Precise Mass, Orbital motion and Stellar properties of the M-dwarf binary LP 349-25AB

LP~349$-$25 is a well studied close stellar binary system comprised of two late M dwarf stars, both stars close to the limit between star and brown dwarf. This system was previously identified as a source of GHz radio emission. We observed LP~349$-$25AB in 11 epochs in 2020$-$2022, detecting both components in this nearby binary system using the Very Long Baseline Array (VLBA). We fit simultaneously the VLBA absolute astrometric positions together with existing relative astrometric observations derived from optical/infrared observations with a set of algorithms that use non-linear least-square, Genetic Algorithm and Markov Chain Montecarlo methods to determine the orbital parameters of the two components. We find the masses of the primary and secondary components to be 0.08188 $\pm$ 0.00061 $M\odot$, and 0.06411 $\pm$ 0.00049 $M_\odot$, respectively, representing one of the most precise mass estimates of any UCDs to date. The primary is an UCD of 85.71$\pm$0.64 M$_{Jup}$, while the secondary has a mass consistent with being a Brown Dwarf of 67.11 $\pm$ 0.51 $M_{Jup}$. This is one of the very few direct detections of a Brown Dwarf with VLBA observations. We also find a distance to the binary system of 14.122 $\pm$ 0.057 pc. Using Stellar Evolutionary Models, we find the model-derived stellar parameters of both stars. In particular, we obtain a model-derived age of 262 Myr for the system, which indicates that LP~349$-$25AB is composed of two pre-main-sequence stars. In addition, we find that the secondary star is significantly less evolved that the primary star.