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A proper motion study of the globular cluster ω Centauri
A proper motion study is presented of the globular cluster ωCentauri. Based on 100 plates obtained with the Yale-Columbia 66 cm (26inch) refractor, and ranging in epoch from 1931 till 1935 and 1978 till1983, differential proper motions are obtained for 9847 stars. Thelimiting magnitude (photographic) is 16.0 for the centre of the cluster,16.5 for the outer parts. The optical system of the Yale-Columbia 26inch refractor (before and after its relocation from South Africa toAustralia) is reconstructed. The reconstruction model is analyzed forpossible systematic effects that changes in the objective may haveintroduced in the proper motions. The precisions of the proper motionsrange from an average of 0.1 milli arcsecond (mas) per year for thebrightest to an average of 0.65 mas yr-1 for the fainteststars. These precisions are high enough to enable the study of internalkinematics of the cluster. 9256 stars, situated within 29.'5 (0.6rt) from the cluster centre and with colour information, areused in cluster- membership determinations, identifying 7853 probablecluster members. Of the remaining 338 stars within 29.'5, those withoutcolour information, an estimated 243 are cluster members. The surfacedensity profile as obtained from integrated membership probabilities,and corrected for losses due to crowding, shows an increase continuingtill much closer to the cluster centre than shown in earlier studies. Aninternal 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) isfound for members near the cluster centre, decreasing to 0.3 masyr-1 (7.5 km s-1 at 5.1 kpc) in the outer regions.There is an indication of anisotropy in the proper motion dispersionsover the intermediate part of the cluster halo, where the dispersions inthe radial direction are systematically higher than in the transversedirection. The proper motions also show a pronounced differentialrotation, which is not reflected in tests performed on the reconstructedtelescope model, and which looks similar to the rotation noticed inradial velocity studies of the cluster. A comparison between velocitydispersions obtained in radial velocity studies with the present resultsindicates a distance for the cluster of 4.5 kpc, compared to theexpected distance of 5.1 kpc. This result should, however, beinterpreted with caution, given a range of possible complications thatmay have affected the dispersion determinations for both the propermotions and the radial velocities, and the possibility that the velocitydistribution is intrinsically anisotropic. The absolute proper motion ofthe cluster is derived through linking the differential proper motionsand positions with the ICRF as defined by the Hipparcos catalogue andextended to a denser grid of stars by the Tycho-2 catalogue. Assuming acluster distance of 5.1 kpc, a tangential velocity of 142 ± 19 kms-1 is derived from the observed cluster proper motion. Aphotometric analysis of the measurements used to obtain the astrometricparameters provides new periods, zero points and mean magnitudes for 153variable stars in the cluster. A compilation of B and V photometry(required for the astrometric reductions) is used in an analysis ofabundance variations among the cluster stars as well. Complete Tables 2,3, 5 and 9 are available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp:://cdsweb.u-strasbg.fr/Abstract.html
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| Photoelectric B, V observations of stars in Omega Centauri
Photoelectric BV data are presented for 71 stars brighter than V = 15.6in Omega Cen. The observation and reduction techniques are described,and sources of error are discussed. A comparison with previousphotometry of 14 of these stars by Cannon and Stobie (1973) shows goodagreement, apart from a small discrepancy in V.
| A BV Photoelectric Sequence of Stars in Omega-Centauri
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