Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. I. DENIS J220002.05-303832.9AB

We present the discovery of the common proper motion M9+L0 binary DENIS J220002.05-303832.9AB, identified serendipitously with the SpeX near-infrared imager/spectrograph. Spectral types are derived from resolved near-infrared spectroscopy of the well-separated (1.09"+/-0.06") components and comparison to equivalent data for M and L dwarf spectral standards. Physical association is deduced from the angular proximity of the sources, their common proper motion, and their similar spectrophotometric distances (35+/-2 pc). The estimated distance of this pair implies a projected separation of 38+/-3 AU, wider than typical separations for other M dwarf/L dwarf binaries but consistent with the maximum separation/total system mass trend previously identified by Burgasser et al. We discuss the DENIS 2200-3038AB system in context with other low-mass binaries and its role in the study of dust formation processes and activity trends across the transition between the M and L dwarf spectral classes.