Toward Complete Statistics of Massive Binary Stars: Penultimate Results From the Cygnus Ob2 Radial Velocity Survey

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 days, but it is not scale-free. Inflections in the cumulative distribution near 6 days, 14 days, and 45 days 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 β −0.22 provides a crude approximation over P = 1.4-2000 days, as does a piece-wise linear function with a break near 45 days. The cumulative period distribution flattens at P > 45 days, 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 days 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 days. Completeness corrections imply a binary fraction near 55% for P < 5000 days. 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 −1 attributed to atmospheric fluctuations.

The effect of pair-instability mass loss on black-hole mergers

“…Initial parameters for massive binary stars (progenitors of neutron stars and black holes; M ZAMS 7-10 M ⊙ ) are guided by recent observations of O/B binaries (Sana et al 2012;Kobulnicky et al 2014). The primary mass is chosen from a three component broken power-law initial mass function with a rather flat power-law exponent for massive stars (α IMF = −2.3), a flat mass ratio distribution is used to calculate the secondary mass, binaries are assumed to form predominantly on close (∝ log P orb −0.5 ) and rather circular orbits (∝ e −0.42 ).…”

Physical properties of seven binary and higher-order multiple OB systems

“…There are, of course, other studies of individual binaries where the orbit of a third body is measured interferometrically (Mayer et al 2014) and of members of associations (Kiminki et al 2007;Kiminki & Kobulnicky 2012;Kobulnicky et al 2014;Sana et al , 2013.…”

A dearth of short-period massive binaries in the young massive star forming region M 17

“…Direct measurements of the orbital period distribution further confirm the relative abundance of short-period binaries, with at least 20% of all OB-type binaries having a period of less than a week; and 40% to 50%, a period of less than a month Kobulnicky et al 2014;Dunstall et al 2015;Almeida et al 2017).…”

“…This is in stark contrast to the overall properties of small and large samples of fully formed, mainsequence massive stars (Kouwenhoven, et al 2007;Sana et al 2012Sana et al , 2013aKiminki & Kobulnicky 2012;Kobulnicky et al 2014;Dunstall et al 2015;Almeida et al 2017). These works have indeed established that 40% to 50% of OB-type binaries have a period of one month or less, with large RV amplitudes (∆v rad > 100 km s −1 ).…”

“…The distribution corresponding to P cutoff ≈ 9 mo is intermediate between those of P cutoff ≈ 8 yr and 1 month and is not shown for clarity. such as, Cyg OB2 (Kobulnicky et al 2014), the Carina nebula (Sana & Evans 2011) and the Tarantula region in the Large Magellanic Cloud Dunstall et al 2015).…”