Direct Imaging Explorations for Companions from the Subaru/IRD Strategic Program II; Discovery of a Brown-dwarf Companion around a Nearby mid-M~dwarf LSPM~J1446+4633

Abstract

We report the discovery of a new directly-imaged brown dwarf companion with Keck/NIRC2+pyWFS around a nearby mid-type M~dwarf LSPM~J1446+4633 (hereafter J1446). The $L^{\prime }$-band contrast ($4.5\times 10^{-3}$) is consistent with a $\sim 20-60M_{\mathrm{Jup}}$ object at 1--10~Gyr and our two-epoch NIRC2 data suggest a $\sim 30\%$ ($\sim 3.1\ddot{I}ƒ)$ variability in its $L^{\prime }$-band flux. We incorporated Gaia DR3 non-single-star catalog into the orbital fitting by combining the Subaru/IRD RV monitoring results, NIRC2 direct imaging results, and Gaia proper motion acceleration. As a result, we derive ${59.8}_{-1.4}^{+1.5}{M}_{\mathrm{Jup}}$ and $\approx 4.3\mathrm{au}$ for the dynamical mass and the semi-major axis of the companion J1446B, respectively. J1446B is one of the intriguing late-T~dwarfs showing variability at $L^{\prime }$-band for future atmospheric studies with the constrained dynamical mass. Because the J1446 system is accessible with various observation techniques such as astrometry, direct imaging, and high-resolution spectroscopy including radial velocity measurement, it has a potential as a great benchmark system to improve our understanding for cool dwarfs.

11 pages, 7 figs, 4 tables, accepted for publication in AJ