Carnegie Observatories, 813 Santa Barbara Street, Pasadena, California 91101, USA.
Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA.
Nature. 2016 Jan 14;529(7585):181-4. doi: 10.1038/nature16168. Epub 2016 Jan 4.
A knowledge of stellar ages is crucial for our understanding of many astrophysical phenomena, and yet ages can be difficult to determine. As they become older, stars lose mass and angular momentum, resulting in an observed slowdown in surface rotation. The technique of 'gyrochronology' uses the rotation period of a star to calculate its age. However, stars of known age must be used for calibration, and, until recently, the approach was untested for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for old field stars whose ages have been determined with asteroseismology. The data for the cluster agree with previous period-age relations, but these relations fail to describe the asteroseismic sample. Here we report stellar evolutionary modelling, and confirm the presence of unexpectedly rapid rotation in stars that are more evolved than the Sun. We demonstrate that models that incorporate dramatically weakened magnetic braking for old stars can--unlike existing models--reproduce both the asteroseismic and the cluster data. Our findings might suggest a fundamental change in the nature of ageing stellar dynamos, with the Sun being close to the critical transition to much weaker magnetized winds. This weakened braking limits the diagnostic power of gyrochronology for those stars that are more than halfway through their main-sequence lifetimes.
恒星年龄的知识对于我们理解许多天体物理现象至关重要，但年龄的确定却很困难。随着恒星年龄的增长，它们会失去质量和角动量，导致表面自转速度观测到的减缓。“星旋计时学”技术利用恒星的自转周期来计算其年龄。然而，该方法需要已知年龄的恒星进行校准，并且直到最近，该方法才在古老恒星（年龄超过 10 亿年）上进行了测试。现在已经知道了一个中等年龄的疏散星团（NGC 6819；25 亿年）中恒星的自转周期，以及一些通过星震学确定了年龄的古老场星的自转周期。该星团的数据与以前的周期-年龄关系一致，但这些关系无法描述星震学样本。在这里，我们报告了恒星演化模型，并证实了比太阳更成熟的恒星中存在出乎意料的快速自转。我们证明，与现有模型相比，那些纳入了对古老恒星显著削弱的磁场制动的模型，可以重现星震学和星团的数据。我们的发现可能表明老化恒星发电机性质发生了根本性变化，太阳接近向磁化风弱得多的关键转变。这种减弱的制动限制了星旋计时学对那些已经度过主序星寿命一半以上的恒星的诊断能力。
