The Hipparcos proper motion link to the extragalactic reference system using NPM and SPM

We describe the use of the Lick Northern Proper Motion (NPM) program and its southern-sky complement, the Yale/San Juan Southern Proper Motion (SPM) program, to determine the Hipparcos proper motion link to the extragalactic reference system defined by galaxies. In total similar to 9,100 Hipparcos stars were chosen for the NPM link solution and similar to 4,100 for the SPM solution. The typical accuracy of the NPM absolute proper motions is 5 mas yr(-1). For the SPM proper motions it varies from 2 to 4 mas yr(-1). The link solutions indicate a presence of magnitude equation in both the NPM and SPM. In the case of the NPM three different approaches (i.e., introducing a magnitude cutoff, additional linear magnitude terms, and including a fictitious zone of avoidance) were examined to alleviate the magnitude equation problem. The SPM proper motions show a notable correlation between the postfit residuals and the size of the magnitude equation correction applied. It is quite likely that galaxies have a magnitude equation different from that affecting the stellar images. We note an improvement in the spin solution with re-calibrated galaxy magnitudes and, subsequently, changed magnitude-equation corrections for the galaxies.