ACCURATE POSITIONS FOR RADIO STARS AS DETERMINED FROM CCD OBSERVATIONS IN THE EXTRAGALACTIC REFERENCE FRAME

This paper presents positions for 155 radio stars determined directly in the extragalactic reference system from CCD scan observations made with the Flagstaff Astrometric Scanning Transit Telescope (FASTT). All the observations were made in 1994-1997 with an accuracy of similar t o+/-60 mas in each coordinate. When these positions are compared with those of 21 radio stars with well known VLBI or VLA radio positions, t here is good agreement, wherein no significant zero-point differences in position were found (+/-15 mas in each coordinate), and the dispers ions in (optical-radio) differences (similar to+/-72 mas in each coord inate) can be explained by the formal errors in the positional and pro per motion data. Moreover, if these (optical-radio) offsets are plotte d against right ascension and declination, no systematic trends are fo und in either coordinate. Improved optical positions for the radio sta rs (similar to+/-30 mas in each coordinate) can be obtained by averagi ng the positions presented in this paper with those determined with th e CAMC and Bordeaux meridian circles. When these combined positions ar e compared with their radio counterparts, external errors in the radio positions of radio stars can be calculated and are <+/-10 mas and sim ilar to+/-40 mas in each coordinate, respectively, for VLBI and VLA ra dio observations. The former is consistent with the errors quoted for VLBI radio star positions, while the latter is about 30% larger. None of the (combined optical-radio) offsets are significant at the >3 sigm a level; but, nonetheless, several stars (HD 26337, HD 283447, b Per, and KQ Pup) have marginally significant (>2 sigma) offsets. (C) 1997 A merican Astronomical Society.