A PHOTOMETRIC ANALYSIS OF THE INTERMEDIATE-AGE OPEN CLUSTER NGC-5822

Photoelectric photometry of stars in NGC 5822 on the UBVR and DDO systems is presented. The B V data are used to calibrate a photographic survey of the cluster, while UBV and DDO data are used to constrain the cluster reddening and metallicity. Taken in conjunction with previous photometric analyses, it is found that the cluster has a reddening of E(B-V) = 0.15 +/- 0.015 (pe) as defined for the stars at the turnoff, and a metallicity relative to the sun of [Fe/H] = -0.15 +/- 0.05 (pe). Taking into account a rich population of cluster binaries, the photometric distance modulus is estimated to be (m-M) = 9.85 +/- 0.15 (pe), in excellent agreement with (m-M) = 9.8 obtained from main-sequence fitting to the semiempirical ZAMS of VandenBerg and Poll [AJ, 98, 1451 (1989)] normalized to a solar color of (B- V) = 0.65. Comparisons to isochrones without convective overshoot indicate an age of 1.25 X 10(9) yr and are generally in agreement with observation if the scatter near the turnoff results from a combination of a modest hydrogen-exhaustion hook, binaries, and field-star contamination. The color and luminosity of the red giants are incompatible with the theoretical predictions, but the ratio of red giants to turnoff stars is consistent with the claim that NGC 5822 is slightly younger than the age associated with the clump-red giant transition phase for intermediate- to low-mass stars. Comparison with convective-overshoot models yields a better match to the red giant branch and an age of 1.2 X 10(9) or 2.0 X 10(9) yr, depending upon the choice of isochrones. It is shown that when isochrones of the appropriate metallicity are consistently normalized, the nonovershoot models do not adequately match the observations of intermediate-age clusters in that they cannot reproduce the shape of the CMD or the distribution of stars near the turnoff, nor do they predict the apparent trend of luminosity with age found for the red giant clump.