A UBVI AND uvbyCaHβ ANALYSIS OF THE INTERMEDIATE-AGE OPEN CLUSTER, NGC 5822

NGC5822 is a richly populated, moderately nearby, intermediate-age open cluster covering an area larger than the full moon on the sky. A CCD survey of the cluster on the UBVI and uvbyCaH beta systems shows that the cluster is superposed upon a heavily reddened field of background stars with E(B - V) >0.35 mag, while the cluster has small and uniform reddening at E(b - y) = 0.075 +/- 0.008 mag or E(B - V) = 0.103 +/- 0.011 mag, based upon 48 and 61 probable A and F dwarf single-star members, respectively. The errors quoted include both internal photometric precision and external photometric uncertainties. The metallicity derived from 61 probable single F-star members is [Fe/H] = -0.058 +/- 0.027 (sem) from m(1) and 0.010 +/- 0.020 (sem) from hk, for a weighted average of [Fe/H]=-0.019 +/- 0.023, where the errors refer to the internal errors from the photometry alone. With reddening and metallicity fixed, the cluster age and apparent distance modulus are obtained through a comparison to appropriate isochrones in both V I and BV, producing 0.9 +/- 0.1 Gyr and 9.85 +/- 0.15, respectively. The giant branch remains dominated by two distinct clumps of stars, though the brighter clump seems a better match to the core-He-burning phase while the fainter clump straddles the first-ascent red giant branch. Four potential new clump members have been identified, equally split between the two groups. Reanalysis of the UBV two-color data extending well down the main sequence shows it to be optimally matched by reddening near E(B - V) = 0.10 rather than the older value of 0.15, leading to [Fe/H] between - 0.16 and 0.00 from the ultraviolet excess of the unevolved dwarfs. The impact of the lower reddening and younger age of the cluster on previous analyses of the cluster is discussed.