An Adaptive Optics Survey of M8-M9 stars: Discovery of 4 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 has allowed for the first time direct adaptive optics (AO) guiding on M8-M9 very low 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 4.2 AU and similar mass ratios (∆K < 0.8 mag; 0.85 < q < 1.0). One system, however, did have the largest ∆K = 2.38 mag and sep = 14.4 AU yet observed for a VLM star with a brown dwarf companion. Based on our initial flux limited (Ks < 12 mag) survey of 20 M8-M9 stars over 14 : 26 < RA < 4 : 30 hours from the sample of Gizis et al. (2000) we find a binary fraction in the range 14 − 24% for M8-M9 binaries with sep > 3 AU. This is likely consistent with the 23±5% measured for more massive (M0-M6) stars over the same separation range. It appears M8-M9 binaries have a much smaller semi-major axis distribution peak (∼ 4 AU; with no systems wider than 15 AU) compared to M and G stars which have a broad peak at larger ∼ 30 AU separations. Subject headings: instrumentation: adaptive optics — binaries: general — stars: evolution — stars: formation — stars: low-mass, brown dwarfs