An increased rate of large flares at intermediate rotation periods for mid-to-late M dwarfs

TL;DR

This study analyzes flare activity in mid-to-late M dwarfs, revealing an increased flare rate at intermediate rotation periods likely due to changes in magnetic field geometry.

Contribution

It provides the first evidence of increased flare rates at intermediate Rossby numbers in mid-to-late M dwarfs, linking flare activity to magnetic field evolution.

Findings

Flare rate increases from rapid to intermediate rotators.

Flare rate also increases from slow to intermediate rotators.

Magnetic field geometry changes may drive flare rate variations.

Abstract

We present an analysis of flares in mid-to-late M dwarfs in the MEarth photometric survey. We search 3985155 observations across 2226 stars, and detect 54 large ($\Delta m \geq 0.018$) 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.