The effects of age on red giant metallicities derived from the near-infrared Ca II triplet

Abstract

We have obtained spectra with a resolution of 2.5 in the region of ≈7500–9500 for 116 red giants in five galactic globular clusters and six old open clusters (five with published metallicities and one previously unmeasured). The signal-to-noise (S/N) ratio lies within the range 20 ≤ S/N ≤ 85. We measure the equivalent widths of the infrared Ca ii triplet absorption lines in each star and compare them with cluster metallicities taken from the literature. With globular cluster abundances on the Carretta & Gratton scale, and open cluster abundances taken from the compilation of Friel and collaborators, we find a linear relation between [Fe/H] and Ca ii line strength spanning the range −2 [Fe/H]−0.2 and 2.5 (age/Gyr) 13. The reference abundance scales appear to be consistent with each other at the 0.1 dex level. Alternate choices for metallicity scales can introduce curvature into the relation between the [Fe/H] and Ca ii equivalent width. No evidence for an age effect on the metallicity calibration is observed. Using this calibration, we find the metallicity of the massive, old, open cluster Trumpler 5 to be [Fe/H]=−0.56 ± 0.11. This is the first spectroscopic abundance measurement for Trumpler 5, and is lower by ≈0.3 dex than estimates based on the cluster colour–magnitude diagram. Considering the 10 clusters of known metallicity shifted to a common distance and reddening, we find that the additional error introduced by the variation of horizontal branch/red clump magnitude with metallicity and age is of the order of ±0.05 dex, which can be neglected in comparison with the intrinsic scatter in the method. The results are discussed in the context of abundance determinations for red giants in Local Group galaxies.