Astrometric Positions and Proper Motions of 19 Radio Stars

We have used the Very Large Array, linked with the Pie Town Very LongBaseline Array antenna, to determine astrometric positions of 19 radio stars inthe International Celestial Reference Frame (ICRF). The positions of thesestars were directly linked to the positions of distant quasars through phasereferencing observations. The positions of the ICRF quasars are known to 0.25mas, thus providing an absolute reference at the angular resolution of ourradio observations. Average values for the errors in our derived positions forall sources were 13 mas and 16 mas in R.A. and declination respectively, withaccuracies approaching 1-2 mas for some of the stars observed. Differencesbetween the ICRF positions of the 38 quasars, and those measured from ourobservations showed no systematic offsets, with mean values of -0.3 mas in R.A.and -1.0 mas in declination. Standard deviations of the quasar positiondifferences of 17 mas and 11 mas in R.A. and declination respectively, areconsistent with the mean position errors determined for the stars. Our measuredpositions were combined with previous Very Large Array measurements taken from1978-1995 to determine the proper motions of 15 of the stars in our list. Withmean errors of approximately 1.6 mas/yr, the accuracies of our proper motionsapproach those derived from Hipparcos, and for a few of the stars in ourprogram, are better than the Hipparcos values. Comparing the positions of ourradio stars with the Hipparcos catalog, we find that at the epoch of ourobservations, the two frames are aligned to within formal errors ofapproximately 3 mas. This result confirms that the Hipparcos frame is inertialat the expected level.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March 1