THE GAP IN THE COLOR-MAGNITUDE DIAGRAM OF NGC 2420 - A TEST OF CONVECTIVE OVERSHOOT AND CLUSTER AGE

Theoretical isochrones have been constructed using the OPAL opacities specifically to study the color-magnitude diagram of the open star clu ster NGC 2420. This cluster provides a rare test of core convection in intermediate-mass stars. At the same time, its age is of interest bec ause of its low metallicity and relatively high Galactic latitude for an open cluster. The excellent color-magnitude diagram constructed by Anthony-Twarog et al. (1990) allows a detailed fit of the isochrones t o the photometric data. We discuss the importance of convective oversh oot at the convective core edge in determining the morphology of the g ap located near the main-sequence turnoff. We find that given the assu mptions made in the models, a modest amount of overshoot (0.23H(p)) is required for the best fit. Good agreement is achieved with all featur es of the turnoff gap for a cluster age of 2.4 +/- 0.2 Gyr. We note th at a photometrically complete luminosity function near the main-sequen ce turnoff and subgiant branch would also provide an important test of the overshoot models.