WIYN open cluster study. II. UBVRI CCD photometry of the open cluster NGC 188

We present high-precision UBVRI CCD photometry of the old open cluster NGC 188. Our color-magnitude diagram extends from near the red giant branch tip to as faint as similar to 5 mag below the main-sequence turnoff. From an a nalysis of these data along with published photometry for M67, we draw the following conclusions: (1) From the UBV two-color diagram, we find a redden ing of E(B-V) = 0.04 +/- 0.02 for M67 and E(B-V) = 0.09 +/- 0.02 for NGC 18 8. (2) Based on main-sequence fitting to solar abundance isochrones, the di stance moduli turn out to be (m - M)(v) = 9.69 +/- 0.11 for M67 and (m - M) (v) = 11.44 +/- 0.08 for NGC 188. (3) The comparison of the CMDs to theoret ical isochrones indicates that an amount of core convective overshoot equiv alent to 0.10 of a pressure scale height is appropriate for M67, while no o vershoot is required to fit the CMD of NGC 188. These isochrones suggest th at NGC 188 is 3.0 +/- 0.7 Gyr older than M67. (4) There is a clear indicati on of mass segregation in both M67 and NGC 188, with the most massive stars (M/M. > 1.1) being more centrally concentrated than those that are the lea st massive (0.8 greater than or equal to M/M. > 0.65).