Rotation-activity relations and flares of M dwarfs with K2 long- and short-cadence data

Abstract

Using light curves obtained by the K2 mission, we study the relation between stellar rotation and magnetic activity with special focus on stellar flares. Our sample comprises 56 bright and nearby M dwarfs observed by K2 during campaigns C0-C18 in long-and short-cadence mode. We derive rotation periods for 46 M dwarfs and measure photometric activity indicators such as amplitude of the rotational signal, standard deviation of the light curves, and the basic flare properties (flare rate, flare energy, flare duration, and flare amplitude). We found 1662 short-cadence flares, 363 of which have a long-cadence counterpart with flare energies of up to 5.6 · 10 34 erg. The flare amplitude, duration, and frequency derived from the short-cadence light curves differ significantly from those derived from the long-cadence data. The analysis of the short-cadence light curves results in a flare rate that is 4.6 times higher than the long-cadence data. We confirm the abrupt change in activity level in the rotation-activity relation at a critical period of ∼10 d when photometric activity diagnostics are used. This change is most drastic in the flare duration and frequency for short-cadence data. Our flare studies revealed that the highest flare rates are not found among the fastest rotators and that stars with the highest flare rates do not show the most energetic flares. We found that the superflare frequency (E ≥ 5 · 10 34 erg) for the fast-rotating M stars is twice higher than for solar like stars in the same period range. By fitting the cumulative FFD, we derived a power-law index of α = 1.84 ± 0.14, consistent with previous M dwarf studies and the value found for the Sun.

The rotation–activity relation of M dwarfs: From K2 to TESS and PLATO

“…CU Cnc is a known eclipsing M dwarf binary (Delfosse et al 1999a) with a direct imaging companion candidate (Beuzit et al 2004), while CV Cnc is itself a visual binary (Delfosse et al 1999b). In Raetz et al (2020), we showed, in a systematic study comparing rotation and activity in short‐ and long‐cadence K2 data of M dwarfs, that the highest flare rate in our sample was observed on CU Cnc. This strengthens the hypothesis that flares might be induced by the presence of a close companion.…”

“…Only 56 of the 430 targets in our K2 M dwarf sample were observed by K2 in the short‐cadence mode. A comparison of the long‐ and short‐cadence data is presented in detail by Raetz et al (2020). Our study of the rotation–activity relation of M dwarfs will strongly benefit from the observation with other existing and upcoming photometric space missions, that is, TESS and PLATO .…”

Flares Big and Small: a K2 and TESS View of ASAS-SN Superflares

“…For each of the stars that we were able to extract a rotation rate, we found them to be fast rotators, with a period less than 6 days, but more than 1 day. These lie in the range of stars on which studies such as Mondrik et al (2018), Newton et al (2016) and Raetz et al (2020) have found the most active flaring behaviour. It is also in line with theoretical predictions of a superflaring rate at faster rotation periods.…”

The corona of GJ 1151 in the context of star–planet interaction

“…Other slowly-rotating M dwarfs have been found to flare occasionally, see for example Raetz et al (2020), so the fact that GJ 1151 as a low-activity star happens to flare in the XMM-Newton observation is not extraordinary.…”