前主序星的自转和活动
Rotation and activity of pre-main-sequence stars
摘要：
我们提出了一个研究旋转（VSINI）和色球活动（Halpha EW）的基础上获得的高分辨率光谱与MIKE上6.5米麦哲伦粘土望远镜的广泛的一套。我们的目标是年轻的恒星协会Eta Cha，TW Hydrae，Beta Pic和Tuc-Hor中的74颗F-M矮星，年龄从6到30百万年。虽然大多数F星和G星的H α EW与纯光球吸收一致，但大多数K星和M星显示色球发射。通过比较我们样本中的Halpha EW与文献中的结果，我们看到了一个清晰的演化序列：色球活动从金牛座T阶段到主序列稳步下降。使用活动作为一个年龄指标，我们发现一个合理的年龄范围为10-40万年的图克-霍尔协会。在500万年到3000万年之间，我们没有在总样本中看到旋转制动的证据，因此角动量是守恒的，与年轻的恒星相反。这种差异表明在5-10百万年的旋转调节的变化，可能是因为磁盘制动不能运行比典型的磁盘寿命更长，允许对象自旋。在我们的样本中，旋转-活动关系是平坦的;与主序星相反，慢旋转星没有线性相关。我们认为，这是因为年轻恒星产生磁场的方式与主序星完全不同，而不仅仅是饱和的太阳型发电机的结果。通过将我们的自转速度与已发表的恒星子集的自转周期进行比较，我们确定了Eta Cha和TWA星协的年龄分别为1300万年和900万年，与以前的估计一致。由此我们得出结论：Baraffe等（1998）的恒星演化模型的恒星半径与观测半径在± 15%以内是一致的。（节略）
英文摘要：
We present a study of rotation (vsini) and chromospheric activity (Halpha EW) based on an extensive set of high-resolution optical spectra obtained with MIKE on the 6.5m Magellan Clay telescope. Our targets are 74 F-M dwarfs in the young stellar associations Eta Cha, TW Hydrae, Beta Pic, and Tuc-Hor, spanning ages from 6 to 30 Myr. While the Halpha EW for most F and G stars are consistent with pure photospheric absorption, most K and M stars show chromospheric emission. By comparing Halpha EW in our sample to results in the literature, we see a clear evolutionary sequence: Chromospheric activity declines steadily from the T Tauri phase to the main sequence. Using activity as an age indicator, we find a plausible age range for the Tuc-Hor association of 10-40 Myr. Between 5 and 30 Myr, we do not see evidence for rotational braking in the total sample, thus angular momentum is conserved, in contrast to younger stars. This difference indicates a change in the rotational regulation at 5-10 Myr, possibly because disk braking cannot operate longer than typical disk lifetimes, allowing the objects to spin up. The rotation-activity relation is flat in our sample; in contrast to main-sequence stars, there is no linear correlation for slow rotators. We argue that this is because young stars generate their magnetic fields in a fundamentally different way from main-sequence stars, and not just the result of a saturated solar-type dynamo. By comparing our rotational velocities with published rotation periods for a subset of stars, we determine ages of 13 (7-20) Myr and 9 (7-17} Myr for the Eta Cha and TWA associations, respectively, consistent with previous estimates. Thus we conclude that stellar radii from evolutionary models by Baraffe et al. (1998) are in agreement with the observed radii within +-15%. (abridged)
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