The proper-motion signal of unresolved binaries in the Hipparcos catalogue

We present an investigation of the differences between quasi-instantaneous stellar proper motions from the Hipparcos catalogue and long-term proper mo tions determined by combining Hipparcos and the Astrographic Catalogue. Our study is based on a sample of about 12000 stars of visual magnitude from 7 to 10 in two declination zones on the northern and equatorial sky. The dis tribution of the proper-motion differences shows an excess of large deviati ons. This is caused by the influence of orbital motion of unresolved binary systems. The proper-motion deviations provide statistical evidence for 360 astrometric binaries in the investigated zones, corresponding to about 240 0 such binaries in the entire Hipparcos catalogue, in addition to those alr eady known. In order to check whether the observed deviations are compatibl e with standard assumptions on the basic parameters of binary stars, me mod el the impact of orbital motion on the observed proper motions ill a Monte Carlo simulation. Mie show that the simulation yields an acceptable approxi mation of the observations, if a binary frequency between 70% and 100% is a ssumed, i.e. if most of the stars in the sample are assumed to have a compa nion. Thus Hipparcos astrometric binaries confirm that the frequency of Iro n-single stars among field stars is very high. We also investigate the infl uence of the mass function for the secondary component on the result of the simulation. A constant mass function and mass functions with moderate incr ease towards low masses lead to results, which are compatible with the obse rved proper-motion effects. A high preponderance of very-low-mass or subste llar companions as produced, for example, by a M-1 power law is not in agre ement with the frequency of proper-motion deviations in our sample of stars .