Studying the Variability of M Dwarfs from Hours to Months to Decades with the CTIO 0.9m and TESS

M dwarfs account for 3 out of every 4 stars in the Solar Neighborhood and presumably offer enduring and stable environments for planetary systems. In our project, ATLAS (A Trail to Life Around Stars), we are compiling a variability catalog of M dwarfs within 25 pc to identify the most and least variable stars in a volume-complete sample. Here we present our assessment of stellar variability at different timescales: short-term (minutes to hours; due to flares), mid-term (days to months; due to stellar rotation), and long-term (years to decades; due to stellar cycles). We have identified 1750 M dwarf systems in the southern sky within 25 pc via Gaia DR3 parallaxes. These long-term results are based on the 24-year RECONS effort at the SMARTS 0.9m at CTIO. We present long-term variability results on ~400 M dwarfs and highlight in detail 32 M dwarfs with confirmed exoplanets. Of these, 6 vary < 2%, 25 vary between 2-6%, and only 1, Proxima Centauri, varies by > 6%. We augment these results with mid- and short-term results from TESS (~90% coverage of our sample). TESS observed 23 out of 32 ATLAS exoplanet hosts and found 17 varying by <1% and 5 showing flares. Overall, it is clear that some M dwarfs are more photometrically stable than others across all timescales, suggesting that specific stars warrant further attention in the search for habitable worlds. This effort has been supported by the NSF through grant AST-2108373 and via observations made possible by the SMARTS Consortium