An updated look at binary characteristics of massive stars in the CYGNUS OB2 association

Abstract

This work provides a statistical analysis of the massive star binary characteristics in the Cygnus OB2 association using radial velocity information of 114 B3-O5 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 α = 0.1 ± 0.5, β = 0.2 ± 0.4, and γ = -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 10 4 years, then 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 < 26 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 and 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. © 2012. The American Astronomical Society. All rights reserved..