Discovery of a Low-mass Companion to the Accelerating Star HIP 53005 with Strongly Conflicting Mass Estimates

We present the discovery of a low-mass companion located at ρ ∼ 0 . ″ 85 ( r proj ≈ 62 au) from the early type 1.3 Gyr old star HIP 53005 using direct-imaging data from the Subaru and Keck Telescopes and astrometry from the Hipparcos–Gaia Catalog of Accelerations. The companion, HIP 53005 C, is a component of a multiple system also including a ≈ 12 . ″ 4 separation M-dwarf companion inducing a negligible proper motion acceleration. HIP 53005 C’s position on color–magnitude diagrams, the fit of its spectral energy distribution to atmosphere models, and its location on an empirical mass-magnitude diagram all suggest that it lies at the M/L transition and near the hydrogen-burning limit (∼80 M Jup ). However, our orbital fitting combining direct-imaging relative astrometry with proper motion acceleration favors a much higher dynamical mass of ∼185 M Jup . An additional unseen, more closely orbiting companion below the detection limit (at ρ ≲ 0 . ″ 2 )) may explain this discrepancy. Alternatively, HIP 53005C could be a low-mass binary like Gliese 229Bab, making this system an intriguing laboratory for studying multiple star formation.