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7 February 2003 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
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Abstract
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 < 100 MJUPITER) objects are very cool (Teff<3000K) and have very low luminosities (V ⪆ 20 at D = 20 pc) but are red enough (V-I ~ 4) for the Houkupa'a curvature WFS to guide on the reddest (λeff~ 0.8 μm) photons. This is the only high-resolution (FWHM~0.1") survey ever 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 (Δ Ks<0.8 mag; 0.85<q<1.0). However, 2 systems (LHS 2397a, and 2M2331016-040618) have large Δ Ks>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 (Mtot<0.18 Msun) 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.
© (2003) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Laird Miller Close, Nicholas Siegler, and Melanie E. Freed "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", Proc. SPIE 4839, Adaptive Optical System Technologies II, (7 February 2003); https://doi.org/10.1117/12.458958
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