Detection of Nine M8.0-L0.5 Binaries: The Very Low Mass Binary Population and its Implications for Brown Dwarf and VLM Star Formation

Use of the highly sensitive Hokupa'a/Gemini curvature wavefront sensor has allowed direct adaptive optics (AO) guiding on very low mass (VLM) stars with SpT=M8.0-L0.5. A survey of 39 such objects detected 9 VLM binaries. Most of these systems are tight (separation <5 AU) and have similar masses (Delta Ks<0.8 mag; 0.85<q<1.0). We find a sensitivity corrected binary fraction in the range 15+/-7% for M8.0-L0.5 stars with separations >2.6 AU. This is slightly less than the 32+/-9% measured for more massive M0-M4 dwarfs over the same separation range (Fischer & Marcy 1992). It appears M8.0-L0.5 binaries (as well as L and T dwarf binaries) have a much smaller semi-major axis distribution peak (~4 AU) than early M binaries. We also find no VLM binary systems (defined here as systems with M_tot<0.185 solar masses) with separations >15 AU. We briefly explore possible reasons why VLM binaries are slightly less common, nearly equal mass, and much more tightly bound compared to more massive binaries. We find that a kick during the ejection of a forming VLM binary from a close triple or quadruple encounter (imparting a differential kick of ~3 km/s between the members of the binary) could reproduce the observed cut-off in the semi-major axis distribution at ~20 AU. However, the estimated binarity (~<5%; Bate et al. 2002) produced by such "ejection scenarios" is below the 9-15% observed. Similarly, the dynamical decay models of Sterzik & Durisen (1998); Durisen, Sterzik, & Pickett (2001) also cannot produce a VLM binary fraction above ~5%. Our estimate of a fragmentation-produced VLM binary semi-major axis distribution contains a significant fraction of ``wide'' VLM binaries with a>20 AU in contrast to observation. Hence more detailed theoretical work will be needed to explain this very interesting VLM binary population.