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 set quantitative limits on binary distributions. A Galaxy model is combined with models of binary distributions and of the observation process to derive the expected numbers of the different kinds of binary solutions in the Catalogue (component, acceleration, orbital and stochastic solutions). The observed number of component (C) and acceleration (G) solutions among bright (V <∼ 7–8) main-sequence stars are not consistent with a model having the same binary frequency and distributions as found by Duquennoy & 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, which suggests that many more orbits could be determined by analysis of the Hipparcos intermediate astrometric data. Future space astrometry missions will be able to probe binaries with separations of 1–10 AU at distances of several kpc by means of their curved motions.