Discovery of a resolved white dwarf–brown dwarf binary with a small projected separation: SDSS J222551.65+001637.7AB

We present the confirmation of SDSS J222551.65+001637.7AB as a closely separated, resolved, white dwarf–brown dwarf binary. We have obtained spectroscopy from GNIRS and seeing-limited Ks-band imaging from NIRI on Gemini North. The target is spatially resolved into its constituent components: a 10926 ± 246 K white dwarf, with log g = 8.214 ± 0.168 and a mass of 0.66^+0.11_-0.06 M_⊙, and an L4 brown dwarf companion, which are separated by 0.9498 ± 0.0022 arcsec. We derive the fundamental properties of the companion from the Sonora–Bobcat evolutionary models, finding a mass of 25–53 M_Jup and a radius of 0.101–0.128 R_⊙ for the brown dwarf, at a confidence level of 1σ. We use wdwarfdate to determine the age of the binary as 1.97^+4.41_-0.76 Gyr. A kinematic analysis shows that this binary is likely a member of the thick disc. The distance to the binary is 218⁺¹⁴_-13 pc, and hence the projected separation of the binary is 207⁺¹³_-12 au. Whilst the white dwarf progenitor was on the main sequence the binary separation would have been 69 ± 5 au. SDSS J222551.65+001637.7AB is the third closest spatially resolved white dwarf–brown dwarf binary after GD 165AB and PHL 5038AB.