Coupling emitted light and chemical yields from stars: A basic constraint to population synthesis models of galaxies

In this paper we emphasize the close connection between the chemical and sp ectrophotometric evolution of stellar systems: chemical yields from stars c orrespond to a precise fraction of their emitted light. We translate this c oncept quantitatively. Starting from simple stellar populations, we derive useful analytical relations to calculate the stellar fuel consumption (emit ted light) as a function of basic quantities predicted by stellar models, i .e. the mass of the core and the chemical composition of the envelope. The final formulas explicate the relation between integrated light contribution (total or limited to particular evolutionary phases), chemical yields and stellar remnants. We test their accuracy in the case of low- and intermedia te-mass stars, and indicate the way to extend the analysis to massive stars . This formalism provides an easy tool to check the internal consistency be tween the different stellar inputs adopted in galaxy models: the fuel compu ted by means of the analytical formulas (corresponding to a given set of ch emical yields) should be compared to the exact values given by the luminosi ty integration along the stellar evolutionary tracks or isochrones (corresp onding to a given set of spectrophotometric models). Only if both estimates of the fuel are similar, the stellar inputs can be considered self-consist ent in terms of their energetics. This sets an important requirement to gal axy models, also in consideration of the fact that different sources of inp ut stellar data are frequently used to model their spectro-photometric and chemical evolution.