Synthetic and observed photometric indices for globular clusters in the Galaxy and M31

Abstract

Buzzoni's (1989) grid of synthetic spectral energy distributions, representative of old stellar populations, was used to derive colours in different photometric systems, and to compare the theoretical predictions with the observational data referring to about 120 globular clusters in the Galaxy and to 159 objects of the globular cluster system of M31. Synthetic and observed indices display an overall agreement in the composite planes of two-colour diagrams, thus in agreement with the standard evolutionary scenario leading, for globular clusters, to old stellar populations consistent with an age of 15 Gyr and a Salpeter initial mass function (IMF). The two main parameters modulating the cluster colour distributions are, as known, metallicity and horizontal branch morphology, while IMF slope and mass loss rate from stars in the red-giant branch and asymptotic-giant branch evolutionary stages produce only minor, although not negligible, effects on the integrated colours. The M31 and Galactic cluster populations are found to be substantially similar, at least as far as the spectral energy distribution characteristics in the U-V spectral range are concerned. In general, the model predictions are fully consistent with the cluster metallicity values given in the current metallicity scale for Galactic globular clusters (Zinn & West 1984) with some exceptions for the IR photometry, where the models predict V-K values systematically bluer (by 0.1-0.2mag) than the observed ones for metal-poor clusters. The question is discussed in some detail with reference to different data sources.