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 sub(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 ( 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, K sub(s) < 1 mag). This result supports the hypothesis that wide (a > 20 AU), very low mass (VLM; M sub(tot) < 0.19 M sub( )) binary systems are rare. The projected semimajor axis distribution of these systems peak at 65 AU, and we report a sensitivity-corrected binary fraction of 9 super(+) sub(-) super(4) sub(3)% for stars with primaries of spectral type M6.0-M7.5 with separations 3 AU and mass ratios q 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.