Rotation and activity of preymain-sequence stars

We present a study of rotation ( v sin i) and chromospheric activity ( H alpha 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 H alpha 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 similar to 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(-6)(+7) and 9(-2)(+8) 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 to within +/- 15%.