SIMBAD references

In this work we analyse the role of low- and intermediate-mass stars in contributing to the chemical enrichment of the interstellar medium. First we present new sets of stellar yields basing on the results of updated evolutionary calculations, which extend from the ZAMS up to the end of the AGB phase (Girardi et al., 2000A&AS..141..371G; Marigo et al., 1999A&A...344..123M). These new yields, that present a significant dependence on metallicity, are then compared to those of other available sets (Renzini & Voli, 1981A&A....94..175R; van de Hoek & Groenewegen, 1997A&AS..123..305V). The resulting differences are explained in terms of different model assumptions - i.e. treatment of convective boundaries, mass loss, dredge-up, hot-bottom burning - and further discussed on the basis of important empirical constraints which should be reproduced by theory - i.e. the initial-final mass relation, white dwarf mass distribution, carbon star luminosity function, and chemical abundances of planetary nebulae. We show that present models are able to reproduce such constraints in a satisfactory way.