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 cluster NGC 6553 using WFPC2, NICMOS, and STIS. Our primary motivation is to calibrate the STIS broadband LP magnitude against V555 and I814 magnitudes for stars of known metallicity and absolute (visual) magnitude, for application to our study of LMC globular clusters. NGC 6553 has been shown in earlier studies to have a very unusual color-magnitude diagram, so we also use our data to investigate the reddening, distance, luminosity function, and structure of this cluster. We deduce a higher metallicity and smaller distance modulus than did some previous studies, but we emphasize that very large patchy extinction on small angular scales prohibits accurate determination of 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 that extinction does not, however, explain the tilt, which is presumably a metallicity effect. The color-magnitude diagram shows an apparent second turnoff some 1.5 mag fainter than that of the cluster. We show that this is most likely the background Galactic bulge: however, in that case, the color-magnitude 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.