CCD astrometry of faint compact extragalactic radio sources. III.

Abstract

Optical positions relative to the Hipparcos Catalogue (ESA 1997) have been obtained for the optical counterparts of 38 faint (B 20−24) southern compact extragalactic radio sources (CERS). Many of these sources are not detectable { or their coordinates not adequately measurable { by means of direct photography with a wide- eld telescope, so the positions were determined using a multi-step procedure involving CCD and photographic observations. This method responds both to the requirement of attaining small positional errors, and the need to refer the positions to a standard astrometric system. Estimated precisions as good as 50 mas were achieved. For 16 of the above objects we are proposing a completely new optical identi cation or providing a revised nder. A comparison of our optical positions with available VLBI radio positions is presented. The residuals obtained are in most cases consistent with the precision of our optical data, and provide an evaluation of these CERS as radio/optical frame link sources (or their identi cation as astrophysically interesting astrometric outliers). Twenty nine of the CERS observed are either de ning or candidate ducial objects of the present International Celestial Reference System (ICRS, Ma et al. 1998), so the optical data we have obtained for them is potentially useful to help mantain and possibly improve the current link of the Hipparcos reference frame to the ICRS.