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Description
We present the results of a multiplicity survey of 91 stars spanning masses of ~0.2-10 M_{sun}_ 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'' (~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 0.04 M_{sun}_ (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 ~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.
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