An Updated Look at Binary Characteristics of Massive Stars in the Cygnus OB2 Association

TL;DR

This study statistically analyzes the binary characteristics of massive stars in Cygnus OB2, revealing a high binary fraction, distributions favoring massive companions, and implications for distance measurements in young clusters.

Contribution

It provides updated binary fraction estimates and detailed distributions of orbital parameters for massive stars in Cygnus OB2, using Monte Carlo simulations and radial velocity data.

Findings

Binary fraction is approximately 44% for P<1000 days.

Extrapolated binary fraction could be as high as 90%.

Secondaries contribute about 16% of V-band light, affecting distance estimates.

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-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 10^4 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.