The evolution of the V-K colours of single stellar populations

Abstract

We discuss how the integrated V-K colours of single stellar populations evolve with age and metallicity. Some of the uncertainties associated to the properties of the underlying stellar models are throughfully discussed. First, we present the effects that different formulations for the mass-loss rates produce on the initial-final mass relation, and hence on the integrated colours. Second, we describe the colour changes occurring at the onset of the AGB and RGB stars. The onset of the AGB is marked by a colour jump to the red, whose amplitude is highly dependent on the metallicity and mass-loss rates adopted in the models. The overluminosity with respect to the standard Mc-L relation, that occurs in the most massive AGB stars, causes a smoothing of this colour jump. On the other hand, we find that a transient red phase takes place ~1.5x10^8 yr after the RGB develops. We then discuss the possible reasons for the transition of V-K colours (from ~1.5 to 3) that takes place in LMC clusters of SWB type IV. A revision of the ages attributed to the clusters reveals that the transition may not be as fast as originally suggested. A gradual (or delayed) transition of the colours, as predicted by models which include the overluminosity of the most massive AGB stars, describes better the data than the sudden colour-jump predicted by classical models.