A Benchmark White Dwarf–Ultracool Dwarf Wide Field Binary

Abstract

We present the discovery and multi-wavelength characterisation of VVV J1438-6158 AB, a new field wide-binary system consisting of a $4.6^{+5.5}_{-2.4}~{\rm Gyr}$ and Teff = 9500 ± 125 K DA white dwarf (WD) and a Teff = 2400 ± 50 K M8 ultracool dwarf (UCD). The projected separation of the system is a = 1236.73 au (∼13.8″), and although along the line-of-sight towards the Scorpius-Centaurus (Sco-Cen) 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 MJup (∼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 RJup, 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 characterised by a large mass-ratio (q = 0.15 ± 0.04), which likely indicates a formation pathway similar to that of stellar binary systems.