PI: Imants Platais, US Naval Observatory, firstname.lastname@example.org
Address: 3450 Massachusetts Ave., NW, Washington, DC 20392-5420, USA
CoI: Terrence M. Girard, Yale University
CoI: William F. van Altena, Yale University
Title: Deep Astrometry of Key Open Clusters
Abstract: We propose to continue determination of cluster membership to very faint magnitudes for the set of old open clusters (NGC 6791, 6819, and 7789) as defined by the NSF-funded WIYN Open Cluster Study (WOCS). Key elements of this proposal are i) depth, ii) field coverage - out to two tidal radii, and iii) astrophysical significance of the clusters. By combining numerous old first-epoch plates taken at the Mayall 4-m telescope with recent CCD Mosaic Imager frames, we will be able to derive accurate proper motions down to V~23. This is a significant improvement over the typical open-cluster proper-motion study's limit of ~16. We have already demonstrated that the ``plate + CCD-Mosaic" astrometry can produce very accurate proper motions (standard error 0.5 mas/yr or better). This combination of limiting magnitude and proper- motion precision is allowing us to obtain complete Present Day Mass Functions down to ~0.5M_\sun, essentially eliminating uncertainty due to field stars. Membership information at the low-mass end of the main sequence also provides a valuable tool for testing calibrations of the theoretical temperature-color transformation, over this difficult range. Also, faint galaxies seen in the fields of these clusters provide the means to obtain absolute proper motions which, coupled with radial velocities and a distance, are essential to calculating their Galactic orbits. An important spin-off of our program is the astrometric calibration of the NOAO CCD Mosaic Imager. It appears to show the signs of chip-to- chip gap drift which, if unaccounted for, may have undesirable consequences in astronomical applications. These observations will provide further evidence as to the CCD Mosaic's geometric stability.
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