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 Issue A&A Volume 464, Number 2, March III 2007 581 - 599 Galactic structure, stellar clusters, and populations http://dx.doi.org/10.1051/0004-6361:20054396

A&A 464, 581-599 (2007)
DOI: 10.1051/0004-6361:20054396

## A brown dwarf desert for intermediate mass stars in Scorpius OB2?

M. B. N. Kouwenhoven1, A. G. A. Brown2, and L. Kaper1

1  Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
e-mail: t.kouwenhoven@sheffield.ac.uk; [kouwenho;lexk]@science.uva.nl
2  Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: brown@strw.leidenuniv.nl

(Received 21 October 2005 / Accepted 21 November 2006)

Abstract
We present observations of 22 intermediate-mass stars in the Scorpius-Centaurus OB association, obtained with the NAOS/CONICA system at the ESO Very Large Telescope. This survey was performed to determine the status of (sub)stellar candidate companions of Sco OB2 member stars of spectral type A and late-B. The distinction between companions and background stars is made on the basis of a comparison to isochrones and additional statistical arguments. We are sensitive to companions with an angular separation of 0.1''-11'' (13-1430 AU) and the detection limit is  mag. We detect 62 stellar components of which 18 turn out to be physical companions, 11 candidate companions, and 33 background stars. Three of the 18 confirmed companions were previously undocumented as such. The companion masses are in the range , corresponding to mass ratios . We include in our sample a subset of 9 targets with multi-color ADONIS observations from Kouwenhoven et al. (2005, A&A, 430, 137). In the ADONIS survey secondaries with  mag were classified as companions; those with  mag as background stars. The multi-color analysis in this paper demonstrates that the simple  mag criterion correctly classifies the secondaries in ~80% of the cases. We reanalyse the total sample (i.e. NAOS/CONICA and ADONIS) and conclude that of the 176 secondaries, 25 are physical companions, 55 are candidate companions, and 96 are background stars. Although we are sensitive (and complete) to brown dwarf companions as faint as  mag in the semi-major axis range 130-520 AU, we detect only one, corresponding to a brown dwarf companion fraction of ( ). However, the number of brown dwarfs is consistent with an extrapolation of the (stellar) companion mass distribution into the brown dwarf regime. This indicates that the physical mechanism for the formation of brown dwarf companions around intermediate mass stars is similar to that of stellar companions, and that the embryo ejection mechanism does not need to be invoked in order to explain the small number of brown dwarf companions among intermediate mass stars in the Sco OB2 association.

Key words: binaries: visual -- binaries: general -- stars: formation -- stars: low mass, brown dwarfs -- open clusters and associations: individual: Sco OB2