A benchmark white dwarf–ultracool dwarf wide field binary.

We present the discovery and multiwavelength characterization of VVV J1438-6158 AB, a new field wide-binary system consisting of a |$4.6^{+5.5}_{-2.4}~{\rm Gyr}$| and |$T_{\rm eff} = 9500\pm 125~\mathrm{ K}$| DA white dwarf (WD) and a |$T_{\rm eff} = 2400\pm 50~\mathrm{ K}$| M8 ultracool dwarf (UCD). The projected separation of the system is |$a = 1236.73~{\rm au}$| (⁠|$\sim 13.8\, \mathrm{ arcsec}$|⁠), and although along the line of sight towards the Scorpius–Centaurus stellar association, VVV J1438-6158 AB is likely to be a field star, from a kinematic 6D probabilistic analysis. We estimated the physical and dynamical parameters of both components via interpolations with theoretical models and evolutionary tracks, which allowed us to retrieve a mass of |$0.62\pm 0.18~\mathrm{ M}_\odot$| for the WD, and a mass of |$98.5\pm 6.2~\mathrm{ M}_{\rm Jup}$| (⁠|$\sim 0.094\pm 0.006~\mathrm{ M}_\odot$|⁠) for the UCD. The radii of the two components were also estimated at |$0.01309\pm 0.0003~\mathrm{ R}_{\odot }$| and |$1.22\pm 0.05~\mathrm{ R}_{\rm Jup}$|⁠ , respectively. VVV J1438-6158 AB stands out as a benchmark system for comprehending the evolution of WDs and low-mass companions given its status as one of the most widely separated WD+UCD systems known to date, which likely indicates that both components may have evolved independently of each other, and also being characterized by a large-mass ratio (⁠|$q = 0.15\pm 0.04$|⁠), which likely indicates a formation pathway similar to that of stellar binary systems. [ABSTRACT FROM AUTHOR]