The Bonn contribution to the extragalactic link of the Hipparcos proper motion system

In order to calibrate the proper motions of the Hipparcos astrometry satellite, our group has mea- sured accurate absolute proper motions of Hipparcos stars in small elds around optically bright extragalactic radio sources or bright galaxies with star-like features. In addi- tion, we also use elds where relative proper motions are calibrated by measurements of large numbers of stars and galaxies on wide-eld plates. The median internal accu- racy of our relative proper motions, based on photographic plates with epoch dierences up to 100 years (typically 70 years), is 1.0 milliarcsec/year (mas/a), while the calibra- tion to an inertial system in each of the 13 elds has a median uncertainty of 1.3 mas/a. We compute the rotation from the Hipparcos proper motions (median internal errors 0.9 mas/a) to the ex- tragalactic reference frame represented by our absolute proper motions, using 88 stars in common. The three com- ponents of the angular velocity vector have internal er- rors of 0.3 mas/a. Our rotation solution has been used together with those of independent groups for the extra- galactic calibration of the Hipparcos proper motion system (Kovalevsky et al. 1996). It compares favourably with the adopted mean solution.