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

Abstract

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, 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 ±0.05 dex for [Fe/H] ZW 84 ∼ < −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.

Integrated colours of Milky Way globular clusters and horizontal branch morphology

“…Where available, metallicities were compiled from the homogeneous data set of Rutledge et al (1997aRutledge et al ( , 1997b. They are based on the combined strengths of the λ8498, λ8542, and λ8662 Ca II triplet lines of red giant branch (RGB) stars that are probable GC members.…”

“…They are based on the combined strengths of the λ8498, λ8542, and λ8662 Ca II triplet lines of red giant branch (RGB) stars that are probable GC members. Rutledge et al (1997aRutledge et al ( , 1997b followed the techniques of Armandroff & Da Costa (1991) for measuring the equivalent widths of these lines, correcting them to a reference point on the RGB equal to the V magnitude of the horizontal branch, and correlating them against various [Fe/H] measurements. The metallicities given in column 5 of Table 2 of Rutledge et al (1997b) were adopted in this study, this column containing their "most probable" [Fe/H] value on the scale of Zinn & West (1984;ZW84).…”

The chemical evolution of subpopulations in the Carina dwarf spheroidal

“…This procedure (cutting at 3 σ), after additional rejection of stars with too low a S/N to measure equivalent widths, left 437 radial velocity members to be analysed. Finally, metallicities were derived from the reduced equivalent width W (Rutledge et al 1997, Cole et al 2004 of the CaT, which we calibrated against a common reference dataset of galactic globular clusters, obtained during the same observational runs.…”

Chemical Evolution in the Carina Dwarf 1Spheroidal

“…After rejection of foreground stars and stars with too low a S/N, 487 radial velocity members were analysed. Finally, metallicities were derived from the reduced equivalent width W ′ ([1], [5]). …”

“…Analyses of the faint Carina dSph have revealed that it contains a variety of stellar populations (e.g., [4]), exhibiting prominent old (>11 Gyr) and intermediateage (5)(6) populations. This implies that Carina must have undergone several star forming (SF) episodes with at least three significant pulses.…”