VLBI and Optical Astrometry of Southern Extragalactic Radio Sources and Radio Stars

Abstract

Mark III very long baseline interferometry (VLBI) has been used to determine the positions of ten compact extragalactic radio sources in the southern hemisphere. These positions are relative to the Jet Propulsion Laboratory VLBI celestial reference frame and are accurate at the 0.01" level, more than an order of magnitude improvement compared to their previous radio positions. First epoch VLBI positions have also been determined for two relatively weak radio stars in the southern hemisphere [the RS CVn close binaries HD 26337 (EI Eri) and HD 77137 (TY Pyx) ]. Optical positions relative to the Perth 70 Catalogue with rms uncertainties better than 0.2" have been found for the extragalactic sources by measurement of Schmidt plates. Optical positions with similar accuracy have been determined for the radio stars, and proper motions, with accuracy of ~0.003"/yr, have been found by measurement of Schmidt plates and reference to catalogs. Comparison of these optical and radio positions indicates that for the southern hemisphere area covered, there is no mean global bias larger than ~0.05" between the VLBI radio reference frame and the "J2000.0 Perth 70 catalogue." In addition, there are no obvious regional effects larger than our optical measurement accuracy of ~0.2". The VLBI positions of the ten extragalactic sources will help strengthen the southern extragalactic reference frame and the VLBI positions for the two optically bright radio stars will be useful for the link of the Hipparcos reference system to the extragalactic reference frame. We have determined the angular size of the radio-emitting region associated with HD 77137 to be ~ 1 mas (milliarcsec), approximately the physical size of the binary system, or 7 times the radius of the individual stars. The corresponding brightness temperature is 3X10^9^ K and is consistent with the gyrosynchrotron emission process.