The Giant Branches of Open and Globular Clusters in the Infrared as Metallicity Indicators: A Comparison with Theory

Tiede, G.P., Martini, P., and Frogel, J.A. 1997, AJ, 114, 694

We apply the giant branch slope - [Fe/H] relation derived by Kuchinski et al. [AJ, 109, 1131 (1995)] to a sample of open clusters. We find that the slope of the giant branch in K vs. (J-K) color-magnitude diagrams correlates with [Fe/H] for open clusters as it does for metal-rich globular clusters but that the open cluster data are systematically shifted to less negative values of giant branch slope, at constant [Fe/H]. We use isochrone models to examine the theoretical basis for this relationship and find that for a given value of [Fe/H], the slope of the relationship remains constant with decreasing population age but the relation shifts to less negative values of giant branch slope with decreasing age. Both of these theoretical predictions agree with the trends found in the data. Finally, we derive new coefficients for the giant branch slope - [Fe/H] relation for specific members of 3 populations, metal-rich globular clusters, bulge stars and open clusters.