Ground-based CCD astrometry with wide-field imagers

Abstract

The solar-age open cluster M 67 (C0847+120, NGC 2682) is a touchstone in studies of the old Galactic disk. Despite its outstanding role, the census of cluster membership for M 67 at fainter magnitudes and their properties are not well-established. Aims. Using proprietary and archival ESO data, we have obtained astrometric, photometric, and radial velocities of stars in a 34 × 33 arcmin² field centered on the old open cluster M 67. The two-epoch archival observations separated by 4 years and acquired with the Wide-Field Imager at the 2.2 m MPG/ESO telescope were reduced with our new astrometric techniques, as described in the first paper in this series. The same observations served to derive calibrated BV I photometry in M 67. Radial velocities were measured using the archival and new spectroscopic data obtained at the VLT. We have determined relative proper motions and membership probabilities for ∼2400 stars. The precision of proper motions for optimally exposed stars is 1.9 mas yr−¹ , gradually degrading down to ∼5 mas yr−¹ at V = 20. Our relatively precise proper motions at V > 16 were first obtained in this magnitude range for M 67. Radial velocities were measured for 211 stars in the same field. We also present a detailed comparison with recent theoretical isochrones from several independent groups. For the M 67 area, we provide positions, calibrated BV I photometry, relative proper motions, membership probabilities, and radial velocities. We demonstrate that ground-based CCD mosaic observations just a few years apart are producing proper motions, allowing reliable membership determination. We have produced a catalog that is electronically available to the astronomical community.