A VI_C color-magnitude diagram of the globular cluster NGC 6352 from Hubble Space Telescope planetary camera observations

The globular cluster NGC 6352, which on the basis of its Galactic position, radial velocity, and [Fe/H] is a member of the (thick) disk system of globular clusters, has been observed to below the main-sequence turnoff (MSTO) using the Hubble Space Telescope. These observations, which were obtained before the repair mission, were analyzed using the flux-conserving iterative/recursive deconvolution algorithm developed at the University of North Carolina. This algorithm can produce more precise photometry than standard PSF-fitting methods with the aberrated images. The V, (V-I_C) color-magnitude diagram constructed from these observations places the cluster turnoff at V=18.80±0.10. Observations with the CTIO 0.9 m telescope have been used to photometer the brighter stars in the cluster, providing a calibration of the HST data and an estimate of the cluster's reddening, E(B-V)=0.21±0.03. From spectroscopic observations of the strengths of the Ca II triplet lines in red giants in the cluster and from previous measurements in the literature, we find that NGC 6352 is only slightly more metal rich (Δ[Fe/H]=0.08±0.05) than the prototypical disk globular cluster, 47 Tuc. From the difference in V magnitude between the horizontal branch and the MSTO, we find that NGC 6352 is essentially the same age as 47 Tuc (formally, older by 0.7±2.2 Gyr). Comparisons with the latest Yale isochrones support this result and yield 14.5±2 Gyr for the age of 47 Tuc from the photometry of Hesser et al. [PASP, 99, 739 (1987)], which is consistent with other determinations. The old age obtained for NGC 6352 provides additional evidence that the disk system of globular clusters is very old and is in fact older or comparable in age to several globular clusters populating the Galactic halo.