Toward Complete Statistics of Massive Binary Stars: Penultimate Results from the Cygnus OB2 Radial Velocity Survey

TL;DR

This study analyzes the orbital periods and properties of 48 massive binary star systems in Cygnus OB2, revealing key physical scales, a roughly uniform period distribution, and a binary fraction of about 55%, with implications for star formation physics.

Contribution

It provides the first detailed orbital solutions for 23 new massive binary systems and characterizes the period, mass ratio, and eccentricity distributions in Cygnus OB2.

Findings

Orbital period distribution is approximately uniform in log P for P<45 days.

Features in the distribution suggest physical scales at ~0.2, ~0.4, and ~1 AU.

Binary fraction is estimated to be around 55% for P<5000 days.

Abstract

We analyze orbital solutions for 48 massive multiple-star systems in the Cygnus OB2 Association, 23 of which are newly presented here, to find that the observed distribution of orbital periods is approximately uniform in log P for P<45 d, but it is not scale-free. Inflections in the cumulative distribution near 6 d, 14, d, and 45 d, suggest key physical scales of ~0.2, ~0.4, and ~1 A.U. where yet-to-be-identified phenomena create distinct features. No single power law provides a statistically compelling prescription, but if features are ignored, a power law with exponent beta = -0.22 provides a crude approximation over P=1.4 -- 2000 d, as does a piece-wise linear function with a break near 45 d. The cumulative period distribution flattens at P > 45 d, even after correction for completeness, indicating either a lower binary fraction or a shift toward low-mass companions. A high degree of similarity (91% likelihood) between the Cyg OB2 period distribution and that of other surveys suggests that the binary properties at P<25 d are determined by local physics of disk/clump fragmentation and are relatively insensitive to environmental and evolutionary factors. Fully 30% of the unbiased parent sample is a binary with period P < 45 d. Completeness corrections imply a binary fraction near 55% for P < 5000 d. The observed distribution of mass ratios 0.2<q<1 is consistent with uniform, while the observed distribution of eccentricities 0.1<e<0.6 is consistent with uniform plus an excess of e ~ 0 systems. We identify six stars, all supergiants, that exhibit aperiodic velocity variations of ~30 km/s attributed to atmospheric fluctuations.