The proper motions of stars observed by the ESA astrometry satellite Hipparcos contain an unknown angular velocity relative to a non-rotating system. In order to provide absolute proper motions for the final Hipparcos catalogue, the proper motion system of Hipparcos had to be calibrated externally by a link to distant extragalactic sources with negligible proper motions. The problem was named, and the present solution was proposed by Brosche (1980).
Lindegren & Kovalevsky (1995) discussed the strategies of various groups for the extragalactic link of the Hipparcos proper motions. They described the method proposed to combine the different link solutions into a unique solution. Kovalevsky et al. (1996) have carried out this merging given the results of the various groups. They present the definitive extragalactic calibration of the Hipparcos proper motions, where the components of the angular velocity vector have estimated internal errors of 0.25 mas/a. The present paper describes the link project of the Sternwarte Bonn, which has earlier been called the ``Small Extragalactic Link'' (Brosche & Geffert 1988). An intermediate solution based on a smaller set of absolute proper motions and on preliminary Hipparcos data (Brosche et al. 1995a) is superseded by the results given below.
We have measured accurate relative proper motions, based on
photographic plates with epoch differences up to 100 years, of stars
from the Hipparcos Input Catalogue (Turon et al. 1992) in small (up to
) fields around optically bright QSO's,
BLLac objects, and bright galaxies with well-defined optical
pointlike features. The fictitious proper motions of the extragalactic
objects are used to correct the proper motions of the Hipparcos stars
to an inertial system. In a different approach we use relative proper
motions in fields of our globular cluster program (see e.g. Tucholke
et al. 1994 and references therein), where absolute proper motions of
stars have been measured on Schmidt plates with respect to a large
number of galaxies. In Geffert et al. (1996) the proper motions of
Hipparcos stars were calibrated via measurements from plates of the
Lick NPM project and additional plates from the same telescope. The
data are described in Sect. 2.
The Hipparcos Science Team has provided our group with the data for the subset of Hipparcos stars which are of interest for us. They form part of the intermediary catalogue called H37Cr, which includes the final reductions for the astrometric parameters, but does not contain a number of problematic double/multiple stars, whose reduction was still pending at that time. We determine the angular velocity vector of the rotation from the H37Cr proper motion system to our extragalactically calibrated system. This procedure, the resulting vector components and a discussion of the residuals are given in Sect. 3. In Sect. 4 we discuss various sources of error. In the concluding Sect. 5 we compare our solution with the mean solution adopted for the calibration of the Hipparcos catalogue (Kovalevsky et al. 1996).