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D. A. Boboltz, A. L. Fey, K. J. Johnston, R. G. Gaume (USNO), M. J. Claussen (NRAO), C. de Vegt (Hamburg Observatory)
The current realization of the International Celestial Reference Frame (ICRF) is defined by the VLBI positions of 212 extragalactic objects and is the IAU sanctioned fundamental astronomical reference frame. At optical wavelengths, the HIPPARCOS catalog now serves as the primary realization of the extragalactic frame. The link between the HIPPARCOS catalog and the ICRF was accomplished through observations of 12 radio stars and achieved an alignment of ±0.6 mas at epoch 1991.25. However, the frame tie between the ICRF and the HIPPARCOS catalog is degrading with time to the point that overlay between radio and optical data is now at a level greater than ~10 mas. Upcoming astrometric satellite missions such as FAME and SIM will likely define frames with internal accuracies that are better than the extragalactic VLBI frame by an order of magnitude, and these frames may define the next generation ICRF. We present here our observations of 19 radio stars using the VLA in A-configuration in conjunction with the Pie Town VLBA antenna. These observations are part of a long-term program (since 1978) to obtain accurate astrometric radio positions, parallaxes, and proper motions for ~50 radio stars which can be used to connect the current ICRF to future astrometric satellite frames. We compare the current positions of the 19 stars observed using the VLA+PT link with those of previous observations using the VLA from 1978 through 1995 to obtain revised estimates of the proper motions of these 19 stars, and we compare the proper motions to the corresponding HIPPARCOS values.