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 (016) 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 nearinfrared). 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.