Collaborative Research: Rethinking the Fundamentals of Massive Star Clusters

球状星团是由数十万颗恒星通过重力捆绑在一起的紧密星团。有数百个球状星团围绕着我们所在的银河系运行。球状星团是追踪宇宙最早条件的化石，当时星系刚刚开始形成。在单个球状星团之间观察到了有趣的差异，这为我们银河系的形成提供了线索。在球状星团恒星中，属于不同化学元素的原子浓度不同，挑战了关于恒星的年龄、起源和生命周期的传统智慧。研究人员将使用一些世界上最大和设备最好的望远镜进行观测，以测试恒星、星团和星系如何形成的理论模型。主要工作包括：(A)建立星团之间自我丰富的统一场景；(B)通过新的动力学和恒星人口调查了解球状星团的恒星含量；以及(C)研究球状星团和处于球状星团质量函数极端上限的星系之间的关系。将进行新的光学和近红外光度和光谱观测，以探索可测量量(如质光比和丰度异常)如何依赖于年龄、金属丰度、恒星初始质量函数和动力学演化。许多有趣的趋势，其中许多混淆了标准解释，已经被发现：1)银河系球状星系团中的丰度扩散和在河外球状星系团系统中发现的质量-金属丰度关系；2)观察到的星系恒星晕的性质，以及中年星系团气体的初始测量，每一个都挑战了有利于自富化的机制；3)质量/光比对金属丰度的依赖与恒星演化模型的预期相反；4)已经确定了一个包括超致密矮星在内的致密恒星系统的不断扩大的动物园，它可能是由矮星系的潮汐剥离形成的，也可能是球状星团合并形成的；5)超致密矮星的质量光比升高可以用大质量的中央黑洞来解释。研究人员将澄清这些问题，测试恒星演化和群体合成模型，并追踪致密恒星系统的起源。由于球状星团比以前认识的更复杂，了解它们的性质以及它们与其他类型恒星系统的关系在天体物理学中具有广泛的意义；从球状星团得出的推断为恒星、星系、黑洞和暗物质的形成和演化提供了理论依据。这项基础性工作影响了从恒星形成到宇宙学的前沿研究。为了有效地促进这项研究的更广泛影响，将继续与非营利性组织“科学伙伴”建立伙伴关系。这一非常成功的试点计划已经吸引了数千名来自不同背景的K-12学生，包括弱势群体。一项积极的专业和公共宣传计划将继续下去，在将女性、本科生和少数族裔纳入前沿研究方面的记录得到了证实。将扩大让本科生参与大规模集群研究的成功试点计划。学生和博士后将接受优秀的培训，获得世界级的设施和高级研究人员，参与研究过程的方方面面。一份用户友好的新发现的致密恒星系统及其基本性质的遗留目录将在网上公开提供。

A Globular Cluster is a tight group of hundreds of thousands of stars bound together by gravity. There are hundreds of Globular Clusters orbiting around our own Milky Way galaxy. Globular Clusters serve as fossils that trace conditions in the universe at the earliest times, when galaxies were just starting to form. Intriguing differences are observed between individual Globular Clusters, which provide clues about the formation of our Galaxy. In Globular Cluster stars, the concentrations of atoms belonging to different chemical elements vary in ways that challenge conventional wisdom about the ages, origins and life cycles of stars. The investigators will make observations with some of the world's largest and best-equipped telescopes to test theoretical models of how stars, star clusters and galaxies come to be.The principal work involves (a) developing a unified scenario for self-enrichment among star clusters; (b) understanding the stellar content of Globular Clusters through novel dynamical and stellar population investigations; and (c) studying the relationship between Globular Clusters and galaxies at the extreme upper limit of the Globular Cluster mass function. New optical and near-infrared photometric and spectroscopic observations will be made to explore how measurable quantities (such as mass-to-light ratios and abundance anomalies) depend on age, metallicity, stellar initial mass function, and dynamical evolution. Intriguing trends, many of which confound standard interpretations, have already been identified: 1) self-enrichment appears to be implicated in abundance spreads in Galactic Globular Clusters and in a mass-metallicity relation found in extragalactic Globular Cluster systems; 2) observed properties of galaxy stellar halos, and initial measurements of gas in intermediate-age clusters, each challenge the favored mechanism for self-enrichment; 3) the dependence of mass-to-light ratio on metallicity is opposite to that expected from stellar evolution models; 4) an expanding zoo of compact stellar systems has been identified, including ultra compact dwarfs, that could be formed either by tidal stripping of dwarf galaxies or from Globular Cluster mergers; 5) the elevated mass-to-light ratios of ultra compact dwarfs may be explained by massive central black holes. The investigators will clarify each of these issues, test stellar evolution and population synthesis models, and trace the origins of compact stellar systems.Since Globular Clusters are more complex than formerly realized, understanding their properties and how they relate to other types of stellar systems has wide implications in astrophysics; inferences drawn from Globular Clusters inform theories of the formation and evolution of stars, galaxies, black holes, and dark matter. This fundamental work impacts forefront research from star formation to cosmology. To promote the broader impacts of this research effectively, the partnership with the non-profit organization, "Science Buddies", will be continued. This highly successful pilot program has already engaged thousands of K-12 students from diverse backgrounds, including disadvantaged groups. A vigorous program of professional and public outreach will continue, with a proven record of including women, undergraduates, and minorities in forefront research. A successful pilot program to involve undergraduates in Massive Clusters research will be expanded. Students and post-docs will receive excellent training with access to world-class facilities and to senior researchers, participating in all aspects of the research process. A user-friendly, legacy catalog of newly discovered compact stellar systems and their fundamental properties will be publicly available online.