An Increased Rate of Large Flares at Intermediate Rotation Periods for Mid-to-late M Dwarfs

Abstract

We present an analysis of flares in mid-to-late M dwarfs in the MEarth photometric survey. We search 3,985,155 observations across 2226 stars, and detect 54 large (Δ m  ≥ 0.018) flares in total, distributed across 34 stars. We combine our flare measurements with recent activity and rotation period results from MEarth to show that there is an increase in flares per observation from low Rossby number ( R o  < 0.04, rapid rotators) to intermediate Rossby number (0.04 <  R o  < 0.44, intermediate rotators) at the 99.85% confidence level. We additionally find an increased flare rate from the high Rossby number population ( R o  > 0.44, slow rotators) to the intermediate population at the 99.97% level. We posit that the rise in flare rate for intermediate R o could be due to changing magnetic field geometry on the surface of the star.