SIMBAD references

The analysis of the observations of double stars made by the ESA satellite Hipparcos has involved a very specific processing to derive the relevant astrometric parameters. This required to distinguish between several categories of double stars according to the separation and orbital motion. We show that for close pairs with orbital period less than about 20-years, the concept of photocentric orbit of an astrometric binary needs to be generalized to benefit fully from the accuracy of Hipparcos. We introduce a point more naturally associated with the Hipparcos observations, the hippacentre, whose orbital path is not longer similar to the relative keplerian orbit of the components, unlike that of the photocentre. For systems with separation larger than about 0.3", it is possible to determine separately the mass and the intensity ratio of the components from the absolute path of the hippacentre on the sky. For smaller separations the scale of the photocentric orbit is recovered as a limiting case. The scope of this paper is to set forth the principles of the method and to explore its possibilities and limitations from extensive simulations.