Abstract
We characterize the binary population in the young and nearby OB association Scorpius OB2 (Sco OB2) using available observations of visual, spectroscopic, and astrometric binaries with intermediate-mass primaries. We take into account observational biases by comparing the observations with simulated observations of model associations. The available data indicate a large binary fraction (> 70% with 3σ confidence), with a large probability that all intermediate mass stars in Sco OB2 are part of a binary system. The binary systems have a mass ratio distribution of the form f_q(q) ∝ qγ_q, with γq ≈ -0.4. Sco OB2 has a semi-major axis distribution of the form f_a(a) ∝ aγ_a with γa ≈ -1.0 (Öpik's law), in the range 5 {R}_⊙⪉ a ⪉ 5× 106 {R}_⊙. The log-normal period distribution of Duquennoy & Mayor [1991, A&A, 248, 485] results in too few spectroscopic binaries, even if the model binary fraction is 100%. Sco OB2 is a young association with a low stellar density; its current population is expected to be very similar to the primordial population. The fact that practically all stars in Sco OB2 are part of a binary (or multiple) system demonstrates that multiplicity is a fundamental factor in the star formation process, at least for intermediate mass stars.
Appendix A is only available in electronic form at http://www.aanda.org