# Poking the Beehive From Space: K2 Rotation Periods For Praesepe

## TL;DR

This study measures rotation periods of low-mass stars in Praesepe, compares them with models, and explores the influence of binarity and magnetic braking on stellar rotation at 650 million years.

## Contribution

It provides the first extensive rotation period measurements for Praesepe low-mass stars and compares these with angular-momentum evolution models, highlighting the role of binarity and magnetic braking.

## Key findings

- Half of fast rotators are confirmed or candidate binaries.
- No clear bimodal rotation distribution for >0.5 M_sun stars.
- Slower rotation in early M dwarfs than predicted, indicating increased braking efficiency.

## Abstract

We analyze {\it K2} light curves for 794 low-mass ($1 > M_* > 0.1$ $M_{\odot}$) members of the $\approx$650-Myr-old open cluster Praesepe, and measure rotation periods ($P_{rot}$) for 677 of these stars. We find that half of the rapidly rotating $>$0.3 $M_{\odot}$ stars are confirmed or candidate binary systems. The remaining $>0.3$ $M_{\odot}$ fast rotators have not been searched for companions, and are therefore not confirmed single stars. We found previously that nearly all rapidly rotating $>$0.3 $M_{\odot}$ 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 $P_{rot}$ distributions. We also compare the observed $P_{rot}$ distribution in Praesepe to that predicted by models of angular-momentum evolution. We do not observe the clear bimodal $P_{rot}$ distribution predicted by Brown (2014) for $>$0.5 $M_{\odot}$ stars at the age of Praesepe, but 0.25$-$0.5 $M_{\odot}$ 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. (2015), 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.