An adaptive optics survey of M6.0-M7.5 Stars: Discovery of three very low mass binary systems including two probable hyades members

A survey of 30 nearby M6.0-M7.5 dwarfs with K_s < 12 mag utilizing the Hokupa'a adaptive optics system at the Gemini North Telescope has discovered three new binary systems. All three systems have separations between 0″.12 and 0″.29 (3-10 AU) with similar mass ratios (q > 0.8, ΔK_S < 0.7). This result gives further support to the suggestion that wide (a > 20 AU), very low mass (M_tot < 0.185 M_⊙) binary systems are exceedingly rare or perhaps even nonexistent. The semimajor axis distribution of these systems peaks at ∼5 AU, tighter than more massive M and G binary distributions, which have a broad peak at separations of ∼30 AU. We find a sensitivity-corrected binary fraction in the range 5_-2⁺⁴% for M6.0-M7.5 stars with separations a > 3 AU. This binary frequency is less than the ∼32% measured among early M dwarfs over the same separation range. Two of the low-mass binaries are probable Hyades open cluster members based on proper motions, cluster membership probabilities, radial velocities, and near-IR photometry. LP 415-20 has the distinction of being the tightest (3.6 AU) multiple system ever spatially resolved in the cluster, and the companions of LP 415-20 and LP 475-855 are among the least massive objects ever resolved in the Hyades, with estimated masses of 0.081 _-0.10^+0.009 and 0.082_-0.009^+0.009 M_⊙.