The mass ratio distribution of B-type visual binaries in the Sco OB2 association

A sample of 115 B-type stars in the Sco OB2 association is examined for existence of visual companions in the J and K-s bands, using the ADONIS near-infrared adaptive optics system and coronograph. Practically all the components in the separation range 0.3-6.4 (45-900 AU) and magnitudes down to K = 16 were detected. The K and J-K photometry of the primaries and differential photometry and astrometry of the 96 secondaries are presented. Ten secondaries are new physical components, as inferred from the photometric and statistical criteria, while the rest of the newly detected objects are faint background stars. After a small correction for detection incompleteness and a conversion of the fluxes into masses, an unbiased distribution of the components mass ratio q was derived. The power law f(q) proportional to q(-0.5) fits the observations well, whereas a q(-1.8) distribution, which corresponds to a random pairing of stars, is rejected. The companion star fraction is 0.20 +/- 0.04 per decade of separation which is comparable to the highest measured binary fraction among low-mass PMS stars and similar to 1.6 times higher than the binary fraction of low-mass dwarfs in the solar neighborhood and in open clusters in the same separation range.