VbyCaHβ CCD Photometry of Clusters. VIII. The Super-Metal-Rich, Old Open Cluster NGC 6791

Abstract

CCD photometry on the intermediate-band vbyCaHβ system is presented for the metal-rich, old open cluster NGC 6791. Preliminary analysis led to [Fe/H] above +0.4 with an anomalously high reddening and an age below 5 Gyr. A revised calibration between (b - y)0 and [Fe/H] at a given temperature shows that the traditional color-metallicity relations underestimate the color of the turnoff stars at high metallicity. With the revised relation, the metallicity from hk and the reddening for NGC 6791 become [Fe/H] = +0.45 ± 0.04 and E(b - y) = 0.113 ± 0.012 or E(B - V) = 0.155 ± 0.016. Using the same technique, reanalysis of the photometry for NGC 6253 produces [Fe/H] = +0.58 ± 0.04 and E(b - y) = 0.120 ± 0.018 or E(B - V) = 0.160 ± 0.025. The errors quoted include both the internal and external errors. For NGC 6791, the metallicity from m1 is a factor of 2 below that from hk, a result that may be coupled to the consistently low metal abundance from DDO photometry of the cluster and the C-deficiency found from high-dispersion spectroscopy. E(B - V) is the same value predicted from Galactic reddening maps. With E(B - V) = 0.15 and [Fe/H] = +0.45, the available isochrones predict an age of 7.0 ± 1.0 Gyr and an apparent modulus of (m - M) = 13.60 ± 0.15, with the dominant source of the uncertainty arising from inconsistencies among the isochrones. The reanalysis of NGC 6253 with the revised lower reddening confirms that on both the hk and m1 metallicity scales, NGC 6253, while less than half the age of NGC 6791, remains at least as metal-rich as NGC 6791, if not richer.