An Adaptive Optics Survey of M8-M9 Stars: Discovery of Four Very Low Mass Binaries with at Least One System Containing a Brown Dwarf Companion

Use of the highly sensitive Hokupa'a/Gemini curvature wavefront sensor hasallowed for the first time direct adaptive optics (AO) guiding on M8-M9 verylow mass (VLM) stars. An initial survey of 20 such objects (SpT=M8-M9)discovered 4 binaries. Three of the systems have separations of less than 4.2AU and similar mass ratios (Delta K<0.8 mag; 0.853 AU. This is likely consistent with the 23+/-5% measured for moremassive (M0-M6) stars over the same separation range. It appears M8-M9 binarieshave a much smaller semi-major axis distribution peak (~4 AU; with no systemswider than 15 AU) compared to M and G stars which have a broad peak at larger\~30 AU separations.Comment: Accepted in Astrophysical Letters (March). 12 pages, 2 figure