THE AGE OF NGC-3680 AND A TEST OF CONVECTIVE OVERSHOOT

A new CCD BV color-magnitude diagram has been derived for the intermed iate age open cluster NGC 3680. The quality of photometry coupled with the best-to-date knowledge of cluster memberships allows for a detail ed isochrone fit to the CMD. The theoretical isochrones have been cons tructed using the Yale Rotating Evolution Code (YREC) in its non-rotat ing 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 convect ive 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 d istance modulus of V-0-M(V)=10.20 and reddening E(B-V)=0.075 which lea ds 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 dec ision 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 36 80 is 1.6+/-0.15 Gyr. The adopted overshoot parameter ostensibly is un certain by +/-0.05H(p). For comparison, the same stellar models with o vershoot of 0.25H(p) were fit to the color-magnitude diagram of the cl uster NGC 752, which is similar in age and composition to NGC 3680. Th e age estimate for NGC 752 is 1.6+/-0.2 Gyr, assuming V-0-M(V)=8.20 an d E(B-V)=0.030 for the cluster. (C) 1997 American Astronomical Society .