EDP Sciences
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Volume 474, Number 1, October IV 2007
Page(s) 77 - 104
Section Galactic structure, stellar clusters, and populations
DOI http://dx.doi.org/10.1051/0004-6361:20077719

A&A 474, 77-104 (2007)
DOI: 10.1051/0004-6361:20077719

The primordial binary population. II.

Recovering the binary population for intermediate mass stars in Scorpius OB2
M. B. N. Kouwenhoven1, 2, A. G. A. Brown3, S. F. Portegies Zwart2, 4, and L. Kaper2

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)

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 $3\sigma$ 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 $f_q(q) \propto q^{\gamma_q}$, with $\gamma_q \approx$ -0.4. Sco OB2 has a semi-major axis distribution of the form $f_a(a) \propto a^{\gamma_a}$ with $\gamma_a \approx$ -1.0 (Öpik's law), in the range $5\,{R}_\odot\la a \la 5\times 10^6 \,{R}_\odot$. 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.

Key words: Galaxy: open clusters and associations: individual: Scorpius OB2 -- stars: binaries: visual -- stars: binaries: general -- stars: formation

© ESO 2007

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