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 (≲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, ΔK_s < 1 mag). This result supports the hypothesis that wide (a > 20 AU), very low mass (VLM; M_tot < 0.19 M_⊙) binary systems are rare. The projected semimajor axis distribution of these systems peak at ∼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 ≳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.