Heavy-element abundances in normal spiral galaxies

We present a model based on standard hypotheses but including edge effects and other geometrical considerations. The model generalizes previous approaches and shows in a schematic way the consequences of diffusive effects in generating a negative gradient of heavy-element abundances, as suggested by the data for the case of oxygen. The model incorporates an initial mass function (IMF) factorized in two terms: a mass-dependent factor and a position-dependent function. By assuming that the ejection of material in a supernova burst is limited to a region around each star, local equations are derived. The results of the calculations show that the metallicity depends on the position function essentially through the steepness of the decreasing shape at its edge. We found that the oxygen abundance falls, roughly, a factor of 4 between a galactocentric distance of a few kpc and the edge of the disk.