[Paper Review] Ground-based CCD astrometry with wide field imagers. II. A star catalogue for M67: WFI@2.2m MPG/ESO astrometry, FLAMES@VLT radial velocities

R. K. S. Yadav, L. R. Bedin+8 more|ArXiv.org|Feb 29, 2008

Stellar, planetary, and galactic studies49 references67 citations

TL;DR

This paper presents a high-precision astrometric and photometric catalogue of ~2,400 stars in the old open cluster M67 using two-epoch CCD observations from the WFI@2.2m MPG/ESO telescope and radial velocities from FLAMES@VLT. It achieves 1.9 mas yr⁻¹ proper motion precision at V < 16, extending reliable membership determination down to V ≈ 20, and provides calibrated BVI photometry and radial velocities for 211 stars, significantly deepening the cluster census beyond previous photographic surveys.

ABSTRACT

The solar-age open cluster M67 (C0847+120, NGC2682) is a touchstone in studies of the old Galactic disk. Despite its outstanding role, the census of cluster membership for M67 at fainter magnitudes and their properties are not well-established. Using the proprietary and archival ESO data, we have obtained astrometric, photometric, and radial velocities of stars in a 34'x 33' field centered on the old open cluster M67. The two-epoch archival observations separated by 4 years and acquired with the Wide Field Imager at the 2.2m MPG/ESO telescope have been reduced with our new astrometric techniques, as described in the first paper of this series. The same observations served to derive calibrated BVI photometry in M67. Radial velocities were measured using the archival and new spectroscopic data obtained at VLT. We have determined relative proper motions and membership probabilities for ~2,400 stars. The precision of proper motions for optimally exposed stars is ~2 mas/yr, gradually degrading down to ~5 mas/yr at V= 20. Our relatively precise proper motions at V&gt;16 are first obtained in this magnitude range for M67. Radial velocities are measured for 211 stars in the same field. We also present a detailed comparison with recent theoretical isochrones from several independent groups. For M67 area we provide positions, calibrated BVI photometry, relative proper motions, membership probabilities, and radial velocities. We demonstrate that the ground-based CCD mosaic observations just a few years apart are producing proper motions, allowing a reliable membership determination. We produced a catalogue that is made electronically available to the astronomical community.

Motivation & Objective

• To improve the completeness and accuracy of cluster membership for M67 at faint magnitudes (V > 16), where previous proper motion surveys based on photographic plates were limited.
• To demonstrate that ground-based CCD mosaic astrometry with just a 4-year baseline can achieve sub-mas/yr precision, enabling reliable proper motion measurements deeper than prior surveys.
• To produce a comprehensive, electronically available catalogue of positions, calibrated BVI photometry, proper motions, membership probabilities, and radial velocities for stars in a 34×33 arcmin² field centered on M67.
• To validate the use of global plate solutions and empirical PSF modeling for wide-field astrometry in clusters with low spatial density, overcoming limitations of local transformation methods.
• To provide a foundational dataset for future studies of stellar evolution, dynamical evolution, and chemical composition in the solar-analog cluster M67.

Proposed method

• Two-epoch archival CCD observations from the WFI@2.2m MPG/ESO telescope, separated by 4 years, were reduced using advanced astrometric techniques developed in Paper I, including empirical PSF modeling to ensure accurate PSF representation across the full field of view.
• Asterisk-based astrometric solution was applied to derive relative proper motions using a global plate solution, avoiding the instability of local reference frame transformations in low-density regions.
• Calibrated BVI photometry was derived from the same CCD frames using aperture photometry with PSF-convolved reference stars and extinction corrections.
• Radial velocities were measured from archival and new spectroscopic data obtained with the FLAMES instrument at the VLT, using high-resolution echelle spectroscopy.
• Membership probabilities were computed by combining proper motion and radial velocity information, with a Bayesian approach to distinguish cluster members from field stars.
• The final catalogue was made publicly available in electronic format to support ongoing and future studies of M67.

Experimental results

Research questions

• RQ1 Can ground-based CCD astrometry with a 4-year baseline achieve sub-mas/yr precision in proper motions for a dense field like M67, enabling reliable membership determination at V > 16?
• RQ2 To what extent can global plate solutions and empirical PSF modeling improve astrometric accuracy in wide-field imaging of open clusters with low spatial density?
• RQ3 How do the derived proper motions and radial velocities compare with theoretical isochrones, and what do they imply about convective core overshoot in M67?
• RQ4 What is the completeness and reliability of cluster membership at V ≈ 20, and how does it compare to previous photographic surveys limited to V ≈ 16?
• RQ5 Can the combination of deep photometry, precise astrometry, and radial velocities significantly improve the identification of cluster members and the characterization of stellar populations in M67?

Key findings

• The astrometric precision reaches 1.9 mas yr⁻¹ for optimally exposed stars at V < 16, degrading to ~5 mas yr⁻¹ at V = 20, enabling reliable proper motion measurements down to V ≈ 20.
• The study provides the first high-precision proper motions for M67 stars at V > 16, extending the depth of reliable membership determination by up to 5 magnitudes beyond previous photographic surveys.
• Radial velocities were successfully measured for 211 stars down to V = 16, significantly enhancing the ability to distinguish cluster members from field stars.
• The calibrated BVI photometry confirms that convective core overshoot in M67 is small, consistent with previous findings by Sandquist (2004) and Vandenberg & Stetson (2004).
• The catalogue includes positions, BVI photometry, proper motions, membership probabilities, and radial velocities for ~2,400 stars, with the full dataset made publicly available to the astronomical community.
• The results demonstrate that ground-based CCD mosaic observations with a few-year baseline can produce astrometry of sufficient precision to decontaminate the cluster color-magnitude diagram down to V ≈ 20.