A New Look at the Binary Characteristics of Massive Stars

• Published in: The Astrophysical journal 2007-11, Vol.670 (1), p.747-765
• Main Authors: Kobulnicky, Henry A, Fryer, Chris L
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/522073

We constrain the properties of massive binaries by comparing radial velocity data on early-type stars in Cygnus OB2 with the expectations of Monte Carlo models. Our comparisons test several popular prescriptions for massive binary parameters. We explore a range of true binary fraction, F, a range of power-law slopes, a, describing the distribution of companion masses between the limits q sub(low) and 1, and a range of power-law slopes, beta , describing the distribution of orbital separations between the limits r sub(in) and r sub(out). We also consider distributions of secondary masses described by a Miller-Scalo type IMF and by a two-component IMF that Includes a substantial "twin" population with M sub(2) [unk] M sub(1). Several seemingly disparate prescriptions for massive binary characteristics can be reconciled by adopting carefully chosen values for F, r sub(in), and r sub(out). We show that binary fractions F < 0.7 are less probable than F greater than or equal to 0.8 for reasonable choices of r sub(in) and r sub(out). Thus, the true binary fraction is high. For F = 1.0 and a distribution of orbital separations near the canonical "Opik's law distribution (i.e., flat; beta = 0), the power-law slope of the mass ratio distribution is alpha = -0.6 to 0.0. For F [unk] 0.8, alpha is somewhat larger, in the range -0.4 to 1.0. In any case, the secondary star mass function is inconsistent with a Miller-Scalo-like IMF unless the lower end is truncated below similar to 2-4 M [unk]. In other words, massive stars preferentially have massive companions. The best-fitting models are described by a Salpeter or Miller-Scalo IMF for 60% of secondary star masses with the other similar to 40% of secondaries having M sub(2) [unk] M sub(1), i.e., "twins." These model parameters simultaneously predict the fraction of Type Ib/c supernovae to be 30%-40% of all core-collapse supernovae, in agreement with recent observational estimates.