EDP Sciences
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Volume 382, Number 1, January IV 2002
Page(s) 92 - 103
Section Formation, structure and evolution of stars
DOI http://dx.doi.org/10.1051/0004-6361:20011542

A&A 382, 92-103 (2002)
DOI: 10.1051/0004-6361:20011542

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

N. Shatsky1, 2 and A. Tokovinin3, 1

1  Sternberg Astronomical Institute, Universitetskii pr. 13, Moscow 119899, Russia
2  Royal Observatory of Belgium, Av. Circulaire 3, Bruxelles 1180, Belgium
3  Cerro-Tololo Inter-American Observatory, La Serena, Casilla 603, Chille
    e-mail: atokovinin@ctio.noao.edu

(Received 25 September 2001 / Accepted 30 October 2001 )

A sample of 115 B-type stars in the Sco OB2 association is examined for existence of visual companions in the J and $K_{\rm s}$ bands, using the ADONIS near-infrared adaptive optics system and coronograph. Practically all the components in the separation range $0\farcs3$- $6\farcs4$ ( 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 $f(q)\propto q^{-0.5}$ 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 $0.20\pm0.04$ per decade of separation which is comparable to the highest measured binary fraction among low-mass PMS stars and ~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.

Key words: binaries: visual -- stars: statistics -- formation

Offprint request: N. Shatsky, kolja@sai.msu.ru

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