THE BONN CONTRIBUTION TO THE EXTRAGALACTIC LINK OF THE HIPPARCOS PROPER MOTION SYSTEM

In order to calibrate the proper motions of the Hipparcos astrometry s atellite, our group has measured accurate absolute proper motions of H ipparcos stars in small fields around optically bright extragalactic r adio sources or bright galaxies with star-like features. In addition, we also use fields where relative proper motions are calibrated by mea surements of large numbers of stars and galaxies on wide-field plates. The median internal accuracy of our relative proper motions, based on photographic plates with epoch differences up to 100 years (typically 70 years), is 1.0 milliarcsec/year (mas/a), while the calibration to an inertial system in each of the 13 fields has a median uncertainty o f 1.3 mas/a. We compute the rotation from the Hipparcos proper motions (median internal errors 0.9 mas/a) to the extragalactic reference fra me represented by our absolute proper motions, using 88 stars in commo n. The three components of the angular velocity vector have internal e rrors of 0.3 mas/a. Our rotation solution has been used together with those of independent groups for the extragalactic calibration of the H ipparcos proper motion system (Kovalevsky et al. 1996). It compares fa vourably with the adopted mean solution.