The open cluster NGC 7789. II. CCD VI photometry

A (V, V- I)-diagram for the intermediate-age open cluster NGC 7789 has been derived from CCD observations of more than 15,000 stars within approximate to 18' of the cluster center. From the brightest giants and blue straggler s at V- II to the faintest lower main-sequence stars that were observed (at V similar to 21, M-V similar to 9), the color-magnitude diagram is well de fined. A prominent clump of core helium-burning stars is evident at V = 13. 0, and the upper end of the main sequence shows a fairly pronounced curvatu re to the red, which is indicative of significant convective core overshoot ing. Indeed, comparisons with up-to-date stellar models show that it is not possible to explain the observed morphology in the vicinity of the turnoff unless the overshooting is quite extensive. Interestingly, if sufficient o vershooting is assumed in order to match the main-sequence data, it is not possible to reproduce the cluster's extended giant branch unless the cluste r age is at least 1.6 Gyr (assuming a metallicity in the range -0.2 less th an or equal to [Fe/H] less than or equal to 0.0). This, in turn, requires t hat the cluster have an apparent distance modulus (m - M)(V) less than or e qual to 12.2. Thus, sometime within the past few hundred million years, the ignition of helium burning in NGC 7789 has switched from a quiescent to an explosive ("flash") phenomenon, and the length of the cluster's red giant branch has been steadily increasing with the passage of time since then. Fr om main-sequence fits to models that have been carefully normalized to the Sun, we infer a reddening 0.35 less than or equal to E(V - I) less than or equal to 0.38.