SIMBAD references

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.