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

M. B.N. Kouwenhoven*, A. G.A. Brown, S. F. Portegies Zwart, L. Kaper

*Corresponding author for this work

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215 Citations (Scopus)


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) ∝ qγq, with γq ≈ -0.4. Sco OB2 has a semi-major axis distribution of the form fa(a) ∝ aγa with γa ≈ -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.

Original languageEnglish
Pages (from-to)77-104
Number of pages28
JournalAstronomy and Astrophysics
Issue number1
Publication statusPublished - Oct 2007
Externally publishedYes


  • Galaxy: open clusters and associations: individual: Scorpius OB2
  • Stars: binaries: general stars: formation
  • Stars: binaries: visual

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