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 a nd its southern-sky complement, the Yale/San Juan Southern Proper Moti on (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 sim ilar to 4,100 for the SPM solution. The typical accuracy of the NPM ab solute proper motions is 5 mas yr(-1). For the SPM proper motions it v aries from 2 to 4 mas yr(-1). The link solutions indicate a presence o f magnitude equation in both the NPM and SPM. In the case of the NPM t hree different approaches (i.e., introducing a magnitude cutoff, addit ional linear magnitude terms, and including a fictitious zone of avoid ance) were examined to alleviate the magnitude equation problem. The S PM proper motions show a notable correlation between the postfit resid uals and the size of the magnitude equation correction applied. It is quite likely that galaxies have a magnitude equation different from th at affecting the stellar images. We note an improvement in the spin so lution with re-calibrated galaxy magnitudes and, subsequently, changed magnitude-equation corrections for the galaxies.