Statistics of Hipparcos binaries: probing the 1-10 AU separation range

The observation of double stars by Hipparcos is modelled in order to check the statistics of double-star solutions in the Hipparcos Catalogue and to s et quantitative limits on binary distributions. A Galaxy model is combined with models of binary distributions and of the observation process to deriv e the expected numbers of the different kinds of binary solutions in the Ca talogue (component, acceleration, orbital and stochastic solutions). The ob served number of component (C) and acceleration (G) solutions among bright (V less than or similar to 7-5) main-sequence stars are not consistent with a model having the same binary frequency and distributions as found by Duq uennoy & Mayor (1991) for solar-type field stars. The number of companions per primary is estimated to 25 +/- 5% for semi-major axes from 1 to 10 AU, and to 43 +/- 20% for the range 10-100 AU. The fitted model predicts about three times as many orbital solutions as actually found in the catalogue, w hich suggests that many more orbits could be determined by analysis of the Hipparcos intermediate astrometric data. Future space astrometry missions w ill be able to probe binaries with separations of 1-10 AU at distances of s everal kpc by means of their curved motions.