M Dwarf Variability and Periodicities in Praesepe

212 M dwarfs in the Praesepe cluster have been monitored photometrically for three observing seasons. It is found that Praesepe M dwarfs earlier than ~M4 often have significant photometric variations, while variability is not detected for >M4. Time series analysis was performed on 147 of the targets having likely variability in order to study possible periodicities. For 83% of these targets, we detected no periodicities; these null results included targets with published photometric periods from earlier work. Our detected periods ranged from 20 to 45 days, and we are not able to confirm any of the 1–5 day periods in Praesepe periods reported by Schultz et al., which we attribute to the very different observing cadences of the two studies. We conjecture that our more widely spaced data cannot adequately sample the Schultz et al. periodicities before the growth and decay of spots have a chance to ruin the coherence. The new periods we find in the range 20–45 days (in targets that do not overlap with those from Schultz having shorter periods) have very small false alarm probabilities. We argue that rotation is unlikely to be responsible for these 20–45 day periods. Perhaps short activity cycles in the Praesepe M dwarfs play a role in generating such periodicities.