Galactic Globular Cluster Metallicity Scale from the CA II Triplet II. Rankings, Comparisons, and Puzzles

Abstract

We compare our compilation of the redew calcium index for 71 Galactic globular clusters to the widely used markcite{zinn84}Zinn and West (1984 ApJS, 55, 45) fe scale and to Carretta and Gratton's (1997 A&AS, 121, 95) scale from high-dispersion spectra analyzed with Kurucz (1992, private communication) model atmospheres. We find our calcium ranking to be tightly correlated with each comparison set, in a non-linear and a linear fashion, respectively. By combining our calcium index information with the Zinn and West ranking, we are able to rank the globular clusters in our sample with a typical precision of $pm 0.05$ dex for $feZW84 ≲ -0.5$; for clusters more metal rich than this, the ranking is less precise. The significant differences between these metallicity scales raise important questions about our understanding of Galactic formation and chemical enrichment processes. Furthermore, in spite of the apparent improvement in metallicity ranking for the Galactic globular clusters that results from our addition of information from the ca triplet lines to the potpourri of other metallicity indicators, caution -- perhaps considerable -- may be advisable when using redew as a surrogate for metallicity, especially for systems where ranges in age and metallicity are likel. (SECTION: Stellar Clusters and Associations)