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 wave-front sensor has allowed, for the first time, direct adaptive optics guiding on M8-M9 very low mass (VLM) stars. An initial survey of 20 such objects (spectral type = M8-M9) discovered four binaries. Three of the systems have separations of less than 4.2 AU and similar mass ratios (ΔK < 0.8 mag; 0.85 < q < 1.0). One system, however, did have the largest mass ratio (ΔK = 2.38 mag) and separation (14.4 AU) yet observed for a VLM star with a brown dwarf companion. Based on our initial flux-limited (K_s < 12 mag) survey of 20 M8-M9 stars over 14^h26^m < R.A. < 4 ^h30^m from the sample of Gizis et al., we find a binary fraction in the range of 14%-24% for M8-M9 binaries with separations of less than 3 AU. This is likely consistent with the 23% ± 5% measured for more massive (M0-M6) stars over the same separation range. It appears that M8-M9 binaries have a much smaller semimajor axis distribution peak (∼4 AU; with no systems wider than 15 AU) compared with M and G stars that have a broad peak at larger ∼30 AU separations.