The metal-rich globular cluster NGC 6553: Observations with WFPC2, STIS, and NICMOS

We present a Hubble Space Telescope study of the metal-rich globular cluste r NGC 6553 using WFPC2, NICMOS, and STIS. Our primary motivation is to cali brate the STIS broadband LP magnitude against V-555 and I-814 magnitudes fo r stars of known metallicity and absolute (visual) magnitude, for applicati on to our study of LMC globular clusters. NGC 6553 has been shown in earlie r studies to have a very unusual color-magnitude diagram, so we also use ou r data to investigate the reddening, distance, luminosity function, and str ucture of this cluster. We deduce a higher metallicity and smaller distance modulus than did some previous studies, but we emphasize that very large p atchy extinction on small angular scales prohibits accurate determination o f the parameters of this cluster. The horizontal branch of NGC 6553 in (V, V-I) is tilted at an angle close to that of the reddening vector. We show t hat extinction does not, however, explain the tilt, which is presumably a m etallicity effect. The color-magnitude diagram shows an apparent second tur noff some 1.5 mag fainter than that of the cluster. We show that this is mo st likely the background Galactic bulge: however, in that case, the color-m agnitude diagram of NGC 6553 is not a good match to that of the field bulge population. The cluster is probably more metal-rich than is the mean field bulge star.