SDSS J042348.57-041403.5AB: A Brown dwarf binary straddling the L/T transition

We present the discovery of SDSS J042348.57-041403.5 as a closely separated (0″.16) brown dwarf binary, resolved by the Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer. Physical association is deduced from the angular proximity of the components and constraints on their common proper motion. SDSS 0423-0414AB appears to be composed of two brown dwarfs with spectral types L6 ± 1 and T2 ± 1. Hence, this system straddles the transition between L dwarfs and T dwarfs, a unique evolutionary phase of brown dwarfs characterized by substantial shifts in spectral morphology over an apparently narrow effective temperature range. Binarity explains a number of unusual properties of SDSS 0423-0414, including its overluminosity and high effective temperature compared to other early-type T dwarfs, and possibly its conflicting spectral classifications (L7.5 in the optical, T0 in the near-infrared). The relatively short estimated orbital period of this system (∼ 15-20 yr) and the presence of Li i absorption in its combined light spectrum make it an ideal target for both resolved spectroscopy and dynamical mass measurements. SDSS 0423-0414AB joins a growing list of late-L/ early-T dwarf binaries, the high percentage of which (∼50%) may provide a natural explanation for observed peculiarities across the L/T transition.