AN ADAPTIVE OPTICS MULTIPLICITY CENSUS OF YOUNG STARS IN UPPER SCORPIUS

We present the results of amultiplicity survey of 91 stars spanning masses of similar to 0.2-10M(circle dot) in the Upper Scorpius star-forming region, based on adaptive optics imaging with the Gemini North telescope. Our observations identified 29 binaries, 5 triples, and no higher order multiples. The corresponding raw multiplicity frequency is 0.37 +/- 0.05. In the regime where our observations are complete-companion separations of 0 ''.1-5 '' (similar to 15-800 AU) with magnitude limits ranging from K < 9.3 at 0 ''.1 to K < 15.8 at 5 '' -the multiplicity frequency is 0.27(-0.04.)(+ 0.05) For similar separations, the multiplicity frequency in Upper Scorpius is comparable to that in other dispersed star-forming regions, but is a factor of two to three higher than in denser star-forming regions or in the field. Our sample displays a constant multiplicity frequency as a function of stellar mass. Among our sample of binaries, we find that both wider (> 100 AU) and higher-mass systems tend to have companions with lower companion-to-primary mass ratios. Three of the companions identified in our survey are unambiguously substellar and have estimated masses below 0.04M(circle dot) (two of them are new discoveries from this survey-1RXS J160929.1-210524b and HIP 78530B-although we have reported them separately in earlier papers). These three companions have projected orbital separations of 300-900 AU. Based on a statistical analysis factoring in sensitivity limits, we calculate an occurrence rate of 5-40 M-Jup companions of similar to 4.0% for orbital separations of 250-1000 AU, compared to < 1.8% at smaller separations, suggesting that such companions are more frequent on wider orbits.