Astrometric Positions and Proper Motions of 19 Radio Stars

We have used the Very Large Array, linked with the Pie Town Very Long Baseline Array antenna, to determine the astrometric positions of 19 radio stars in the International Celestial Reference Frame (ICRF). The positions of these stars were directly linked to the positions of distant quasars through phase-referencing observations. The positions of the ICRF quasars are known to 0.25 mas, thus providing an absolute reference at the angular resolution of our radio observations. Average values for the errors in our derived positions for all sources were 13 and 16 mas in α cos δ and δ, respectively, with accuracies approaching 1–2 mas for some of the stars observed. Differences between the ICRF positions of the 38 quasars and those measured from our observations showed no systematic offsets, with mean values of -0.3 mas in α cos δ and -1.0 mas in δ. Standard deviations of the quasar position differences of 17 and 11 mas in α cos δ and δ, respectively, are consistent with the mean position errors determined for the stars. Our measured positions were combined with previous Very Large Array measurements taken from 1978 to 1995 to determine the proper motions of 15 of the stars in our list. With mean errors of ≈1.6 mas yr-1, the accuracies of our proper motions approach those derived from Hipparcos and, for a few of the stars in our program, are better than the Hipparcos values. Comparing the positions of our radio stars with the Hipparcos Catalogue, we find that at the epoch of our observations, the two frames are aligned to within formal errors of approximately 3 mas. This result confirms that the Hipparcos frame is inertial at the expected level.