A KECK LGS AO SEARCH FOR BROWN DWARF AND PLANETARY MASS COMPANIONS TO UPPER SCORPIUS BROWN DWARFS

We searched for binary companions to 20 young brown dwarfs in the Upper Scorpius association (145 pc, 5 Myr, nearest OB association) with the Laser Guide Star adaptive optics system and the facility infrared camera NIRC2 on the 10 m Keck II telescope. We discovered a 0 ''.14 companion (20.9 +/- 0.4 AU) to the <0.1 M-circle dot object SCH J16091837-20073523. From spectral deconvolution of integrated-light near-IR spectroscopy of SCH1609 using the SpeX spectrograph (Rayner et al. 2003), we estimate primary and secondary spectral types of M6 +/- 0.5 and M7 +/- 1.0, corresponding to masses of 79 +/- 17 M-Jup and 55 +/- 25 M-Jup at an age of 5 Myr and masses of 84 +/- 15 M-Jup and 60 +/- 25 M-Jup at an age of 10 Myr. For our survey objects with spectral types later than M8, we find an upper limit 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 and similar young regions to set the strongest constraints to date on binary fraction for young substellar objects and very low mass stars. The binary fraction for low-mass (<40 M-Jup) brown dwarfs in Upper Sco is similar to that for T dwarfs in the field; for higher mass brown dwarfs and very low mass stars, there is an excess of medium-separation (10-50 AU projected separation) young binaries with respect to the field. These medium-separation binaries will likely survive to late ages.