The age of NGC 3680 and a test of convective overshoot

A new CCD BV color-magnitude diagram has been derived for the intermediate age open cluster NGC 3680. The quality of photometry coupled with the best-to-date knowledge of cluster memberships allows for a detailed isochrone fit to the CMD. The theoretical isochrones have been constructed using the Yale Rotating Evolution Code (YREC) in its non-rotating mode and the OPAL opacities (circa 1991). Four sets of isochrones have been calculated: one for the standard stellar model calibrated to the Sun and three others for models with different amounts of convective overshoot at the edge of the convective core, namely, 0.15, 0.20. and 0.25H(p), where H-p is the pressure scale height at the core edge. All four sets of theoretical isochrones were adjusted to an adopted distance modulus of V-0-M(V)=10.20 and reddening E(B-V)=0.075 which leads to ages of 1.3+/-0.15, 1.5+/-0.15, 1.6+/-0.15, and 1.7+/-0.15 Gyr, respectively. The uncertainties in age mainly reflect a subjective decision in differentiating a good fit from a poorer one. The model with a convective overshoot of 0.20H(p) seems to yield the best fit to the sharply curved upper main sequence. Hence, the estimated age of NGC 3680 is 1.6+/-0.15 Gyr. The adopted overshoot parameter ostensibly is uncertain by +/-0.05H(p). For comparison, the same stellar models with overshoot of 0.25H(p) were fit to the color-magnitude diagram of the cluster NGC 752, which is similar in age and composition to NGC 3680. The age estimate for NGC 752 is 1.6+/-0.2 Gyr, assuming V-0-M(V)=8.20 and E(B-V)=0.030 for the cluster. (C) 1997 American Astronomical Society.