疏散星团深MMT凌日巡天M37 III：5.5亿年恒星自转
Deep MMT Transit Survey of the Open Cluster M37 III: Stellar Rotation at 550 Myr
摘要：
在对年龄约5.5亿年的疏散星团M37进行深（14.5 ~< r ~< 23）20夜凌日行星巡天的过程中，我们测量了575颗位于星团主序附近质量为0.2 Msun ~< M ~< 1.3 Msun的恒星的自转周期。这是年龄超过5亿年的星团的最大旋转周期样本。使用这个丰富的样本，我们调查的旋转周期，颜色和光度变化的幅度之间的关系。M >~ 0.8 Msun的恒星在周期和质量之间表现出紧密的相关性，较重的恒星旋转得更快。有一组4颗恒星的P > 15天，远高于这个关系，如果这是真的，将对恒星角动量演化理论提出重大挑战。在0.8 Msun以下，恒星继续遵循周期-质量相关，但有一个快速旋转的宽尾巴，随着质量的减少而扩展到更短的周期。我们将这些结果与其他疏散星团的观测结果相结合，以检验低主序星角动量演化的标准理论。我们发现，该模型再现了太阳质量恒星的观测结果，但对于0.6 ~< M ~<1.0Msun的恒星，模型的结果与观测结果有明显的差异。我们还发现，对于晚K到早M的矮星在这个集群中的快速旋转往往是蓝色的比慢旋转在B-V，但红色比慢旋转在V- I_{C}。这一结果支持了低质量主序星的温度和半径的观测值与预测值之间的显著差异是由于恒星活动的假设。
英文摘要：
In the course of conducting a deep (14.5 ~< r ~< 23), 20 night survey for transiting planets in the rich ~550 Myr old open cluster M37 we have measured the rotation periods of 575 stars which lie near the cluster main sequence, with masses 0.2 Msun ~< M ~< 1.3 Msun. This is the largest sample of rotation periods for a cluster older than 500 Myr. Using this rich sample we investigate a number of relations between rotation period, color and the amplitude of photometric variability. Stars with M >~ 0.8 Msun show a tight correlation between period and mass with heavier stars rotating more rapidly. There is a group of 4 stars with P > 15 days that fall well above this relation, which, if real, would present a significant challenge to theories of stellar angular momentum evolution. Below 0.8 Msun the stars continue to follow the period-mass correlation but with a broad tail of rapid rotators that expands to shorter periods with decreasing mass. We combine these results with observations of other open clusters to test the standard theory of lower-main sequence stellar angular momentum evolution. We find that the model reproduces the observations for solar mass stars, but discrepancies are apparent for stars with 0.6 ~< M ~< 1.0 Msun. We also find that for late-K through early-M dwarf stars in this cluster rapid rotators tend to be bluer than slow rotators in B-V but redder than slow rotators in V-I_{C}. This result supports the hypothesis that the significant discrepancy between the observed and predicted temperatures and radii of low-mass main sequence stars is due to stellar activity.
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