Dynamical masses for two M1 + mid-M dwarf binaries monitored during the SPHERE-SHINE survey

We present orbital fits and dynamical masses for HIP 113201AB and HIP 36985AB, two M1 + mid-M dwarf binary systems monitored as part of the SPHERE-SHINE survey. To robustly determine the age of both systems via gyrochronology, we undertook a photometric monitoring campaign for HIP 113201 and GJ 282AB, the two wide K star companions to HIP 36985, using the 40 cm Remote Observatory Atacama Desert telescope. Based on this monitoring and gyrochronological relationships, we adopt ages of 1.2 ± 0.1 Gyr for HIP 113201AB and 750 ± 100 Myr for HIP 36985AB. These systems are sufficiently old that we expect that all components of these binaries have reached the main sequence. To derive dynamical masses for all components of the HIP 113201AB and HIP 36985AB systems, we used parallel-tempering Markov chain Monte Carlo sampling to fit a combination of radial velocity, direct imaging, and Gaia and HIPPARCOS astrometry. Fitting the direct imaging and radial velocity data for HIP 113201 yields a primary mass of 0.54 ± 0.03 M⊙, fully consistent with its M1 spectral type, and a secondary mass of 0.145 ± M⊙. The secondary masses derived with and without including HIPPARCOS-Gaia data are all considerably more massive than the 0.1 M⊙ mass estimated from the photometry of the companion. Thus, the dynamical impacts of this companion suggest that it is more massive than expected from its photometry. An undetected brown dwarf companion to HIP 113201B could be a natural explanation for this apparent discrepancy. At an age >1 Gyr, a 30 MJup companion to HIP 113201B would make a negligible (<1%) contribution to the system luminosity but could have strong dynamical impacts. Fitting the direct imaging, radial velocity, and HIPPARCOS-Gaia proper motion anomaly for HIP 36985AB, we find a primary mass of 0.54 ± 0.01 M⊙ and a secondary mass of 0.185 ± 0.001 M⊙, which agree well with photometric estimates of component masses, the masses estimated from MK- mass relationships for M dwarf stars, and previous dynamical masses in the literature.