Quantifying the uncertainties of chemical evolution studies - I. Stellar lifetimes and initial mass function

Stellar lifetimes and initial mass function are basic ingredients of chemical evolution models, for which different recipes can be found in the literature. In this paper, we quantify the effects on chemical evolution studies of the uncertainties in these two parameters. We concentrate on chemical evolution models for the Milky Way, because of the large number of good observational constraints. Such chemical evolution models have already ruled out significant temporal variations for the stellar initial mass function in our own Galaxy, with the exception perhaps of the very early phases of its evolution. Therefore, here we assume a Galactic initial mass function constant in time. Through an accurate comparison of model predictions for the Milky Way with carefully selected data sets, it is shown that specific prescriptions for the initial mass function in particular mass ranges should be rejected. As far as the stellar lifetimes are concerned, the major differences among existing prescriptions are found in the range of very low-mass stars. Because of this, the model predictions differ widely for those elements which are produced mostly by very long-lived objects, as for instance He-3 and Li-7. However, we conclude that model predictions of several important observed quantities, constraining the plausible Galactic formation scenarios, are fairly robust with respect to changes in both the stellar mass spectrum and lifetimes. For instance, the metallicity distribution of low-mass stars is nearly unaffected by these changes, since its shape is dictated mostly by the time scale for thin-disk formation.