Multiplicity at the stellar/substellar boundary in Upper Scorpius

We present the results of a high-resolution imaging survey of 12 brown dwarfs and very low mass stars in the closest (similar to 145 pc) young (similar to 5Myr) OB association, Upper Scorpius. We obtained images with the Advanced Camera for Surveys High Resolution Channel on HST through the F555W (V), F775W (i'), and F850LP (z') filters. This survey discovered three new binary systems, including one marginally resolved pair with a projected separation of only 4.9 AU, resulting in an observed binary fraction of 25% +/- 14% at separations greater than or similar to 4 AU. After correcting for detection biases assuming a uniform distribution of mass ratios for m(s)/m(p) > 0.6, the estimated binary fraction is 33% +/- 17%. The binary fraction is consistent with that inferred for higher mass stars in Upper Sco, but the separation and mass ratio distributions appear to be different. All three low-mass binary systems in Upper Sco are tight (< 18 AU) and of similar mass (ms/mp greater than or similar to 0.6), consistent with expectations based on previous multiplicity studies of brown dwarfs and very low mass stars in the field and in open clusters. The implication is that the distinct separation and mass ratio distributions of low-mass systems are set in the formation process or at very young ages, rather than by dynamical disruption of wide systems at ages greater than or similar to 5Myr. Finally, we combine the survey detection limits with low-mass evolutionary models to show that there are no planets or very low mass brown dwarfs with masses > 10M(J) at projected separations > 20 AU or masses > 5M(J) at projected separations > 40 AU orbiting any of the low-mass (0.04 - 0.10 M-circle dot) objects in our sample.