A High Angular Resolution Survey of Massive Stars in Cygnus OB2: JHK Adaptive Optics Results from the Gemini Near-InfraRed Imager

We present results of a high angular resolution survey of massive OB stars in theCygnus OB2 association that we conducted with the NIRI camera and ALTAIR adaptive optics system of the Gemini North telescope. We observed 74 O- and early B-typestars in Cyg OB2 in the JHK infrared bands in order to detect binary and multiplecompanions. The observations are sensitive to equal-brightness pairs at separationsas small as 0.′′08, and progressively fainter companions are detectable out to △K = 9mag at a separation of 2′′. This faint contrast limit due to readnoise continues out to10 arcsec near the edge of the detector. We assigned a simple probability of chancealignment to each companion based upon its separation and magnitude difference fromthe central target star and upon areal star counts for the general star field of Cyg OB2.Companion stars with a field membership probability of less than 1% are assumed tobe physical companions. This assessment indicates that 47% of the targets have atleast one resolved companion that is probably gravitationally bound. Including knownspectroscopic binaries, our sample includes 27 binary, 12 triple, and 9 systems withfour or more components. These results confirm studies of high mass stars in otherenvironments that find that massive stars are born with a high multiplicity fraction.The results are important for the placement of the stars in the H-R diagram, the inter-2 Caballero-Nieves et al.pretation of their spectroscopic analyses, and for future mass determinations throughmeasurement of orbital motion