|
Description
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<45days, but it is not scale-free. Inflections in the cumulative distribution near 6days, 14days, and 45days suggest key physical scales of =~0.2, =~0.4, and =~1A.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=1.4-2000days, as does a piece-wise linear function with a break near 45days. The cumulative period distribution flattens at P>45days, 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<~25days 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<45days. Completeness corrections imply a binary fraction near 55% for P<5000days. 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 ~30km/s attributed to atmospheric fluctuations.
|
