A New Look at the Binary Characteristics of Massive Stars

We constrain the properties of massive binaries by comparing radial velocity data of Cygnus OB2 with 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, \alpha, describing the distribution of companion masses, and a range of power-law slopes, \beta, describing the distribution of orbital separations. We also consider distributions of secondary masses described by a Miller-Scalo type initial mass function and by a two-component IMF that includes a substantial ``twin'' population with M_2 ~ M_1. We show that binary fractions F<0.7 are considerably less probable than F>0.8. 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 - 0.0. For F~0.8, \alpha is somewhat larger, in the range -0.4 - 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 solar masses. 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 M_2 ~ M_1, i.e., ``twins''. These best-fitting 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.