Predicted colours for simple stellar populations

This paper presents theoretical integrated colours of old stellar populations as computed adopting an homogeneous set of stellar models, covering all the major evolutionary phases of globular cluster stars. We show that adopting 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 present 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 Gyr. The stochastic occurrence of luminous post-AGB stars is briefly discussed. We find that for an age $t \sim 15$ Gyr the predicted integrated colours appear in good agreement with available data for both galactic and LMC old globulars. We discuss the uncertainties of integrated colours due to statistical fluctuations in the number of luminous stars, giving for each colour the expected uncertainty as a function of the cluster integrated V-magnitude. Comparison with Kurth et al. (1999) discloses that the still existing differences in the evolutionary results appear of minor relevance as far as integrated cluster colours is concerned. Finally, we show that reasonable uncertainties either in the cluster age or in the efficiency of mass loss have marginal effects on the predicted colours.