Discovery of two very low mass binaries:: Final results of an adaptive optics survey of nearby M6.0-M7.5 stars

We present updated results of a high-resolution, magnitude-limited (K-s < 12 mag) imaging survey of nearby low-mass M6.0-M7.5 field stars. The observations were carried out using adaptive optics at the Gemini North, VLT, Keck II, and Subaru telescopes. Our sample of 36 stars consists predominantly of nearby (less than or similar to 30 pc) field stars, five of which we have resolved as binaries. Two of the binary systems, 2MASSI J0429184-312356 and 2MASSI J1847034+552243, are presented here for the first time. All five of the discovered binary systems have separations between 0.'' 08 and 0.'' 53 (2-9 AU) with similar mass ratios (q > 0: 8, Delta K-s < 1 mag). This result supports the hypothesis that wide (a > 20 AU), very low mass (VLM; M-tot < 0: 19 M-infinity) binary systems are rare. The projected semimajor axis distribution of these systems peak at similar to 5 AU, and we report a sensitivity-corrected binary fraction of 9(-3)(+4) % % for stars with primaries of spectral type M6.0-M7.5 with separations greater than or similar to 3 AU and mass ratios q greater than or similar to 0.6. Within these instrumental sensitivities, these results support the overall trend that both the semimajor axis distribution and binary fraction are a function of the mass of the primary star and decrease with decreasing primary mass. These observations provide important constraints for low-mass binary star formation theories.