M dwarfs dominate the solar neighborhood population, accounting for three of every four stars. Their broad mass range — from 62% down to 8% that of the Sun — creates a rich dynamical laboratory that can be used to challenge stellar and binary formation models. Our Orbital Architectures project is constructing a large sample of orbits for nearby M dwarf systems to establish their distributions in period, mass ratio, semimajor axis, and eccentricity, with the goal of building crucial empirical evidence that will constrain models of multi-star formation and evolution. These orbits have been observed during the 20+ year RECONS astrometry program at the CTIO/SMARTS 0.9m, enhanced by a new speckle interferometry campaign at SOAR with HRCam+SAM to map the shorter-period orbits. Together, these observing efforts will map ~120 orbits of nearby M dwarfs with orbital periods spanning 0 to 30 years, providing the richest set of data ever collected for these ubiquitous stars.
The speckle observations at SOAR resolve systems and provide magnitude differences between components, many of which already have orbits mapped by the RECONS astrometry program. The synergy of 0.9m and SOAR observations allows us to determine individual component masses, and to compare those masses to their fluxes in the Kron-Cousins I band. Here we present a new mass-luminosity relation for M dwarfs in the I band, populated with 40 masses focused on the low-mass end of the M dwarf sequence. This work has been supported by NSF grants AST-0507711, AST-0908402, AST-1109445, AST-141206, and AST-1715551.