The Active Lifetimes of Low-mass Stars and the Implications for the Atmospheres of their Terrestrial Worlds

The plethora of rocky planets identified to transit nearby low-mass stars are our best hope to begin the study of the secondary atmospheres of terrestrial exoplanets. However, these low-mass stars remain active over much longer timescales than their Sun-like counterparts, with potentially transformative, if not entirely destructive implications for these atmospheres. I will present the results of a 6-year photometric and high-resolution spectroscopic monitoring campaign of the volume-complete sample of the 512 stars with masses between 10% and 30% that of the Sun and lying within 15 parsecs. We find that the epoch of spindown is indeed very late, and is strongly dependent on stellar mass. Fully convective M dwarfs initially spin down slowly, with the population of 0.2–0.3 M⊙ rapid rotators evolving from Prot < 2 days at 600 Myr to 2 < Prot < 10 days at 1–3 Gyr before rapidly spinning down to long rotation periods and inactive states. We further find that all low-mass stars share a common distribution of flare rates with energy, but with a normalization that drops abruptly by a factor of 50,000 as they move from their active to inactive states. Because the high-energy emission and particle fluxes remain extreme for a time that is much longer than that over which terrestrial planets likely outgas their secondary atmospheres, we may soon learn that rocky planets end as boring airless orbs if they are so unfortunate as to have been born around a low-mass star. However, a glimmer of hope has also emerged from our survey: We find an intrinsic variability in the spin-down time scale, with a small fraction of stars having substantially spun down by 600 Myr; perhaps this subset has yielded some locations with a little atmosphere.