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 (7 of which were discovered for the first time to be binaries). Most of these systems are tight (separation < 5 AU) and have similar masses (∆Ks < 0.8 mag; 0.85 < q < 1.0). However, 2 systems (LHS 2397a and 2M2331016-040618) have large ∆Ks > 2.4 mag and consist of a VLM star orbited by a much cooler L7-L8 brown dwarf companion. Based on this flux limited (Ks < 12 mag) survey of 39 M8.0-L0.5 stars (mainly from the 2MASS sample of Gizis et al. (2000)) 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) compared to more massive M and G dwarfs which have a broad peak at larger ∼ 30 AU separations. We also find no VLM binary systems (defined here as systems with Mtot < 0.185M⊙) 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 investigate the hypothesis that the lack of wide (a > 20 AU) VLM/brown dwarf binaries may be explained if the binary components were given a significant differential velocity kick. Such a velocity kick is predicted by current “ejection” theories, where brown dwarfs are formed because they are ejected from their embryonic mini-cluster and therefore starved of accretion material. We find that a kick from a close triple or quadruple encounter (imparting a differential kick of ∼ 3 km/s between the members of an escaping 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 15± 7% observed. Similarly, VLM binaries could be the final hardened binaries produced when a mini-cluster decays. However, the models of Sterzik & Durisen (1998); Durisen, Sterzik, & Pickett (2001) also cannot produce a VLM binary fraction above ∼ 5%. The observed VLM binary frequency could possibly be produced by cloud core fragmentation. Although, 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. In summary, more detailed theoretical work will be needed to explain these interesting results which show VLM binaries to be a significantly different population from more massive M & G dwarf binaries. Subject headings: instrumentation: adaptive optics — binaries: general — stars: evolution — stars: formation — stars: low-mass, brown dwarfs