1 Introduction

The principal purpose of this work is to present a catalog of positions and proper motions for stars in the area of the open star cluster in Coma Berenices. The data are based on a compilation of 12 different catalogs. The first exhaustive study of the cluster was carried out by Trumpler (1938), determining definite cluster membership for the brighter stars. For some of the fainter stars the membership remained in doubt. The small number of members, a little more than 50, can be considered to be characteristic for most of the known open clusters. The determination of membership probability based on proper motions is somewhat difficult in this case because the motion of the cluster is not very different from that of the surrounding field stars. Thus it is important to have proper motions of high accuracy for the selection of cluster members which is the aim of this work. The precision of the proper motions determined in this paper is comparable to that of the Hipparcos data. This is due to the large epoch difference covered by the catalogs which contribute to the compilation of data, to the quality of the plate model used in the reduction of a series of photographic plates, and to the validity of the process with which systematic errors with respect to the Hipparcos system were eliminated from the various source data. Initially this work was part of the Ph. D. thesis of C. Abad (Abad 1996), where the PPM Catalogue of Positions and Proper Motions by Bastian & Roeser (1991) was used as reference system. The Hipparcos Catalogue was not available yet at that time. The reduction method employed was that described by Abad (1993). In this work, the reference system and the reduction method were both improved. The reference system is now the Hipparcos Catalogue, and the reduction is based on a model described by Abad (1998). The catalog compiled in this paper is based on 12 source catalogs of different types. They include meridian circle observations, photographic plates, and position observations with CCD detectors. One of the sources is photographic plates obtained with the CIDA 1-meter Schmidt telescope, and it is these plates which define the limits of the area survey. The source catalogs for which we either had the plates or the measured rectangular coordinates X, Y of stellar objects were reduced in the process. We wish to mention in particular the POSS plates whose microdensitometer scans were made available to us by the Space Telescope Science Institute. A list of all sources, giving the respective epoch and indicating whether the date was reduced by us or not is given in Table 1.

  

Table 1: List of used catalogues

$$\begin{tabular} {\vert r\vert c\vert r\vert r\vert c\vert c\... ...oalign{\smallskip} \hline \noalign{\smallskip} \end{tabular}$$