vbyCaH beta CCD photometry of clusters. VIII. The super-metal-rich, old open cluster NGC 6791

摘要

CCD photometry on the intermediate-band vbyCaH beta 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 m(1) 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 m(1) 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.