2007ApJ...662.1254S

We present a study of rotation (vsini) and chromospheric activity (Hα equivalent width) based on an extensive set of high-resolution optical spectra obtained with the MIKE instrument on the 6.5 m Magellan Clay telescope. Our targets are 74 F-M dwarfs in four young stellar associations, spanning ages from 6 to 30 Myr. By comparing Hα EWs 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, and 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_–6 and 9⁺⁸_–2Myr for the η Cha and TWA associations, respectively, consistent with previous estimates. Thus we conclude that stellar radii from evolutionary models by Baraffe et al. ((ref???)1998) are in agreement with the observed radii to within ±15%.