代天体物理学中的三个开放问题：宇宙中最古老的恒星的起源，宇宙黑洞人口中可能存在的“缺失环节”，以及小恒星系统中不可见的“暗物质”存在的极限。因此，对这些有趣的恒星系统的历史和进化的基本理解将为星系的第一个组成部分和银河系的起源提供一个新的视角。

All large galaxies, including our own Milky Way, contain hundreds of 'globular clusters'. Each is a dense group of about a million stars held together by their mutual gravitational attraction. Through a mathematician's eye, they correspond to the deep-rooted problem of understanding the motion of N bodies interacting by long-range forces. Through an astronomer's telescope, they appear as local relics of the ancient universe, as they were among the first stellar structures to emerge at the dawn of the formation of galaxies. Observational astronomers have studied star clusters for decades, but in recent years the quality and range of these observations has leapt beyond the range of existing theory. In particular, the European space observatory Gaia is now measuring the positions and the velocities of thousands of stars in globular clusters of our Galaxy, with unprecedented accuracy. This new generation of data, coupled with measurements by the Hubble Space Telescope and other state-of-the-art and forthcoming astronomical facilities, enable theorists to explore, for the first time, the full 'phase space' of star clusters by studying both the position and velocity of their individual stars.Theorists' models are based on simplifying assumptions, and at present almost always include the ideas that the clusters are spherical, do not rotate, and are composed only of stars - all born at the same time. Three recent developments are undermining these assumptions and requiring us to treat the clusters as they are, and not as we would wish them to be: (i) the realisation that the motions of their stars is much more complex than we expected and that their internal rotation is the rule rather than the exception; (ii) the empirical evidence that the stars in clusters were not all born at once in a single population (an assumption which was once part of their very definition); and (iii) the recent discovery of a numerous 'hybrid' stellar systems which now makes arduous to trace the distinction between 'star clusters' and 'dwarf galaxies'. In addition, star clusters have recently been recognised as prolific cradles for small, 'stellar-mass', black holes and they have been long speculated to be the ideal birth site of larger, 'intermediate-mass', ones. Currently undetected, this class of astrophysical objects is a crucial missing link in the population of cosmic black holes. But new facilities such as the Laser Interferometer Gravitational-Wave Observatory and the Kamioka Gravitational Wave Detector will soon help us tackle this open problem from a new perspective. With a combination of mathematical techniques and numerical simulations, I aim to propose a more realistic description of the dynamics of star clusters, to interpret such new-generation astronomical data. As a result, this research programme will allow me to address three open problems in modern astrophysics: the origin of stars which are among the oldest in the universe, the possible existence a 'missing link' in the population of cosmic black holes, and the limits of the presence of invisible 'dark' matter in small stellar systems.A fundamental understanding of the history and evolution of these intriguing stellar systems will, therefore, provide a new perspective on the first building blocks of galaxies and on the origin of our Milky Way.