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 K-s-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(-0.06)(+0.11) M-circle dot, 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-circle dot for the brown dwarf, at a confidence level of 1 sigma. We use WDWARFDATE to determine the age of the binary as 1.97(-0.76)(+4.41) Gyr. A kinematic analysis shows that this binary is likely a member of the thick disc. The distance to the binary is 218(-13)(+14) pc, and hence the projected separation of the binary is 207(-12)(+13) 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.