SYNTHETIC AND OBSERVED PHOTOMETRIC INDEXES FOR GLOBULAR-CLUSTERS IN THE GALAXY AND M31

Buzzoni's (1989) grid of synthetic spectral energy distributions, repr esentative of old stellar populations, was used to derive colours in d ifferent photometric systems, and to compare the theoretical predictio ns with the observational data referring to about 120 globular cluster s in the Galaxy and to 159 objects of the globular cluster system of M 31. Synthetic and observed indices display an overall agreement in th e 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 in itial mass function (IMF). The two main parameters modulating the clus ter colour distributions are, as known, metallicity and horizontal bra nch morphology, while IMF slope and mass loss rate from stars in the r ed-giant branch and asymptotic-giant branch evolutionary stages produc e only minor, although not negligible, effects on the integrated colou rs. The M 31 and Galactic cluster populations are found to be substant ially similar, at least as far as the spectral energy distribution cha racteristics in the U --> V spectral range are concerned. In general, the model predictions are fully consistent with the cluster metallicit y values given in the current metallicity scale for Galactic globular clusters (Zinn and West 1984) with some exceptions for the IR photomet ry, where the models predict V - K values systematically bluer (by 0.1 -0.2 mag) than the observed ones for metal-poor clusters. The question is discussed in some detail with reference to different data sources.