Multiplicity at the Stellar/Substellar Boundary in Upper Scorpius

We present the results of a high-resolution imaging survey of 12 brown dwarfsand very low mass stars in the closest (~145 pc) young (~5 Myr) OB association,Upper Scorpius. We obtained images with the Advanced Camera for Surveys/HighResolution Camera on HST through the F555W (V), F775W (i'), and F850LP (z')filters. This survey discovered three new binary systems, including onemarginally resolved pair with a projected separation of only 4.9 AU, resultingin an observed binary fraction of 25+/-14% at separations >4 AU. Aftercorrecting for detection biases assuming a uniform distribution of mass ratiosfor q>0.6, the estimated binary fraction is 33+/-17%. The binary fraction isconsistent with that inferred for higher-mass stars in Upper Sco, but theseparation and mass ratio distributions appear to be different. All threelow-mass binary systems in Upper Sco are tight (<18 AU) and of similar mass(q>0.6), consistent with expectations based on previous multiplicity studies ofbrown dwarfs and very low mass stars in the field and in open clusters. Theimplication is that the distinct separation and mass ratio distributions oflow-mass systems are set in the formation process or at very young ages, ratherthan by dynamical disruption of wide systems at ages >5 Myr. Finally, wecombine the survey detection limits with the models of Burrows et al. (1997) toshow that there are no planets or very low-mass brown dwarfs with masses >10M_J at projected separations >20 AU, or masses >5 M_J at projected separations>40 AU orbiting any of the low-mass (0.04-0.10 M_sun) objects in our sample.Comment: Accepted for publication in ApJ; 10 pages, 4 figures in emulateapj forma