A proper motion study of the globular cluster ω Centauri ?

A proper motion study is presented of the globular cluster ω Centauri. Based on 100 plates obtained with the YaleColumbia 66 cm (26 inch) refractor, and ranging in epoch from 1931 till 1935 and 1978 till 1983, differential proper motions are obtained for 9847 stars. The limiting magnitude (photographic) is 16.0 for the centre of the cluster, 16.5 for the outer parts. The optical system of the Yale-Columbia 26 inch refractor (before and after its relocation from South Africa to Australia) is reconstructed. The reconstruction model is analyzed for possible systematic effects that changes in the objective may have introduced in the proper motions. The precisions of the proper motions range from an average of 0.1 milli arcsecond (mas) per year for the brightest to an average of 0.65 mas yr−1 for the faintest stars. These precisions are high enough to enable the study of internal kinematics of the cluster. 9256 stars, situated within 29.′5 (0.6 rt) from the cluster centre and with colour information, are used in cluster-membership determinations, identifying 7853 probable cluster members. Of the remaining 338 stars within 29.′5, those without colour information, an estimated 243 are cluster members. The surface density profile as obtained from integrated membership probabilities, and corrected for losses due to crowding, shows an increase continuing till much closer to the cluster centre than shown in earlier studies. An internal proper motion dispersion of 1.0 to 1.2 mas yr−1 (equivalent to 25 to 29 km s−1 for a distance of 5.1 kpc) is found for members near the cluster centre, decreasing to 0.3 mas yr−1 (7.5 km s−1 at 5.1 kpc) in the outer regions. There is an indication of anisotropy in the proper motion dispersions over the intermediate part of the cluster halo, where the dispersions in the radial direction are systematically higher than in the transverse direction. The proper motions also show a pronounced differential rotation, which is not reflected in tests performed on the reconstructed telescope model, and which looks similar to the rotation noticed in radial velocity studies of the cluster. A comparison between velocity dispersions obtained in radial velocity studies with the present results indicates a distance for the cluster of 4.5 kpc, compared to the expected distance of 5.1 kpc. This result should, however, Send offprint requests to: F.van Leeuwen ? Complete Tables 2, 3, 5 and 9 are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http:://cdsweb.u-strasbg.fr/Abstract.html be interpreted with caution, given a range of possible complications that may have affected the dispersion determinations for both the proper motions and the radial velocities, and the possibility that the velocity distribution is intrinsically anisotropic. The absolute proper motion of the cluster is derived through linking the differential proper motions and positions with the ICRF as defined by the Hipparcos catalogue and extended to a denser grid of stars by the Tycho-2 catalogue. Assuming a cluster distance of 5.1 kpc, a tangential velocity of 142±19km s−1 is derived from the observed cluster proper motion. A photometric analysis of the measurements used to obtain the astrometric parameters provides new periods, zero points and mean magnitudes for 153 variable stars in the cluster. A compilation of B and V photometry (required for the astrometric reductions) is used in an analysis of abundance variations among the cluster stars as well.