uvbyCaHbeta CCD Photometry of Clusters. III. The Most Metal-Rich Open Cluster, NGC 6253

Abstract

CCD photometry on the intermediate-band uvbyCaH-beta system is presented for the old open cluster, NGC 6253. Despite a high level of field star contamination due to its location toward the galactic center, combination of the data from the multiple color indices with the core cluster sample derived from radial star counts leads to the identification of a set of highly probable, single cluster members. Photometric analysis of a select sample of 71 turnoff stars produces a reddening value of E(b-y) = 0.190 +/-0.002 (s.e.m.) or E(B-V) = 0.260 +/- 0.003 (s.e.m.) from 71 stars. The metallicity indices, delta-m1 and delta-hk, both identify this cluster as the most metal-rich object studied on either system to date. Simple extrapolation of the available metallicity calibrations leads to [Fe/H] ranging from +0.7 to +0.9. Metal-rich isochrones with overshoot imply an age between 2.5 and 3.5 Gyr, with an apparent distance modulus between (m-M)= 11.6 and 12.2, depending upon the isochrones used. The improvement in the fit using alpha-enhanced isochrones may indicate that the cluster [Fe/H] is closer to +0.4, but the photometric indices are distorted by an elemental distribution other than a scaled solar. The galacto- centric position of the cluster, in conjunction with data for other clusters and Cepheids, is consistent with the inner disk reaching and maintaining a metallicity well above solar since the early history of the disk, unlike the solar neighborhood.