Abstract

We present the discovery and multiwavelength characterization of VVV J1438-6158 AB, a new field wide-binary system consisting of a 4.6(-2.4)(+5.5) 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 (similar to 13.8arcsec), 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-circle dot for the WD, and a mass of 98.5 +/- 6.2 MJup (similar to 0.094 +/- 0.006 M-circle dot) for the UCD. The radii of the two components were also estimated at 0.01309 +/- 0.0003 R-circle dot 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.