The modelling of intermediate-age stellar populations -: I.: Near-infrared properties

Evolutionary population synthesis predictions for stellar systems with complex star formation histories rest on their major building blocks: single-burst population models. In this paper, we discuss how the integrated properties of intermediate-age single-burst populations, especially in the near-infrared, behave as a function of age and metallicity. Our models take into account all stellar evolutionary phases that affect the evolution of the integrated optical and near-infrared spectrum of such a population. Particular care was dedicated to the Asymptotic Giant Branch (AGB) stars, which can be dominant at near-infrared wavelengths. First, we present a new synthetic model that takes into account the relevant physical processes that control the evolution through the thermally pulsing AGB, namely (i) the mass-loss, (ii) the third dredge-up, and (iii) the envelope burning. We use this model to evaluate the AGB-termination luminosity, carbon star properties as function of initial metallicity and initial mass, and the contribution of these stars to the integrated light. In the isochrones presented in this paper the lifetime and the nature of the AGB stars (oxygen-rich or carbon-rich) are established as consequences of the interplay between the physical processes that control the AGB star evolution. The contribution of these stars to the integrated light of the population is thus obtained in a consistent way. We optimize our models by using a new stellar spectral library that explicitly takes into account the spectral features that characterize only AGB stars in comparison to other cool and luminous stars. We analyze the contribution of the upper AGB to the bolometric and the near-infrared light. Our models reproduce the contributions of luminous AGB stars to the bolometric and K-band light, and the carbon star contribution to the bolometric light as observed in the Magellanic Cloud star clusters in a satisfactory way, without ad hoc correction factors that could force agreement. Second, we describe the changes occurring in the integrated colours when AGB stars first appear. We confirm that, in contrast with the classical point of view, no sharp optical/near-infrared colour jump occurs when AGB stars start to dominate the stellar population. The envelope burning process that affects massive AGB stars, making them overluminous with respect to early standard core mass-luminosity relations, causes a smoothing of the colour evolution for stellar systems dominated by those stars. We reanalyze the observational strategy proposed by Lancon et al. (1999) to identify intermediate-age stellar populations in post-starburst spectra using our new model sets. The new spectrophotometric models constitute a first step in a more extended study aimed at modelling the spectral properties of the galaxies in the near-infrared.