Investigating the Rotational Phase of Stellar Flares on M dwarfs Using K2 Short Cadence Data

TL;DR

This study analyzes K2 short cadence data of 34 M dwarfs to investigate the relationship between stellar rotation phase and flare occurrence, finding no preferred phase for flares despite expectations.

Contribution

It provides the first detailed analysis of rotational phase of flares on M dwarfs using K2 data, challenging previous assumptions about starspot-flare correlations.

Findings

Most stars showed flares lasting 10-90 minutes.

Flares occur across all rotational phases, with no preferred phase.

Rapid rotators tend to have more flares, with activity declining beyond 10-day rotation periods.

Abstract

We present an analysis of K2 short cadence data of 34 M dwarfs which have spectral types in the range M0 - L1. Of these stars, 31 showed flares with a duration between $\sim$10-90 min. Using distances obtained from Gaia DR2 parallaxes, we determined the energy of the flares to be in the range $\sim1.2\times10^{29}-6\times10^{34}$ erg. In agreement with previous studies we find rapidly rotating stars tend to show more flares, with evidence for a decline in activity in stars with rotation periods longer than $\sim$10 days. The rotational modulation seen in M dwarf stars is widely considered to result from a starspot which rotates in and out of view. Flux minimum is therefore the rotation phase where we view the main starspot close to the stellar disk center. Surprisingly, having determined the rotational phase of each flare in our study we find none show any preference for rotational phase. We outline three scenarios which could account for this unexpected finding. The relationship between rotation phase and flare rate will be explored further using data from wide surveys such as NGTS and TESS.