A Hokupa'a/Gemini survey of the lowest mass/faintest guide stars: The very low mass binary population and its implications for brown dwarf formation theories

Use of the highly sensitive Hokupa'a/Gemini curvature wavefront sensor has allowed, for the first time, direct adaptive optics (AO) guiding on very low mass (VLM) stars with cool spectral types (M8.0-L0.5). These low mass (Mass < 100M_Jupiter) objects are very cool (T_eff < 3000K) and very low luminosity (V ≳ 20 at D=20 pc). However, they are red enough (V - I ∼ 4) for the Houkupa'a curvature WFS to guide on the reddest (λ_eff ∼ 0.8μ) photons. This is the only high angular resolution (FWHM ∼ 0.1″) survey of stars cooler than M7 from the ground. We guided on 39 such objects and detected 9 VLM binaries (7 of which were discovered for the first time to be binaries). Most of these systems (55%) are tight (separation < 5 AU) and have similar masses (ΔKs < 0.8 mag; 0.85 < q < 1.0). However, 2 systems (LHS 2397a, and 2M2331016-040618) have large ΔKs > 2.38 mag and consist of a VLM star orbited by a much cooler L6.5-L8.5 brown dwarf companion. Based on our initial flux limited (Ks < 12 mag) survey of 39 M8.0-L0.5 stars (mainly from the sample of Gizis et al. 2000) we find a binary fraction in the range 19 ± 7% for M8.0-L0.5 binaries with separations > 2.6 AU. This is slightly less than the 32 ± 9% measured for more massive (M0-M4) stars 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 stars which have a broad peak at larger ∼ 30 AU separations. We also find no VLM binaries (M_tot < 0.18M_⊙) with separations > 20 AU. We find that a velocity "kick" of ∼ 3 km/s can reproduce the observed cut-off in the semi-major axis distribution at ∼ 20 AU. This kick may have been from the VLM system being ejected from its formation mini-cluster.