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

We compare our compilation of the W' calcium index for 71 Galactic globularclusters to the widely used Zinn and West (1984 ApJS, 55, 45) [Fe/H] scale andto Carretta and Gratton's (1997 A&A Supplement 121, 95) scale fromhigh-dispersion spectra analyzed with Kurucz (1992, private communication)model atmospheres. We find our calcium ranking to be tightly correlated witheach comparison set, in a non-linear and a linear fashion, respectively. Bycombining our calcium index information with the Zinn and West ranking, we areable to rank the globular clusters in our sample with a typical precision of+/- 0.05 dex for [Fe/H] < -0.5 on the Zinn and West scale; for clusters moremetal rich than this, the ranking is less precise. The significant differencesbetween these metallicity scales raise important questions about ourunderstanding of Galactic formation and chemical enrichment processes.Furthermore, in spite of the apparent improvement in metallicity ranking forthe Galactic globular clusters that results from our addition of informationfrom the Ca II triplet lines to the potpourri of other metallicity indicators,caution -- perhaps considerable -- may be advisable when using W' as asurrogate for metallicity, especially for systems where ranges in age andmetallicity are likely.