We have Fourier analyzed 941 K2 light curves of likely members of Praesepe , measuring periods for 86 % and increasing the number of rotation periods ( P ) by nearly a factor of four .
The distribution of P vs . ( V - K _ { s } ) , a mass proxy , has three different regimes : ( V - K _ { s } ) < 1.3 , where the rotation rate rapidly slows as mass decreases ; 1.3 < ( V - K _ { s } ) < 4.5 , where the rotation rate slows more gradually as mass decreases ; and ( V - K _ { s } ) > 4.5 , where the rotation rate rapidly increases as mass decreases .
In this last regime , there is a bimodal distribution of periods , with few between \sim 2 and \sim 10 days .
We interpret this to mean that once M stars start to slow down , they do so rapidly .
The K2 period-color distribution in Praesepe ( \sim 790 Myr ) is much different than in the Pleiades ( \sim 125 Myr ) for late F , G , K , and early-M stars ; the overall distribution moves to longer periods , and is better described by 2 line segments .
For mid-M stars , the relationship has similarly broad scatter , and is steeper in Praesepe .
The diversity of lightcurves and of periodogram types is similar in the two clusters ; about a quarter of the periodic stars in both clusters have multiple significant periods .
Multi-periodic stars dominate among the higher masses , starting at a bluer color in Praesepe ( ( V - K _ { s } ) \sim 1.5 ) than in the Pleiades ( ( V - K _ { s } ) \sim 2.6 ) .
In Praesepe , there are relatively more light curves that have two widely separated periods , \Delta P > 6 days .
Some of these could be examples of M star binaries where one star has spun down but the other has not .