STELLAR POPULATION SYNTHESIS REVISITED

We introduce a model of stellar population synthesis whose main attributes are the method used to compute the distribution of stars in the theoretical color-magnitude diagram (CMD) and the updated stellar library. Existing galaxy synthesis models are limited by their derivations of approximate isochrones in the CMd. Conventional population synthesis (by Tinsley, Bruzual, Guiderdoni and Rocca-Volmerange) cannot be used reliably to study stellar systems forming on short (<< 1 Gyr) time scales. Alternatively, models that use the fuel consumption theorem (Renzini, Renzini and Buzzoni) hide approximations whose consequences on the integrated properties of stellar systems have never been properly evaluated. We circumvent the inconvenience of these techniques by developing a method of isochrone synthesis inspired from studies of the ages and colors of globular clusters. We optimize our models by compiling a library of stellar evolutionary tracks (for solar metallicity) which relies on updated stellar calculations. To limit uncertainties, we determine semi-empirically the positions in the theoretical CMD of stars on the late asymptotic giant branch (AGB). After comparing our isochrone synthesis models with models based on th fuel consumption theorem and computed with the same library of tracks, we conclude that the latter are a reasonable although misleading approximation of population synthesis. Earlier results derived with the fuel consumption theorem (Renzini and Buzzoni) are superseded by the new library of tracks. Our isochrone synthesis models reproduce well the fractional contribution of bright AGB stars to the bolometric light of a burst population as observed in the Magellanic Clouds star clusters. We anlayze the contribution of each stellar evolutionary stage to the integrated UBVRIJKL light of populations with various star formation laws and the Salpeter initial mass function (IMF). The resulting colors from the UV to the near-IR reproduce well the integrated colors of young (0.01-4 Gyr) star clusters in the Large Magellanic Cloud (LMC) and of present-day galaxies of various morphological types. We consider these successful comparisons of the models with observed stellar populations in a wide range of ages (from a few times 10(7) yr to a Hubble time) as an evidence for the appropriateness of the set of evolutionary tracks and of the isochrone synthesis method.