An Updated Look at Binary Characteristics of Massive Stars in the Cygnus Ob2 Association

This work provides a statistical analysis of the massive star binary characteristics in the Cygnus OB2 Association using radial velocity information of 114 B3-O3 primary stars and orbital properties for the 24 known binaries. We compare these data to a series of Monte Carlo simulations to infer the intrinsic binary fraction and distributions of mass ratios, periods, and eccentricities. We model the distribution of mass ratio, log-period, and eccentricity as power-laws and find best fitting indices of alpha=0. 1+/-0. 5, beta=0. 2+/-0. 4, and gamma=-0. 6+/-0. 3 respectively. These distributions indicate a preference for massive companions, short periods, and low eccentricities. Our analysis indicates that the binary fraction of the cluster is 44+/-8% if all binary systems are (artificially) assumed to have P<1000 days; if the power-law period distribution is extrapolated to 10⁴ years, a plausible upper limit for bound systems, the binary fraction is ~90+/-10%. Of these binary (or higher order) systems, ~45% will have companions close enough to interact during pre- or post-main-sequence evolution (semi-major axis ~/<4. 7 AU). The period distribution for P<27 days is not well reproduced by any single power-law owing to an excess of systems with periods around 3-5 days (0. 08-0. 31 AU) and a relative shortage of systems with periods around 7-14 days (0. 14-0. 62 AU). We explore the idea that these longer-period systems evolved to produce the observed excess of short-period systems. The best fitting binary parameters imply that secondaries generate, on average, ~16% of the V-band light in young massive populations. This means that photometrically based distance measurements for young massive clusters & associations will be systematically low by ~8% (0. 16 mag in the distance modulus) if the luminous contributions of unresolved secondaries are not taken into account.