MODELING INTERMEDIATE AGE AND OLD STELLAR POPULATIONS IN THE INFRARED

In this paper we have investigated the spectrophotometric properties o f the Asymptotic Giant Branch (AGB) stars and their contribution to th e integrated infrared emission in simple stellar populations (SSP). Ad opting analytical relations describing the evolution of these stars in the HR diagram and empirical relations for the mass-loss rate and the wind terminal velocity, we were able to model the effects of the dust y envelope around these stars, with a minimal number of parameters. Af ter deriving simple scaling relations which allow us to account for th e metallicity of the star, we computed isochrones at different age and initial metal content. We compare our models with existing infrared c olors of M giants and Mira stars and with IRAS PSC data. The former da ta are fairly well reproduced by our models, though a possible inadequ acy of the adopted atmospheric models is indicated. Though being chara cterized by different metallicity, the isochrones follow a single path in the IRAS two color diagram, fixed by the composition and optical p roperties of the dust mixture. The bulk of the data in the latter diag ram is delimited by the curves corresponding to a mixture of silicate grains and one of carbonaceous grains. We also discuss the effects of detached shells of matter but we do not take into account this phenome non in the present computations. Contrary to previous models, in the n ew isochrones the mass-loss rate, which establishes the duration of th e AGB phase, also determines the spectral properties of the stars. The contribution of these stars to the integrated light of the population is thus obtained in a consistent way. We find that the emission in th e mid infrared is about one order of magnitude larger when dust is tak en into account in an intermediate age population, irrespective of the particular mixture adopted. The dependence of the integrated colors o n the metallicity and age is discussed, with particular emphasis on th e problem of age-metallicity degeneracy. We show that, contrary to the case of optical or near infrared colors, the adoption of a suitable p ass-band in the mid infrared allows a fair separation of the two effec ts. We suggest intermediate redshift elliptical galaxies as possible t argets of this method of solving the age-metallicity dilemma. The new SSP models constitute a first step in a more extended study aimed at m odelling the spectral properties of the galaxies from the ultraviolet to the far infrared.