  We present the results of a near-infrared adaptive optics survey with the aim
to detect close companions to Hipparcos members in the three subgroups of the
nearby OB association Sco OB2: Upper Scorpius (US), Upper Centaurus Lupus (UCL)
and Lower Centaurus Crux (LCC). We have targeted 199 A-type and late B-type
stars in the Ks band, and a subset also in the J and H band. We find 151
stellar components other than the target stars. A brightness criterion is used
to separate these components into 77 background stars and 74 candidate physical
companion stars. Out of these 74 candidate companions, 41 have not been
reported before (14 in US; 13 in UCL; 14 in LCC). Companion star masses range
from 0.1 to 3 Msun. The mass ratio distribution follows f(q) = q^-0.33, which
excludes random pairing. No close (rho < 3.75'') companion stars or background
stars are found in the magnitude range 12 < Ks < 14. The lack of stars with
these properties cannot be explained by low-number statistics, and may imply a
lower limit on the companion mass of ~ 0.1 Msun. Close stellar components with
Ks > 14 are observed. If these components are very low-mass companion stars, a
gap in the companion mass distribution might be present. The small number of
close low-mass companion stars could support the embryo-ejection formation
scenario for brown dwarfs. Our findings are compared with and complementary to
visual, spectroscopic, and astrometric data on binarity in Sco OB2. We find an
overall companion star fraction of 0.52 in this association. This paper is the
first step toward our goal to derive the primordial binary population in Sco
OB2.
