UBVICCD Photometry of the Old Open Cluster Berkeley 17

Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17.Color-magnitude diagrams (CMDs) are constructed and reach well past themain-sequence turn-off for both clusters. Cluster ages are determined by meansof isochrone fitting to the cluster CMDs. These fits are constrained to agreewith spectroscopic metallicity and reddening estimates. Cluster ages aredetermined to be 7.0+/-0.5 Gyr for NGC 188, and 10.0+/- 1.0 Gyr for Berkeley17, where the errors refer to uncertainties in the relative age determinations.These ages are compared to the ages of relatively metal-rich inner halo/thickdisk globular clusters and other old open clusters. Berkeley 17 and NGC 6791are the oldest open clusters with an age of 10 Gyr. They are 2 Gyr younger thanthe thick disk globular clusters. These results confirm the status of Berkeley17 as one of the oldest known open cluster in the Milky Way, and its ageprovides a lower limit to the age of the Galactic disk.Comment: to appear in AJ; 28 pages, 9 figure