J1250+0455AB an ultracool binary in a hierarchical triple system

Abstract

We report the discovery of the ultracool dwarf binary system J1250+0455AB, a low-mass (M${}_{\odot }$${}_{\mathrm{tot}}<$ 0.2 M${}_{\odot }$) system in which the components straddle the M/L dwarf boundary. The binary was resolved through near-infrared adaptive optics imaging with LUCI1-SOUL on the Large Binocular Telescope, revealing a projected angular separation of 0.17 $\pm$ 0.015$\arcsec$, which, combined with a system distance of $71\pm 5.8$\,pc, corresponds to a physical separation of 12.2 $\pm$ 1.5\,AU at a position angle of 84.8 $\pm$ 0.2°. We estimated the orbital period of J1250+0455AB to be 156 $\pm$ 8\,yr, the bolometric luminosities of the primary and secondary luminosities as $\log (L_{\mathrm{bol}}/L_{\odot })=-3.45\pm 0.04$ and $-3.58\pm 0.04$, respectively, with the spectral types of M9 and L0 determined through binary template fitting and spectrophotometric relations. This binary system is part of a hierarchical triple with a separation of 10.44$\arcsec$ from its primary. We estimated the age of the system from the rotational period of the primary star as $0.56_{-0.06}^{+0.07}$ Gyr. Using evolutionary models, for each component we estimate the mass [0.079 $\pm$ 0.002\,M${}_{\odot }$ / 0.072 $\pm$ 0.003\,M${}_{\odot }$], effective temperature [2350 $\pm$ 38\,K / 2200 $\pm$ 43\,K], and radius [0.113 $\pm$ 0.003\,R${}_{\odot }$ / 0.108 $\pm$ 0.002\,R${}_{\odot }$]. Based on the system's binding energy, total mass, and separation, J1250+0455AB is predicted to be a highly stable system, remaining bound for $>$ 10\,Gyr. J1250+0455AB extends the growing population of UCD benchmark systems, providing a new system for refining evolutionary theories at the lowest stellar masses into the substellar regime.

12 pages, 8 figures