A Globular Cluster Metallicity Scale Based on the Abundance of Fe II

Abstract

Assuming that in the atmospheres of low-mass, metal-poor red giant stars, 1-D models based on LTE accurately predict the abundance of iron from Fe II, we derive a globular cluster metallicity scale based on the equivalent widths of Fe II lines measured from high resolution spectra of giants in 16 key clusters lying in the abundance range -2.4 < [Fe/H]II < -0.7. We base the scale largely on the analysis of spectra of 149 giant stars in 11 clusters by the Lick-Texas group supplemented by high-resolution studies of giants in five other clusters. We also derive ab initio the true distance moduli for M5, M3, M13, M92, and M15 as a means of setting stellar surface gravities. We find that [Fe/H]II is correlated linearly with <W'>, the reduced strength of the near infra-red Ca II triplet defined by Rutledge et al, although the actual correlation coefficients depend on the atmospheric model employed. We also discuss how to estimate [X/Fe]-ratios. We suggest that C, N, and O, as well as elements appearing in the spectrum in the singly ionized state, e.g., Ti, Sc, Ba, La and Eu, should be normalized to the abundance of Fe II. Other elements, which appear mostly in the neutral state, but for which the dominant species is nevertheless the ionized state, are probably best normalized to Fe I, but uncertainties remain.