M dwarfs with masses below 0.3 the solar value provide the only opportunity for atmospheric studies of terrestrial exoplanets in the next decade. Stellar flares can remove or alter these planetary atmospheres. Hence, it is essential that we determine both the present flare rate of the host star, and construct the past history of flares throughout its lifetime. We studied the flare rates, rotation periods, and spectroscopic activity indicators of a volume-complete sample of 125 single stars within 15 parsecs and with masses between 0.1-0.3 solar masses, which were observed during the first year of the TESS Mission. We gathered multi-epoch high-resolution spectra and determined the equivalent widths of several chromospheric activity indicators. Thirty-five of our stars had a previously published rotation period; we present 18 new rotation periods from MEarth photometry (spanning 65–180 days) and 20 new rotation periods from TESS photometry (spanning 0.17–5 days). From the TESS time series, we find that stars in this sample share a flare frequency distribution with a communal slope of alpha = 1.98 ± 0.02, but with rates that can differ up to 6 orders of magnitude. Our sample divides into two groups: 26% have H-alpha in emission, a saturated flare rate, and are rapidly rotating. The remaining 74% show little to no H-alpha in emission, exhibit few flares (such that the majority do not show a single flare during the TESS observations), and, when measured, long rotation periods. For 53 of the 89 stars in the second group, the photometric rotation period has not been determined, but we expect all of these stars to have rotation periods in excess of 100 days. Our study provides a means to estimate the flare rate based on either the H-alpha equivalent width or the rotation period, and constrains the radiation environment of the most spectroscopically accessible terrestrial exoplanets. This constraint may be important for understanding future near-term detailed atmospheric studies of terrestrial exoplanets with the next generation of ground-based extremely large telescopes and the James Webb Space Telescope.
This work was supported by the NSF through a Graduate Research Fellowship, NASA through the XRP and TESS GI programs, and through a grant from the John Templeton Foundation.