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

Abstract

Open clusters are collections of stars with a single, well-determined age,\nand can be used to investigate the connections between angular-momentum\nevolution and magnetic activity over a star's lifetime. We present the results\nof a comparative study of the relationship between stellar rotation and\nactivity in two benchmark open clusters: Praesepe and the Hyades. As they have\nthe same age and roughly solar metallicity, these clusters serve as an ideal\nlaboratory for testing the agreement between theoretical and empirical\nrotation-activity relations at $\\approx$600 Myr. We have compiled a sample of\n720 spectra --- more than half of which are new observations --- for 516\nhigh-confidence members of Praesepe; we have also obtained 139 new spectra for\n130 high-confidence Hyads. We have collected rotation periods ($P_{rot}$) for\n135 Praesepe members and 87 Hyads. To compare $H\\alpha$ emission, an indicator\nof chromospheric activity, as a function of color, mass, and Rossby number\n$R_o$, we first calculate an expanded set of $\\chi$ values, with which we can\nobtain the $H\\alpha$ to bolometric luminosity ratio, $L_{H\\alpha}/L_{bol}$,\neven when spectra are not flux-calibrated and/or stars lack reliable distances.\nOur $\\chi$ values cover a broader range of stellar masses and colors (roughly\nequivalent to spectral types from K0 to M9), and exhibit better agreement\nbetween independent calculations, than existing values. We find no difference\nbetween the two clusters in their $H\\alpha$ equivalent width or\n$L_{H\\alpha}/L_{bol}$ distributions, and therefore take the merged $H\\alpha$\nand $P_{rot}$ data to be representative of 600-Myr-old stars. Our analysis\nshows that $H\\alpha$ activity in these stars is saturated for\n$R_o\\leq0.11^{+0.02}_{-0.03}$. Above that value activity declines as a\npower-law with slope $\\beta=-0.73^{+0.16}_{-0.12}$, before dropping off rapidly\nat $R_o\\approx0.4$...\n