SCYON Abstract

Received on July 19 2007

The primordial binary population II: Recovering the binary population for intermediate mass stars in Sco OB2

Authors	M.B.N. Kouwenhoven (^1,2), A.G.A. Brown (³), S.F. Portegies Zwart (^2,4), and L. Kaper (²)
Affiliation	(¹) Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, United Kingdom (²) Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands (³) Leiden Observatory, University of Leiden, P.O. Box 9513, 2300 RA Leiden, The Netherlands (⁴) Section Computer Science, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
Accepted by	Astronomy & Astrophysics
Contact	t.kouwenhoven@sheffield.ac.uk
URL	http://xxx.lanl.gov/abs/0707.2746
Links

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) ∝ q^γ_q, with γ_q ≈ -0.4. Sco OB2 has a semi-major axis distribution of the form f_a(a) ∝ a^γ_a with γ_a ≈ -1.0 (Öpik's law), in the range 5 R(sun) < a < 5x10⁶ R(sun). The log-normal period distribution of Duquennoy & Mayor 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.