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Title:
A New Look at the Binary Characteristics of Massive Stars
Authors:
Kobulnicky, Henry A.; Fryer, Chris L.
Affiliation:
AA(Department of Physics and Astronomy, University of Wyoming, Laramie, WY 82070; ), AB(Theoretical Astrophysics, Los Alamos National Laboratory, Los Alamos, NM 87545; )
Publication:
The Astrophysical Journal, Volume 670, Issue 1, pp. 747-765. (ApJ Homepage)
Publication Date:
11/2007
Origin:
UCP
ApJ Keywords:
Stars: Binaries: General, Stars: Binaries: Spectroscopic, Gamma Rays: Bursts, Stars: Early-Type, Stars: Supernovae: General, Techniques: Radial Velocities, X-Rays: Binaries
DOI:
10.1086/522073
Bibliographic Code:
2007ApJ...670..747K

Abstract

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, α, describing the distribution of companion masses between the limits qlow and 1, and a range of power-law slopes, β, describing the distribution of orbital separations between the limits rin and rout. 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 M2~=M1. Several seemingly disparate prescriptions for massive binary characteristics can be reconciled by adopting carefully chosen values for F, rin, and rout. We show that binary fractions F<0.7 are less probable than F>=0.8 for reasonable choices of rin and rout. 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; β=0), the power-law slope of the mass ratio distribution is α=-0.6 to 0.0. For F~=0.8, α 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 ~2-4 Msolar. 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 ~40% of secondaries having M2~=M1, 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.
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