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 ∼9,100 Hipparcos stars were chosen for the NPM link solution and ∼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.