The main conclusions from this study are:
(1) The observed number of C and G solutions among the bright (-8) main-sequence (spectral type A) stars in the Hipparcos Catalogue cannot be explained by assuming the same frequency and distribution of binaries as found by Duquennoy & Mayor (1991) for nearby solar-type stars. A significantly higher multiplicity is found, at least for semi-major axes in the range 1-100 AU. The estimated multiplicity in the 1-10 AU interval is %, and in the 10-100 AU interval it is %. With reasonable extrapolations to other a this gives a total multiplicity of 0.9-1.2 companions per primary.
(2) The data are consistent with a decreasing mass-ratio distribution (e.g., ) assumed for all separations, but not with flat or increasing distributions.
(3) Assuming a distribution in similar to the ones derived in this study (dashed curves in Fig. 2), we predict three times as many orbital solutions (type O) as given in the Hipparcos Catalogue for the corresponding stellar sample. This indicates that many more orbital solutions could be found by re-analysing the Hipparcos intermediate data, possibly with the aid of new spectroscopic observations for the determination of periods.
(4) The detection probability for binaries with to 10 AU is 10-20 per cent, if the curved motion of the photocentre is used to detect duplicity. This applies, for Hipparcos, to stars at a median distance of 100 pc. This is an interesting and generally difficult range of separations that falls between the spectroscopic and the visual binaries. Future astrometric projects such as GAIA should be able to probe the same separation range at distances of several kpc.
© European Southern Observatory (ESO) 2000
Online publication: October 2, 2000