1] Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Hai Dian District, Beijing 100871, China [2] Department of Astronomy, Peking University, Yi He Yuan Lu 5, Hai Dian District, Beijing 100871, China [3] Key Laboratory for Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100012, China.
1] Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Lu 5, Hai Dian District, Beijing 100871, China [2] Department of Astronomy, Peking University, Yi He Yuan Lu 5, Hai Dian District, Beijing 100871, China.
Nature. 2014 Dec 18;516(7531):367-9. doi: 10.1038/nature13969.
Stars spend most of their lifetimes on the main sequence in the Hertzsprung-Russell diagram. The extended main-sequence turn-off regions--containing stars leaving the main sequence after having spent all of the hydrogen in their cores--found in massive (more than a few tens of thousands of solar masses), intermediate-age (about one to three billion years old) star clusters are usually interpreted as evidence of internal age spreads of more than 300 million years, although young clusters are thought to quickly lose any remaining star-forming fuel following a period of rapid gas expulsion on timescales of order 10(7) years. Here we report, on the basis of a combination of high-resolution imaging observations and theoretical modelling, that the stars beyond the main sequence in the two-billion-year-old cluster NGC 1651, characterized by a mass of about 1.7 × 10(5) solar masses, can be explained only by a single-age stellar population, even though the cluster has a clearly extended main-sequence turn-off region. The most plausible explanation for the existence of such extended regions invokes a population of rapidly rotating stars, although the secondary effects of the prolonged stellar lifetimes associated with such a stellar population mixture are as yet poorly understood. From preliminary analysis of previously obtained data, we find that similar morphologies are apparent in the Hertzsprung-Russell diagrams of at least five additional intermediate-age star clusters, suggesting that an extended main-sequence turn-off region does not necessarily imply the presence of a significant internal age dispersion.
恒星在赫罗图的主序带上度过其大部分生命。在大质量（超过几十万太阳质量）、中等年龄（约一到三十亿年）的星团中，发现了延伸的主序结束区域——其中包含核心中的所有氢都已耗尽的恒星，它们通常被解释为存在超过 3 亿年的内部年龄分布的证据，尽管人们认为年轻的星团在经历了约 10^7 年的快速气体驱逐之后，会迅速失去任何剩余的恒星形成燃料。在这里，我们根据高分辨率成像观测和理论模型的组合报告，在两亿岁的星团 NGC 1651 中，其质量约为 1.7×10^5 个太阳质量，超出主序带的恒星仅可以用单一年龄的恒星种群来解释，尽管该星团有一个明显延伸的主序结束区域。存在这种延伸区域的最合理解释是存在一群快速旋转的恒星，尽管与这种恒星种群混合相关的延长恒星寿命的次要影响目前还不太清楚。从之前获得的数据的初步分析中，我们发现至少另外五个中等年龄星团的赫罗图中也存在类似的形态，这表明延伸的主序结束区域不一定意味着存在显著的内部年龄分散。
