Rethinking the dynamical paradigm of low-mass stellar systems

所有大型星系，包括我们自己的银河系，都包含数百个“球状星团”。每一颗恒星都是由大约100万颗恒星组成的密集星团，它们通过相互引力而聚集在一起。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个物体在长程力作用下的运动。通过天文学家的望远镜，它们看起来像是古代宇宙的当地遗迹，因为它们是星系形成初期出现的第一批恒星结构之一。观测天文学家对星团的研究已经有几十年了，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现在正在以前所未有的精度测量我们银河系球状星团中数千颗恒星的位置和速度。新一代的数据，再加上哈勃太空望远镜和其他最先进的天文设施的测量，使理论家能够首次通过研究各自恒星的位置和速度来探索星团的完整相空间。理论家的模型基于简化的假设，目前几乎总是包括星团是球形的，不旋转，并且只由恒星组成--所有这些都是在同一时间诞生的。最近的三个发展正在破坏这些假设，并要求我们按原样对待星系团，而不是我们希望的那样：(I)意识到它们的恒星的运动比我们预期的要复杂得多，它们的内部自转是规则而不是例外；(Ii)经验证据表明，星系团中的恒星并不都是同时在一个星团中诞生的(这一假设曾经是它们定义的一部分)；(Iii)最近发现了大量的“混合”恒星系统，这使得现在很难追踪“星团”和“矮小星系”之间的区别。此外，星团最近被认为是多产的小质量黑洞的摇篮，长期以来一直被认为是较大的中等质量黑洞的理想诞生地。目前未被发现的这类天体是宇宙黑洞数量中一个关键的缺失环节。但新的设施，如激光干涉仪引力波天文台和神冈引力波探测器，将很快帮助我们从一个新的角度解决这个悬而未决的问题。通过数学技术和数值模拟的结合，我的目标是对星团的动力学提出更现实的描述，以解释这些新一代天文数据。因此，这项研究计划将使我能够解决现代天体物理学中的三个悬而未决的问题：宇宙中最古老的恒星的起源，宇宙黑洞群体中可能存在的一种“缺失的一环”，以及小型恒星系统中不可见的“暗”物质的存在的限度。因此，对这些有趣的恒星系统的历史和演化的基本理解将为星系的最初组成部分和我们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.