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We present results of a high angular resolution survey of massive OB stars inthe Cygnus OB2 association that we conducted with the NIRI camera and ALTAIRadaptive optics system of the Gemini North telescope. We observed 74 O- andearly B-type stars in Cyg OB2 in the $JHK$ infrared bands in order to detectbinary and multiple companions. The observations are sensitive toequal-brightness pairs at separations as small as 0.08 \arcsec, andprogressively fainter companions are detectable out to $\Delta$ K = 9 mag at aseparation of 2 arcsec. This faint contrast limit due to readnoise continuesout to 10 arcsec near the edge of the detector. We assigned a simpleprobability of chance alignment to each companion based upon its separation andmagnitude difference from the central target star and upon areal star countsfor the general star field of Cyg OB2. Companion stars with a field membershipprobability of less than 1% are assumed to be physical companions. Thisassessment indicates that 47% of the targets have at least one resolvedcompanion that is probably gravitationally bound. Including known spectroscopicbinaries, our sample includes 27 binary, 12 triple, and 9 systems with four ormore components. These results confirm studies of high mass stars in otherenvironments that find that massive stars are born with a high multiplicityfraction. The results are important for the placement of the stars in the H-Rdiagram, the interpretation of their spectroscopic analyses, and for futuremass determinations through measurement of orbital motion.

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