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

Abstract

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$\pm$ 246 K white dwarf, with log $g=8.214\pm 0.168$ and a mass of 0.66${}_{-0.06}^{+0.11}$ M${}_{\odot }$, and an L4 brown dwarf companion, which are separated by $0.9498\pm 0.0022$". We derive the fundamental properties of the companion from the Sonora-Bobcat evolutionary models, finding a mass of $25-53$ M${}_{\text{Jup}}$ and a radius of $0.101-0.128$ R${}_{\odot }$ 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 $69\pm 5$ AU. SDSS J222551.65+001637.7AB is the third closest spatially resolved white dwarf-brown dwarf binary after GD 165AB and PHL 5038AB.

9 pages, 6 figures. Accepted for publication in MNRAS

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