Galactic globular-cluster metallicity scale from the Ca II triplet .2. Rankings, comparisons, and puzzles

We compare our compilation of the W' calcium index for 71 Galactic globular clusters to the widely used Zinn and West (1984, ApJS, 55, 45) [Fe/H] scale and to Carretta and Gratton's (1997, A&AS, 121, 95) scale from high-dispersion spectra analyzed with Kurucz (1992, personal communication) model atmospheres. We find our calcium ranking to be tightly correlated with each comparison set, in a nonlinear 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 +/- 0.05 dex for [Fe/H](ZW84) less than or equal to -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 II tripler lines to the potpourri of other metallicity indicators, caution - perhaps considerable - may be advisable when using W' as a surrogate for metallicity, especially for systems where ranges in age and metallicity are likely.