Predicted colours for simple stellar populations II. The case of old stellar clusters

This paper presents theoretical integrated colours of old stellar populatio ns as computed adopting an homogeneous set of stellar models, covering all the major evolutionary phases of globular cluster stars. We show that adopt ing Reimers parameterization of mass loss rates, the choice eta = 0.4 gives synthetic CM diagrams of simple stellar populations in agreement with the typical dependence of globular cluster CM diagrams on metallicity. We prese nt theoretical U - B, B - V, V - R and V - I integrated colours for cluster metallicity in the range Z = 0.0001 to 0.02 and for age between 8 and 15 G yr. The stochastic occurrence of luminous post-AGE stars is briefly discuss ed. We find that for an age t similar to 151 Gyr the predicted integrated c olours appear in good. agreement with available data for both galactic and LMC old globulars. We discuss the uncertainties of integrated colours due t o statistical fluctuations in the number of luminous stars, giving for each colour the expected uncertainty;Is a function of the cluster integrated V- magnitude, Comparison with Kurth et al. (1990) discloses that the still exi sting differences in the evolutionary results appear of minor relevance as far as integrated cluster colours is concerned. Finally, we show that reaso nable uncertainties either in the cluster age or in the efficiency of mass loss have marginal effects on the predicted colours.