Unveiling the distinctive spindown histories of fully convective M dwarfs

Age estimates are challenging for M dwarfs, making it difficult to map how their rotation rates and activity levels change over time. Here I discuss our efforts to uncover this picture using open clusters, wide binaries, and a volume-complete survey of our nearest neighbours. We find that the spindown of fully convective M dwarfs is markedly different from the Skumanich-law spindown of Sun-like stars: small M dwarfs spin down gradually (P~0.1-10 days) for 1-4Gyr (depending on mass), then abruptly transition to long periods (P>70 days) over a timescale of <100Myr. The dispersion in the mass-dependent spindown relation is small (<0.5Gyr) when stellar birth environment, age, and metallicity are controlled, but we have identified some M dwarfs that make the transition at much younger ages—systems that may represent special initial conditions (e.g., high-energy birth environment and initial rotation rate) that led to a shorter saturation lifetime. In addition to offering a unique parameter space for testing theoretical spindown models, our findings have important implications for planetary atmosphere retention and the 'cosmic shoreline' of fully convective M dwarfs, suggesting that many key planets for atmospheric characterization received a historic XUV flux that exceeds the canonical scaling relation by more than a factor of 3.