HIPPARCOS link with Carte du Ciel triple images.

Abstract

This study for the first time investigates the possibility of deriving astrometric positions for bright stars on Carte du Ciel (CdC) plates, whose triple images are not separable. The field from the Bonn Hipparcos Extragalactic Link Programme with the BL Lac object OJ 287 has been selected. The triple images on the CdC plate were modelled by three overlapping elliptical Gaussians. A strong photographic adjacency effect was found increasing the distances between the three images for each star from about 140 microns for faint stars to more than 300 microns for the brightest stars. However, due to the symmetric arrangement of the three images their centre of gravity allows to derive a usable astrometric position. In addition to the CdC plate from the Paris zone taken in 1903 twelve plates from four different telescopes with epochs from 1926 to 1980 were used. The rms errors of the derived proper motions for Hipparcos stars are of the order of 1-2 mas/a, i.e. similar to the errors expected from Hipparcos.