Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA.
1] Leibniz Institute for Astrophysics, An der Sternwarte 16, 14482 Potsdam, Germany [2] Space Science Institute, 4750 Walnut Street #205, Boulder, Colorado 80301, USA.
Nature. 2015 Jan 29;517(7536):589-91. doi: 10.1038/nature14118. Epub 2015 Jan 5.
The ages of the most common stars--low-mass (cool) stars like the Sun, and smaller--are difficult to derive because traditional dating methods use stellar properties that either change little as the stars age or are hard to measure. The rotation rates of all cool stars decrease substantially with time as the stars steadily lose their angular momenta. If properly calibrated, rotation therefore can act as a reliable determinant of their ages based on the method of gyrochronology. To calibrate gyrochronology, the relationship between rotation period and age must be determined for cool stars of different masses, which is best accomplished with rotation period measurements for stars in clusters with well-known ages. Hitherto, such measurements have been possible only in clusters with ages of less than about one billion years, and gyrochronology ages for older stars have been inferred from model predictions. Here we report rotation period measurements for 30 cool stars in the 2.5-billion-year-old cluster NGC 6819. The periods reveal a well-defined relationship between rotation period and stellar mass at the cluster age, suggesting that ages with a precision of order 10 per cent can be derived for large numbers of cool Galactic field stars.
常见恒星（如太阳等低质量恒星和更小的恒星）的年龄很难确定，因为传统的定年方法使用的恒星性质要么随着恒星老化变化很小，要么难以测量。所有冷星的自转速率都会随着恒星不断失去角动量而随时间显著降低。如果经过适当校准，自转可以根据天体旋距学方法可靠地确定它们的年龄。为了校准天体旋距学，必须确定不同质量冷星的自转周期和年龄之间的关系，这最好通过对具有已知年龄的星团中的恒星进行自转周期测量来完成。迄今为止，这种测量仅在年龄小于约 10 亿年的星团中可行，而对于较老恒星的天体旋距学年龄则是根据模型预测推断得出的。在这里，我们报告了在 25 亿岁的 NGC 6819 星团中 30 颗冷星的自转周期测量结果。这些周期在星团年龄时揭示了自转周期和恒星质量之间的明确关系，表明可以为大量银河系场恒星提供精度在 10%左右的年龄。
