Astron. Astrophys. 360, 472-498 (2000)

A proper motion study of the globular cluster  Centauri ^*

F. van Leeuwen ¹, R.S. Le Poole ², R.A. Reijns ², K.C. Freeman ³ and P.T. de Zeeuw <sup>2

1</sup> Institute of Astronomy, The Observatories, Madingley Road, Cambridge, CB3 0HA, UK (fvl@ast.cam.ac.uk)
² Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands (lepoole@strw.leidenuniv.nl; omegacentauri@reijns.com; tim@strw.leidenuniv.nl)
³ Mount Stromlo and Siding Springs Observatories, Private Bag, Weston Creek P.O., A.C.T.2611, Australia (kcf@mso.anu.edu.au)

Received 23 March 2000 / Accepted 15 June 2000

Abstract

A proper motion study is presented of the globular cluster  Centauri. Based on 100 plates obtained with the Yale-Columbia 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 (0.6 r_t) 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 , 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, 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  km 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.

Key words: telescopes celestial mechanics, stellar dynamics Galaxy: globular clusters: individual: NGC 5139

* Complete Tables 2, 3, 5 and 9 are available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strasbg.fr.pub/cats/J/A+A/360/472 (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/360/472

Send offprint requests to: F.van Leeuwen

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Contents

• 1. Introduction
• 2. The Yale-Columbia 66 cm (26 inch) refractor
• 3. The plate material
• 4. Preparation for the measurements
• 5. The cluster centre and flattening parameters
• 6. The plate measurements
• 7. The photometric reductions
  • 7.1. The magnitude and colour reference system
  • 7.2. The photometric calibration
  • 7.3. The variable stars
• 8. The plate reductions
  • 8.1. The positional and proper motion reference frames
  • 8.2. Determination of weights
  • 8.3. Local drift and distortion corrections
  • 8.4. The proper motion determination
  • 8.5. Calibration to ICRF positions and proper motions
• 9. Membership determinations
• 10. Surface and space density
• 11. The cluster rotation
• 12. Internal kinematics
• 13. The intrinsic velocity dispersions
• 14. The Hertzsprung-Russell diagram
  • 14.1. The [Ca/H] abundances
  • 14.2. Membership parameters for known variables in the field
• 15. Conclusions and future prospects
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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