发表机构
American University of Sharjah; Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Padova; Dipartimento di Fisica e Astronomia “Galileo Galilei”, Univ. di Padova; Dipartimento di Tecnica e Gestione dei Sistemi Industriali, Università degli Studi di Padova; NSF’s NOIRLab（沙迦美国大学; 意大利国家天体物理研究所-帕多瓦天文台; 帕多瓦大学伽利略·伽利莱物理与天文学系; 帕多瓦大学工业系统技术与管理系; 美国国家科学基金会NOIR实验室）
机构由 AI 辅助整理，请以论文原文为准。AI 中文总结
本研究结合测光与APOGEE光谱分析M22的恒星族群，发现异常恒星在外部区域占优，其化学增强与轻元素相关，支持自增丰稀释模型而非合并起源。
AI 中文摘要
球状星团（GCs）以拥有表现出轻元素变化的恒星族群而闻名。在银河系球状星团的一个子集中，除了此类“典型”族群外，还包含一个额外富集铁、慢中子俘获过程元素以及总碳氮氧（C+N+O）丰度的群体（即异常恒星）。我们结合多设施测光数据与APOGEE光谱，研究M22中各恒星族群的空间分布、化学性质及形成历史，特别关注其异常成分。我们从星团中心追踪典型与异常族群及其亚族群，直至八个半质量半径处。异常族群在最外层区域日益占据主导地位，而其亚族群之间未检测到显著的径向梯度。我们的化学分析揭示了典型与异常成分内部均存在轻元素反相关，尽管后者向更高的碳（C）、氮（N）和铝（Al）丰度偏移。我们首次表明，异常恒星中铁（Fe）、慢中子俘获过程元素及C+N+O的增强并非均匀一致，而是与其轻元素组成相关：化学上最极端的异常恒星同时也是Fe、Ce最富集且C+N+O最贫乏的。我们在水平分支上识别出一个明显的红色过密区域，可能由最贫氦（He）的恒星占据，并初步将极端水平分支与化学上最富集的异常族群联系起来。这些观测难以与M22是两个球状星团合并的结果相调和。相反，它们定性支持一种由稀释作用调控的自增丰情景，类似于近期为$\omega$Centauri提出的机制，且两者密切的化学对应性表明它们经历了类似的形成历史。
英文摘要
Globular clusters (GCs) are well known to host stellar populations characterized by light-element variations. A subset of Galactic GCs, beyond such 'canonical' populations, contains an additional group enriched in iron, s-process elements, and total C+N+O abundance (i.e., the anomalous stars). We combine multi-facility photometry with APOGEE spectroscopy to investigate the spatial distribution, chemical properties, and formation history of the stellar populations in M22, with particular focus on its anomalous component. We trace the canonical and anomalous populations, together with their subpopulations, from the cluster center out to eight half-mass radii. The anomalous population becomes increasingly dominant in the outermost regions, whereas no significant radial gradients are detected among its subpopulations. Our chemical analysis reveals light-element anticorrelations within both the canonical and anomalous components, although the latter are shifted toward higher C, N, and Al abundances. For the first time, we show that the Fe, s-process, and C+N+O enhancements among anomalous stars are not uniform but correlate with their light-element composition: the most chemically extreme anomalous stars are also the most Fe-, Ce-rich, and C+N+O-poor. We identify a distinct red overdensity on the horizontal branch, likely populated by the most He-poor stars, and tentatively associate the extreme horizontal branch with the most chemically enriched anomalous population. These observations are difficult to reconcile with M22 being a merger between two GCs. Instead, they qualitatively favor a self-enrichment scenario regulated by dilution, similar to that recently proposed for $ω$Centauri, with their close chemical correspondence suggesting that they experienced analogous formation histories.
CommentsAccepted for publication on AJ
