Orbits of M Dwarf Binaries Big and Small: Revealing Their Sizes and Shapes Via a Multi-Pronged Observing Campaign

The ubiquity of the M dwarf stars makes them an essential piece of the story of star formation. Their binary and multi-star system configurations present a valuable opportunity to reveal important clues about the mysteries of how stars form.

We are gathering orbits of M dwarf multiples within 25 pc to establish the distributions of their orbital elements in an effort to understand the complex combination of physical processes molding their formation and dynamical evolution. We focus in particular on eccentricity vs. orbital period, creating a plot of orbit shape as a function of orbit size, using orbits with periods ranging from a few days to 30 years. Our ultimate goal is to secure at least 120 reliable orbits via observations made through a variety of complementary methods: (1) ground-based astrometry mapping years-to-decades–long orbits via the RECONS (REsearch Consortium On Nearby Stars, www.recons.org) program at the SMARTS 0.9m, (2) speckle interferometry to map orbits up to 3 years long at SOAR with HRCam+SAM, (3) spectroscopy to map orbits on day-to-month–long scales with CHIRON at the SMARTS 1.5m, and (4) orbits already available in the literature. Results to date show a tidal circularization period for M dwarfs around 7 days, shorter than the circularization period of solar-type binaries at 12 days. Curiously, we find a paucity of circular orbits for systems with periods longer than 10 years, a key indicator of how stars form.

This effort has been supported by the NSF through grants AST-1715551 and AST-2108373 via observations made possible by the SMARTS Consortium.