Stellar Flares in the TESS Light Curves of Planet-hosting M dwarfs

Abstract

M dwarfs are magnetically active stars that frequently produce flares, which have implications for both stellar evolution and exoplanet studies. Flare occurrence rates and activity levels of M dwarfs correlate with stellar characteristics such as age, mass, and rotation period. We search TESS observations of a known active population of M dwarfs as well as a volume-limited sample of M dwarfs within 15 parsecs. We detect flares in the light curves of these stars, including 276 of 538 M dwarfs within 15 pc, and calculate cumulative flare frequency distributions (FFDs) for each star. Based on flaring behavior, we categorize stars into relatively higher and lower activity groups and fit power laws to their FFDs to compare the power law exponent ($α$) across activity levels. We find $α=1.99 \pm 0.07$ for the combined FFD of the lower activity M dwarfs, compared to averages of $α= 1.94 \pm 0.58$ for highly active stars with 10-100 detected flares, and $α= 2.03 \pm 0.43$ for those with > 100 detected flares, suggesting little evolution in the power law distribution of flares as M dwarfs transition from high to low activity states. The uncertainties for the active star groups reflect the standard deviation of $α$ values across individual stars within each subset. Because stellar flares and associated stellar activity complicate exoplanet observations, we also examine the subset of M dwarfs with JWST transmission spectroscopy follow-up observations in Cycles 1-3. The flares we detect for these targets are consistent with the broader 15 pc sample, providing context for interpreting planetary atmosphere retrievals from JWST spectra.