ar X iv : a st ro - p h / 99 08 20 7 v 1 1 8 A ug 1 99 9 The Chemical Evolution of the Galaxy with Variable IMFs

In this work we explore the effects of adopting initial mass functions (IMFs) variable in time on the chemical evolution of the Galaxy. In order to do that we adopt a chemical evolution model which assumes two main infall episodes for the formation of the Galaxy and which proved to be successful in reproducing the majority of the observational constraints, at least for the case of a constant IMF. Different variable IMFs are tested with this model, all assuming that massive stars are preferentially formed in ambients of low metallicity. This implies that massive stars are formed preferentially at early times and at large galactocentric distances. Our numerical results have shown that all the variable IMFs proposed so far are unable to reproduce all the relevant observational constraints in the Galaxy and that a constant IMF still reproduces better the observations. In particular, variable IMFs of the kind explored here are unable to reproduce the observed abundance gradients even when allowing for changes in other chemical evolution model parameters as, for instance, the star formation rate. As a consequence of this we conclude that the G-dwarf metallicity distribution is best explained by infall with a large timescale and a constant IMF, since it is possible to find variable IMFs of the kind studied here, reproducing the G-dwarf metallicity but this worsen the agreement with other observational constraints.