The primordial binary population. II. Recovering the binary population for intermediate mass stars in Scorpius OB2*

We characterize the binary population in the young and nearby OB association Scorpius OB2 (Sco OB2) using available observations of visual, spectroscopic, and astrometrie 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) ∝ q^γq, with γ_q ≈ -0.4. Sco OB2 has a semi-major axis distribution of the form f_a(a) ∝ a^γa with γ_a ≈ -1.0 (Öpik's law), in the range 5 R_⊙ ≲ a ≲ 5 × 10⁶R _⊙. The log-normal period distribution of Duquennoy & Mayor (1991) 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.