Modeling age and metallicity in globular clusters: a comparison of theoretical giant branch isochrones

In view of the important contributions of red giants to the integrated light in evolutionary synthesis calculations of old stellar populations in clusters and galaxies, we provide a comparison of theoretical isochrones computed from the Padova, Geneva, and Yale evolutionary tracks, respectively. Using the most recent version of the color-calibrated library of theoretical stellar spectra by Lejeune et al. (1999), the isochrones are converted in a uniform manner to the observational color-magnitude diagrams, (M(T1), C-T1) and (M(I), V-I), and compared with observed giant branches of typical globular clusters spanning a large range of metallicities. We find that the three different isochrones of the red giant branch provide significantly different slopes and curvatures, where in general, the differences tend to be larger for metallicities decreasing below [M/H] ~ -1. Throughout the full metallicity range (~ -2<[M/H]<0), the Yale isochrones are the only ones which show very good matches of both the observed giant branches and the associated metallicity scale derived from the color of the RGB, while the Padova isochrones exhibit significant discrepancies for low-metallicities. The effects of these differences on the integrated model colors of single stellar populations, in particular for age and metallicity determinations, are briefly addressed.