The Effect of Age and Stellar Model Choice on Globular Cluster Color-to-metallicity Conversions

The photometric colors of globular clusters (GCs) act as effective proxies for metallicity, since all normally used optical/IR color indices exhibit a nonlinear but monotonic relation between their integrated color and their metallicity. One color index, (g − z) or (F475W–F850LP), has been spectroscopically calibrated in several studies, providing leverage to define color-to-metallicity conversions for other indices. In this paper, building on the work of Hartman et al., we study the GC color-metallicity relation in more detail by testing the dependence of the relations on different suites of stellar models and different assumed GC ages. Though noticeable differences between models exist, we find that the net effect on the derived GCS metallicity distributions is small.