Poking the Beehive from Space: K2 Rotation Periods for Praesepe

Abstract

We analyze {\\it K2} light curves for 794 low-mass ($1 > M_* > 0.1$\n$M_{\\odot}$) members of the $\\approx$650-Myr-old open cluster Praesepe, and\nmeasure rotation periods ($P_{rot}$) for 677 of these stars. We find that half\nof the rapidly rotating $>$0.3 $M_{\\odot}$ stars are confirmed or candidate\nbinary systems. The remaining $>0.3$ $M_{\\odot}$ fast rotators have not been\nsearched for companions, and are therefore not confirmed single stars. We found\npreviously that nearly all rapidly rotating $>$0.3 $M_{\\odot}$ stars in the\nHyades are binaries, but we require deeper binary searches in Praesepe to\nconfirm whether binaries in these two co-eval clusters have different $P_{rot}$\ndistributions. We also compare the observed $P_{rot}$ distribution in Praesepe\nto that predicted by models of angular-momentum evolution. We do not observe\nthe clear bimodal $P_{rot}$ distribution predicted by Brown (2014) for $>$0.5\n$M_{\\odot}$ stars at the age of Praesepe, but 0.25$-$0.5 $M_{\\odot}$ stars do\nshow stronger bimodality. In addition, we find that $>$60\\% of early M dwarfs\nin Praesepe rotate more slowly than predicted at 650 Myr by Matt et al. (2015),\nwhich suggests an increase in braking efficiency for these stars relative to\nsolar-type stars and fully convective stars. The incompleteness of surveys for\nbinaries in open clusters likely impacts our comparison with these models,\nsince the models only attempt to describe the evolution of isolated single\nstars.\n