The primordial binary population. II.^*

Recovering the binary population for intermediate mass stars in Scorpius OB2

¹ Department of Physics and Astronomy, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S3 7RH, UK e-mail: t.kouwenhoven@sheffield.ac.uk
² Astronomical Institute Anton Pannekoek, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: lexk@science.uva.nl
³ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands e-mail: brown@strw.leidenuniv.nl
⁴ Section Computer Science, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands e-mail: spz@science.uva.nl

Received: 25 April 2007
Accepted: 18 July 2007

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 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 , with -0.4. Sco OB2 has a semi-major axis distribution of the form with -1.0 (Öpik's law), in the range . 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.