The discrepancy between dynamical and theoretical mass in the triplet-system 2MASS J10364483+1521394

We combine new Lucky Imaging astrometry from New Technology Telescope/AstraLux Sur with already published astrometry from the AstraLux Large M-dwarf Multiplicity Survey to compute orbital elements and individual masses of the 2MASS J10364483+1521394 triple system belonging to the Ursa-Major moving group. The system consists of one primary low-mass M-dwarf orbited by two less massive companions, for which we determine a combined dynamical mass of M B + C = 0.48 ± 0.14 M ⊙ . We show from the companions’ relative motions that they are of equal mass (with a mass ratio of 1.00 ± 0.03), thus 0.24 ± 0.07 M ⊙ individually, with a separation of 3.2 ± 0.3 AU, and we conclude that these masses are significantly higher (30%) than what is predicted by theoretical stellar evolutionary models. The biggest uncertainty remains the distance to the system, here adopted as 20.1 ± 2.0 pc based on trigonometric parallax, whose ambiguity has a major impact on the result. With the new observational data we are able to conclude that the orbital period of the BC pair is 8.41 +0.04 -0.02 yr.