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

TL;DR

This study used adaptive optics imaging to survey 74 massive stars in Cygnus OB2, revealing a high multiplicity rate with nearly half having gravitationally bound companions, which impacts understanding of massive star formation.

Contribution

First high angular resolution infrared survey of massive stars in Cygnus OB2, providing new data on their binary and multiple star systems.

Findings

47% of targets have at least one probable physical companion

Sample includes 27 binaries, 12 triples, and 9 systems with four or more components

Results support high multiplicity fraction among massive stars

Abstract

We present results of a high angular resolution survey of massive OB stars in the Cygnus 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-type stars in Cyg OB2 in the $JHK$ infrared bands in order to detect binary and multiple companions. The observations are sensitive to equal-brightness pairs at separations as small as 0.08 \arcsec, and progressively fainter companions are detectable out to $\Delta$ K = 9 mag at a separation of 2 arcsec. This faint contrast limit due to readnoise continues out to 10 arcsec near the edge of the detector. We assigned a simple probability of chance alignment to each companion based upon its separation and magnitude difference from the 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 to be physical companions. This assessment indicates that 47% of the targets have at least one resolved companion that is probably gravitationally bound. Including known spectroscopic binaries, our sample includes 27 binary, 12 triple, and 9 systems with four or more components. These results confirm studies of high mass stars in other environments 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 interpretation of their spectroscopic analyses, and for future mass determinations through measurement of orbital motion.