M dwarfs comprise 75% of all stars and commonly host rocky exoplanets, but their magnetic stellar activity significantly impacts the potential habitability of said planets. Here we discuss three projects we conducted to better understand this activity.

We first investigate a sample of 36 wide binaries, where the two “twin” M dwarfs in each pair have BP/RP/J/H/Ks magnitudes differing by <0.10 mag, mass estimates matching within <3%, and presumably the same age and composition. We use short– and long-term photometry, multi-epoch spectroscopy, and archival data to measure rotation periods, photometric variability, and Hα strengths, alongside speckle imaging, radial velocities, and long-term astrometry to find unresolved companions. Four systems received additional X-ray observations and comprised our first project, where we found NLTT 44989 A and B differ by 6× in rotation period, ∼40× in Lx, and have a total Hα inactive/active mismatch. Our second project expanded to all 36 binaries, finding numerous twins hosting functionally identical rotation rates and activity levels. In contrast, mismatches of >25% exist in rotation period, Hα activity, and photometric variability for roughly one out of five twin pairs despite their otherwise identical natures. These results challenge the magnetic predictability of M dwarfs, and we discuss many possible explanations.

Our third project uses 25 years of VRI photometry from the RECONS 0.9m program to discover new long-term stellar activity cycles in nearby M dwarfs. We report cycle detections in 50 stars — including 20 with measured periods spanning 4.0–24.3 yrs — roughly doubling the number of known cycle periods in fully convective M dwarfs. We find cycles with extreme brightness changes of ∼10–20%, others indicating periods beyond 30 yrs, and one in a 40–50 Myr-old pre-main-sequence star. Comparison with our volume-complete subsample yields a cycle occurrence rate lower limit of ≥ 5.7% ± 1.0%, which increases to ≥ 10.2% ± 2.4% for stars between 0.30–0.15 Msol. Activity cycles are therefore a somewhat common outcome of dynamos in fully convective M dwarfs, and can have periods similar to those in solar-like stars.