[Paper Review] Ground-based CCD astrometry with wide field imagers. III. WFI@2.2m proper-motion catalog of the globular cluster omega Centauri

Andrea Bellini, G. Piotto+7 more|Research Padua Archive (University of Padua)|Oct 10, 2008

Stellar, planetary, and galactic studies1 references58 citations

TL;DR

This paper presents a high-precision, CCD-based proper-motion catalog of the globular cluster ω Centauri using archival WFI@2.2m telescope data, achieving astrometric precision of ~7 mas/yr and extending to V ~ 20. It confirms that metal-poor, metal-intermediate, and metal-rich red giant branch populations share the same mean proper motion, providing a critical membership probability catalog for spectroscopic follow-up and multi-population studies down to the cluster turn-off region.

ABSTRACT

Omega Centauri is the most well studied Globular Cluster because of its numerous puzzling features. Intensive spectroscopic follow-up observing campaigns targeting stars at different positions on the color-magnitude diagram promises to clarify some of these peculiarities. To able to target cluster members reliably during spectroscopic surveys and both spatial and radial distributions in the cluster outskirts without including field stars, a high quality proper-motion catalog of omega Cen and membership probability determination are required. The only available wide field proper-motion catalog of omega Cen is derived from photographic plates, and only for stars brighter than B~16. Using ESO archive data, we create a new, CCD-based, proper-motion catalog for this cluster, extending to B~20. We used the high precision astrometric software developed specifically for data acquired by WFI@2.2m telescope and presented in the first paper of this series. We achieved a good cluster-field separation with a temporal base-line of only four years. We corrected our photometry for sky-concentration effects. We provide calibrated photometry for UBVRI wide-band data plus narrow-band filter data centered on H_alpha for almost 360 000 stars. We can confirm that the omega Cen metal-poor and the metal-rich components have the same proper motion, and demonstrate that the metal-intermediate component in addition exhibits the same mean motion as the other RGB stars. We provided membership probability determination for published omega Cen variable star catalogs. Our catalog extends the proper-motion measurements to fainter than the cluster turn-off luminosity, and covers a wide area (~33'x33') around the center of omega Cen. Our catalog is now electronically available to the astronomical community.

Motivation & Objective

• To provide a high-precision, wide-field proper-motion catalog of ω Centauri extending to faint magnitudes (V ~ 20) for reliable membership determination.
• To enable accurate targeting of cluster members in spectroscopic surveys by minimizing contamination from field stars.
• To investigate the kinematic behavior of multiple stellar populations in ω Centauri, particularly across metallicity components.
• To correct photometric measurements for sky-concentration effects and provide calibrated UBVRI and Hα photometry for all stars.
• To provide membership probabilities for existing variable star catalogs in ω Centauri.

Proposed method

• Utilized archival CCD data from the WFI@2.2m telescope at La Silla and Paranal Observatories with a temporal baseline of four years.
• Applied high-precision astrometric software with empirical PSF and local transformation methods to measure star displacements with ~7 mas/yr precision.
• Corrected photometry for sky-concentration effects using a dedicated calibration method to improve photometric accuracy across the wide field of view.
• Combined multi-band photometry (UBVR_C I_C, Hα) for nearly 360,000 stars, with error estimates from 0.02 mag (bright) to 0.05 mag (faint).
• Calculated proper-motion membership probabilities (Pμ) using astrometric and photometric data, with a secondary method for a subset of stars.
• Used a distortion-corrected reference meta-chip to derive precise J2003.29 equatorial and pixel coordinates for all sources.

Experimental results

Research questions

• RQ1 Do the metal-poor, metal-intermediate, and metal-rich red giant branch populations in ω Centauri exhibit distinct proper motions?
• RQ2 Can a wide-field, CCD-based proper-motion catalog extend to the cluster turn-off region with sufficient precision to distinguish cluster members from field stars?
• RQ3 How do sky-concentration effects impact photometric measurements in wide-field imaging, and can they be effectively corrected?
• RQ4 Do the known variable star catalogs in ω Centauri have reliable membership assignments based on proper motion?
• RQ5 Can high-precision astrometry and multi-band photometry constrain the kinematic and structural properties of multiple stellar populations in ω Centauri?

Key findings

• The metal-poor, metal-intermediate, and metal-rich red giant branch populations in ω Centauri all share the same mean proper motion, consistent with being part of the same cluster system.
• The catalog covers a wide area of ~33×33 arcmin² around the cluster center and includes nearly 360,000 stars with measured proper motions down to V ~ 20.
• Photometric errors range from 0.02 mag for bright stars to 0.05 mag for fainter ones, with calibrated UBVRI and Hα magnitudes provided for all sources.
• Proper-motion membership probabilities (Pμ > 90%) were computed for all stars, enabling reliable selection of cluster members for spectroscopic follow-up.
• The study confirms that the metallicity dispersion in ω Centauri does not correlate with kinematic differences among the red giant branch populations.
• Membership probabilities were provided for the Kaluzny et al. (2004) and Weldrake et al. (2007) variable star catalogs, enhancing their utility for future studies.