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

Abstract

ABSTRACT Photometric photometry of stars in NGC 5822 on the UBVR and DDO systems is presented. The BV data are used to calibrate a photometric survey of the cluster, while UBV and DDO data are used to constrain the cluster reddening and metallicity. Taken in conjucntion with previous photometric analyses, it is found that the cluster has a reddening of E(B-V)=0.15 ± 0.015 (p.e.) as defined for the stars at the turnoff, and a metallicity relative to the sun of [Fe/H]=-0.15 ± 0.05 (p.e.). Taking into account a rich population of cluster binaries, the photometric distance modulus is estimated to be (m-M)=9.85 ± 0.15 (p.e.), in excellent agreement with (m-M)=9.8 obtained from main sequence fitting to the semi-empirical ZAMS of van den Bergh & Poll [AJ, 98,1415 (1989)] normalized to a solar color of (B-V)=-0.65. Comparison to isochrones without convective overshoot indicate an age of 1.25 X 109 years and are generally in agreement with observation if the scatter near the turnoff results from a combindation of a modest hydrogen-exhaustion hook, binaries, and field star contamination. The color and luminosity of the red giants are incompatible with the theorietcial 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 109 or 2.0 X 109 years, depending upon the choice of isochrones. It is shown that when isochrones of the appropriate metallicity are consistently normalized, the non-overshoot models do not adequately match the observations of intermedate-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.