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⁺₋₂^5.₄⁵ Gyr and T_eff = 9500 ± 125 K DA white dwarf (WD) and a T_eff = 2400 ± 50 K M8 ultracool dwarf (UCD). The projected separation of the system is a = 1236.73 au (∼ 13.8 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 ± 0.18 M for the WD, and a mass of 98.5 ± 6.2 M_Jup (∼ 0.094 ± 0.006 M) for the UCD. The radii of the two components were also estimated at 0.01309 ± 0.0003 R and 1.22 ± 0.05 R_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 ± 0.04), which likely indicates a formation pathway similar to that of stellar binary systems.