Preliminary Link of the Hipparcos and VLBI Reference Frames

We present a comparison of VLBI and Hipparcos astrometric parameters of several optically bright radio-emitting stars. The systematic discrepancies found in these preliminary data can be removed by performing a single global rotation between the extragalactic and Hipparcos reference frames. Two estimates of this rotation and its time derivative have been made by using seven and six link stars from the two Hipparcos reduction consortia FAST and NDAC, respectively. The three angles and annual rates of rotation between the two frames are determined at better than the milli-arcsec level. The angles of rotation found are relative to a VLBI extragalactic reference frame that is defined by the IERS VLBI coordinates of the extragalactic reference sources used for the differential VLBI measurement of the link stars. The post-fit residuals of the star coordinates and proper motion components after the adjustment of the global rotation indicate that the consistency between the Hipparcos and VLBI astrometric techniques is at the milli-arcsec level. This is the first cross-check between these two astrometric techniques of comparable precision. The level of agreement found is consistent with the expected accuracy of the technique, even though the astrometric parameters used are from preliminary reductions of the data for both techniques. The Hipparcos reference frame has been aligned with a quasi-FK5 frame in the process of its construction. The magnitudes of the rotation angles found are consistent with the degree of alignment known between the VLBI and FK5 frames. This VLBI link of the Hipparcos catalog is preliminary and we describe various improvements that will be implemented before the release of the Hipparcos catalog in 1996. Comparison of the VLBI and Hipparcos trigonometric parallaxes are also discussed.