Subtle Signatures of Multiplicity in Late - Type Dwarf Spectra : the Unresolved M 8 . 5 + T 5 Binary 2 Mass

Evidence is presented that 2MASS J03202839−0446358, a late-type dwarf with discrepant optical (M8:) and near-infrared (L1) spectral types, is an as-yet unresolved stellar/brown dwarf binary with late-type M dwarf and T dwarf components. This conclusion is based on low-resolution, near-infrared spectroscopy that reveals a subtle but distinctive absorption feature at 1.6 μm. The feature, which is also present in the combined light spectrum of the M8.5 + T6 binary SCR 1845−6357, arises from the combination of FeH absorption from an M8.5 primary and pseudo-continuum flux from a T5±1 secondary, as ascertained from binary spectral templates constructed from empirical data. The binary templates provide a far superior match to the overall near-infrared spectral energy distribution of 2MASS J0320−0446 than any single comparison spectra. Laser guide star adaptive optics (LGS AO) imaging observations, including the first application of LGS AO aperture mask interferometry, fail to resolve a faint companion, restricting the projected separation of the system to less than 8.3 AU at the time of observation. 2MASS J0320−0446 is the second very low mass binary to be identified from unresolved, low-resolution, near-infrared spectroscopy, a technique that complements traditional high resolution imaging and spectroscopic methods. Subject headings: stars: binaries: general — stars: fundamental parameters — stars: individual (2MASS J03202839−0446358) — stars: low mass, brown dwarfs