A brown dwarf desert for intermediate mass stars in Sco OB 2 ? ⋆

We present JHK S 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 K S = 17 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 0.03 M ⊙ ≤ M ≤ 1.19 M ⊙ , corresponding to mass ratios 0.06 ≤ q ≤ 0.55. We include in our sample a subset of 9 targets with multi-color ADONIS observations from Kouwenhoven et al. (2005). In the ADONIS survey secondaries with K S < 12 mag were classified as companions; those with K S > 12 mag as background stars. The multi-color analysis in this paper demonstrates that the simple K S = 12 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 K S = 14 mag in the semi-major axis range 130 − 520 AU, we detect only one, corresponding to a brown dwarf companion fraction of 0.5 ± 0.5% (M > ∼ 30 M J). 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.