CCD [CLC][ITAL]uvby[/ITAL][/CLC]Hβ Photometry in Clusters. II. The Nearest Globular Cluster, NGC 6397

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Abstract
CCD photometry extending below the turnoff of the globular cluster NGC 6397 on the uvbyHβ system is presented and analyzed. Restricting the sample to stars with small photometric errors and highly probable cluster membership, the reddening from approximately 1500 stars at the turnoff is E(b-y) = 0.127 ± 0.002 (standard error of the mean), equivalent to E(B-V) = 0.179 ± 0.003. The photometric abundance for the same sample, on the revised spectroscopic scale for globular clusters, is [Fe/H] = -1.82 ± 0.04 (standard error of the mean). From 220 cluster red giants, well-defined relations are established for m1 and c1 as a function of b-y. The reddening from the giants is systematically smaller than found for the turnoff stars, while the m1-based metallicity ranges from [Fe/H] = -2.4 for the bluest stars to greater than -1.7 for the reddest. In conjunction with previous results for ω Cen, M22, M55, and M13, it is concluded that the globular cluster red giants do not follow the same c1 versus b-y and m1 versus b-y relations as the field giants, even if the cluster sample is restricted to CN-weak giants. Comparison of the cluster turnoff stars with field dwarfs of comparable [Fe/H] in the c1 versus b-y diagram implies that, within the uncertainties, there is no difference in age between the two samples. From a handful of field dwarfs with comparable [Fe/H] and accurate parallaxes, the modulus of NGC 6397, (V - MV)0, is calculated to be 12.15. Comparison with isochrones of the appropriate [Fe/H] with [α/H] = +0.3 results in an age of 12.0 ± 0.8 Gyr. The ultraviolet color-magnitude diagram of the cluster is examined and compared with earlier work.