Astrometric Positions and Proper Motions of 19 Radio Stars

Abstract

We have used the Very Large Array, linked with the Pie Town Very Long Baseline Array antenna, to determine 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 mas and 16 mas in R.A. and declination 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 R.A. and -1.0 mas in declination. Standard deviations of the quasar position differences of 17 mas and 11 mas in R.A. and declination 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 1978-1995 to determine the proper motions of 15 of the stars in our list. With mean errors of approximately 1.6 mas/yr, 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 catalog, 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.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March 1

Astrometric control of the inertiality of the Hipparcos catalog

“…An analysis of the radio-interferometric observations (Boboltz et al 2003) performed 9.69 years after (epoch 2000.94) the construction of ICRS (epoch 1991.25) revealed no significant effect in the coordinates of radio stars. However, the rotation components obtained by Boboltz et al (2003) from the coordinate differences for 18 radio stars (Kovalevsky et al 1997) are of considerable interest:…”

“…(11) We changed the signs of the components of the vector ε inferred by Boboltz et al (2003) in order to have Hipparcos catalog differences comparable with those analyzed. To compare results (9)-(11), for example, with the Mean 1 solution, the quantities ε 0x,y,z must be divided by the epoch difference.…”

“…In addition, the results of the Potsdam program were presented by Hirte et al (1996), and the Bonn program by Geffert et al Table 1 gives the components of the vector ω that we calculated using the absolute proper motions of 15 radio stars from Boboltz et al (2003). These authors described in detail the project and the method of referencing the observations of radio stars to quasars and compared these stars with the Hipparcos catalog, but did not determine the components of the vector ω.…”

A NEW VLA-HIPPARCOSDISTANCE TO BETELGEUSE AND ITS IMPLICATIONS

“…We also find that the positions measured with different u-v bin sizes are very similar, and that the positions measured in the image map with JMFIT are very close to those measured in the visibility data. Boboltz et al (2003) find that their formal fitting errors are typically smaller than the scatter of individual position measurements made at a given frequency. These trends suggest that some residual phase errors remain.…”

“…Radio positions of Betelgeuse have been published by Johnston et al (2003), Boboltz et al (2003), and Boboltz et al (2007). The two 1982 positions were obtained at C band (6 cm) with a single 50 MHz channel , while the 2000 and 2003 Boboltz et al observations were made with two 50 MHz channels at X band (3.6 cm) in A configuration with the Pie Town antenna.…”