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Galactic Globular-Cluster Metallicity Scale from the Ca II Triplet. II. Rankings, Comparisons, and Puzzles
Glen A. Rutledge, James E. Hesser and Peter B. Stetson
Publications of the Astronomical Society of the Pacific
Vol. 109, No. 738 (1997 August), pp. 907-919
Published by: Astronomical Society of the Pacific
Stable URL: http://www.jstor.org/stable/40680979
Page Count: 13
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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 ≲-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 triplet 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.
Publications of the Astronomical Society of the Pacific © 1997 The University of Chicago Press