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

TL;DR

This paper reports the discovery of a low-mass companion to star HIP 53005, with conflicting mass estimates from different methods, suggesting possible binarity or additional unseen companions.

Contribution

It presents direct imaging and astrometric data revealing a companion with conflicting mass estimates, highlighting the complexity of characterizing low-mass stellar companions.

Findings

Companion likely near the hydrogen-burning limit (~80 M_Jup) based on photometry.

Orbital fitting suggests a much higher dynamical mass (~185 M_Jup).

Possible unseen close companion or binary nature of the companion.

Abstract

We present the discovery of a low-mass companion located at $\rho$ $\sim$ 0\farcs{}85 ($r_{\rm proj} \approx 62~au$) from the early-type 1.2 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 $\approx$ 12\farcs{}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 ($\sim80~M_{\rm Jup}$). However, our orbital fitting combining direct-imaging relative astrometry with proper motion acceleration favors a much higher dynamical mass of $\sim185\ M_{\rm Jup}$. An additional unseen, more closely-orbiting companion below the detection limit (at $\rho\lesssim0\farcs2$)) 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.