Poking the Beehive from Space: K2 Rotation Periods for Praesepe

We analyze K2 light curves for 794 low-mass ( 1 M * 0.1 M ) members of the 650 Myr old open cluster Praesepe and measure rotation periods (Prot) for 677 of these stars. We find that half of the rapidly rotating 0.3 M stars are confirmed or candidate binary systems. The remaining 0.3 M fast rotators have not been searched for companions, and are therefore not confirmed single stars. We previously found that nearly all rapidly rotating 0.3 M stars in the Hyades are binaries, but we require deeper binary searches in Praesepe to confirm whether binaries in these two co-eval clusters have different Prot distributions. We also compare the observed Prot distribution in Praesepe to that predicted by models of angular-momentum evolution. We do not observe the clear bimodal Prot distribution predicted by Brown for >0.5 M stars at the age of Praesepe, but 0.25−0.5 M stars do show stronger bimodality. In addition, we find that >60% of early M dwarfs in Praesepe rotate more slowly than predicted at 650 Myr by Matt et al., which suggests an increase in braking efficiency for these stars relative to solar-type stars and fully convective stars. The incompleteness of surveys for binaries in open clusters likely impacts our comparison with these models since the models only attempt to describe the evolution of isolated single stars.