Mass Determination of Astrometric Binaries With Hipparcos

Abstract

Among the 11500 known double stars observed by the ESA satellite Hipparcos, there are some 900 binaries for which orbits have been published since the beginning of the century. When the apparent displacement of the components due to the orbital motion is significant over the 37 months of the mission, the five astrometric parameters introduced in the general processing (position, parallax and proper motion), are not sufficient to allow for the motion of the components around the centre of mass of the system. It is therefore necessary to correct each observation in order to recover the astrometric properties of the centre of mass. We show that the knowledge of the orbital motion of a binary, when combined with the Hipparcos observations, may lead to an estimation of the mass and intensity ratios of the components, provided that the period is less than 30 years and the semi-major axis is larger than 0.2 arcsec. For smaller separations, we only get a linear combination of these two quantities.