Dynamical Masses of ε Indi B and C: Two Massive Brown Dwarfs at the Edge of the Stellar–substellar Boundary

We report individual dynamical masses for the brown dwarfs ε Indi B and C, which have spectral types of T1.5 and T6, respectively, measured from astrometric orbit mapping. Our measurements are based on a joint analysis of astrometric data from the Carnegie Astrometric Planet Search and the Cerro Tololo Inter-American Observatory Parallax Investigation, as well as archival high-resolution imaging, and use a Markov chain Monte Carlo method. We find dynamical masses of 75.0 ± 0.82 MJup for the T1.5 B component and 70.1 ± 0.68 MJup for the T6 C component. These masses are surprisingly high for such cool objects and challenge our understanding of substellar structure and evolution. We discuss several evolutionary scenarios proposed in the literature and find that while none of them can provide conclusive explanations for the high substellar masses, evolutionary models incorporating lower atmospheric opacities come closer to approximating our results. We discuss the details of our astrometric model, its algorithm implementation, and how we determine parameter values via Markov chain Monte Carlo Bayesian inference.