WIYN Open Cluster Study. II. UBVRI CCD Photometry of the Open Cluster NGC 188

<p> We present high-precision <em> UBVRI </em> 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 ~5 mag below the main-sequence turnoff. From an analysis of these data along with published photometry for M67, we draw the following conclusions: (1) From the <em> UBV </em> two-color diagram, we find a reddening of <em> E </em> ( <em> B ̶ V </em> ) <em> </em> = 0.04 &plusmn; 0.02 for M67 and <em> E </em> ( <em> B </em> <em> ̶ V </em> ) = 0.09 &plusmn; 0.02 for NGC 188. (2) Based on main-sequence fitting to solar abundance isochrones, the distance moduli turn out to be ( <em> m </em> - M) _v = 9.69 &plusmn; 0.11 for M67 and (m - M) _v = 11.44 &plusmn; 0.08 for NGC 188. (3) The comparison of the CMDs to theoretical isochrones indicates that an amount of core convective overshoot equivalent to 0.10 of a pressure scale height is appropriate for M67, while no overshoot is required to fit the CMD of NGC 188. These isochrones suggest that NGC 188 is 3.0 &plusmn; 0.7 Gyr older than M67. (4) There is a clear indication of mass segregation in both M67 and NGC 188, with the most massive stars (M/M _&odot; &gt; 1.1) being more centrally concentrated than those that are the least massive (0.8 &ge; M/M _&odot; &gt; 0.65).</p>