星团中的扩展主序带
Extended Main Sequences in Star Clusters
AI总结：
本文综述了年轻和中年星团中扩展主序带与扩展主序转折点的观测特征、形成机制及天体物理意义，指出恒星自转是解释这些现象的最合理机制，并讨论了相关研究对恒星磁制动、潮汐锁定双星和磁星形成等问题的启示。
摘要：
扩展主序带（eMSs）和扩展主序转折点（eMSTOs）是星团中经常观测到的引人入胜的现象。这些现象对当前星团“简单恒星族”的标准图景提出了强烈挑战，该图景假定星团是同时诞生且化学均匀的，因此可以用单一、唯一的等龄线来描述。eMSs 和 eMSTOs 的探测为恒星物理学和星团演化提供了宝贵的见解。本综述深入探讨了在年轻（小于6亿年）和中年（6亿至20亿年）星团中观测到的 eMSs 和 eMSTOs 的观测特征、内在机制以及天体物理学意义。目前已提出若干情景或假说来解释这些现象，包括年龄弥散的存在、双星相互作用、变星以及恒星自转速率差异。本综述讨论了当前模型的优势与局限。在现有模型和假说中，恒星自转已被证明是解释 eMSs 和 eMSTOs 出现的最合理机制。关于恒星自转及其与 eMSs 联系的研究开辟了大量引人入胜的途径，例如对恒星磁制动机制的探索、对星团中潮汐锁定双星系统的搜寻，以及双星并合是否可能产生大质量磁星的研究。这些工作已取得宝贵见解，并显著丰富了我们对恒星天体物理学的理解。
英文摘要：
Extended main sequences (eMSs) and extended main-sequence turnoffs (eMSTOs) are fascinating phenomena that are routinely observed in star clusters. These phenomena strongly challenge the current canonical "simple stellar population" picture of star clusters, which postulates that star clusters are coeval and chemically homogeneous and can thus be described by a single, unique isochrone. Detections of eMSs and eMSTOs provide valuable insights into stellar physics and the evolution of star clusters. This comprehensive review delves into the observational characteristics, underlying mechanisms, and astrophysical implications of the eMSs and eMSTOs observed in young (less than 600 million years) and intermediate-age (600 to 2000 million years) star clusters. Several scenarios or hypotheses have been proposed to explain these phenomena, including the presence of an age spread, binary interactions, variable stars, and differences in stellar rotation rates. This review discusses the advantages and limitations of current models. Among contemporary models and hypotheses, stellar rotation has been demonstrated as the most plausible mechanism to explain the occurrence of eMSs and eMSTOs. Research on stellar rotation and its connection to eMSs has opened up a myriad of fascinating avenues, such as investigations of the magnetic braking mechanism in stars, searches for tidally locked binary systems in star clusters, and investigations as to whether binary mergers can give rise to massive magnetars. These endeavors have yielded valuable insights and significantly enriched our understanding of stellar astrophysics.
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