THE FACTORY AND THE BEEHIVE. II. ACTIVITY AND ROTATION IN PRAESEPE AND THE HYADES

Open clusters are collections of stars with a single, well-determined age, and can be used to investigate the connections between angular-momentum evolution and magnetic activity over a star's lifetime. We present the results of a comparative study of the relationship between stellar rotation and activity in two benchmark open clusters: Praesepe and the Hyades. We have compiled a sample of 720 spectra-more than half of which are new observations-for 516 high-confidence members of Praesepe; we have also obtained 139 new spectra for 130 high-confidence Hyads. We have also collected rotation periods (P sub(rot)) for 135 Praesepe members and 87 Hyads. Our analysis shows that H alpha activity in these stars is saturated for R sub(o) [< or =, slant] 0.11 super(+0.02) sub(-0.03). Above that value activity declines as a power-law with slope beta = -0.73 super(+0.16) sub(-0.12), before dropping off rapidly at R sub(o) approximately 0.4. These data provide a useful anchor for calibrating the age-activity-rotation relation beyond 600 Myr.