Matching stellar population models to bulge globular clusters

Abstract

We compare observed color-magnitude diagrams (CMDs) of the three metal-rich bulge globular clusters NGC 6553, NGC 6528, and Terzan 5, and integrated spectral energy distributions (SEDs) of NGC 6528, and 47 Tuc, to theoretical isochrones and model SEDs computed with the code of Bruzual & Charlot (1997, ApJ, in preparation, hereafter BC97). The BC97 models provide the evolution in time of the spectrophotometric properties of simple stellar populations (SSPs) for a wide range of stellar metallicity. These models allow us to compare predictions based on different sets of evolutionary tracks and various choices of the stellar spectral libraries with observational data. We conclude that: (a) At least for solar metallicity models, the serni-empirical flux corrections applied by Lejeune et al. (1997, A&A, in press; 1997, A&A, in preparation) to available grids of synthetic stellar spectra improve the agreement between population model predictions and observations. (b) The adopted reddening and distance moduli for the three clusters seem well deterrnined, since the theoretical isochrones fit quite well the observed CMDs. (c) The overall metallicity of these clusters is close to solar. (d) Based on our CMD and SED models we estimate that the ages of NGC 6553 and NGC 6528 must be ≈ 12±2 Gyr. These are the only two clusters in our sample with main sequence photometry. From the UV-optical SED of 47 Tuc we estimate an age of ≈ 14±2 Gyr.