An adaptive optics multiplicity census of young stars in Upper Scorpius

We present the results of a multiplicity survey of 91 stars spanning masses of ∼0.2-10 M _☉ 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 _☉ (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.