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We searched for binary companions to 20 young brown dwarfs in the UpperScorpius association (145 pc, 5 Myr, nearest OB association) with the the LaserGuide Star adaptive optics system and the facility infrared camera NIRC2 on the10 m Keck II telescope. We discovered a 0.14" companion (20.9+-0.4 AU) to the<0.1 MSun object SCH J16091837-20073523. From spectral deconvolution ofintegrated-light near-IR spectroscopy of SCH1609 using the SpeX spectrograph(Rayner et al. 2003), we estimate primary and secondary spectral types ofM6+-0.5 and M7+-1.0, corresponding to masses of 79+-17 MJup and 55+-25 MJup atan age of 5 Myr and masses of 84+-15 MJup and 60+-25 MJup at an age of 10 Myr.For our survey objects with spectral types later than M8, we find an upperlimit on the binary fraction of <9% (1-sigma) at separations of 10 -- 500 AU.We combine the results of our survey with previous surveys of Upper Sco andsimilar young regions to set the strongest constraints to date on binaryfraction for young substellar objects and very low mass stars. The binaryfraction for low mass (<40 MJup) brown dwarfs in Upper Sco is similar to thatfor T dwarfs in the field; for higher mass brown dwarfs and very low massstars, there is an excess of medium-separation (10-50 AU projected separation)young binaries with respect to the field. These medium separation binaries willlikely survive to late ages.

A Keck LGS AO Search for Brown Dwarf and Planetary Mass Companions to Upper Scorpius Brown Dwarfs