PI: Imants Platais, US Naval Observatory, email@example.com
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.
National Optical Astronomy Observatory, 950 North Cherry Avenue, P.O. Box 26732, Tucson, Arizona 85726, Phone: (520) 318-8000, Fax: (520) 318-8360