THRESHOLDS AND THE CHEMICAL EVOLUTION OF GALACTIC DISKS .2. RADIAL FLOWS

We introduce radial flows into the chemical evolution models of Chamch am, Pitts & Tayler. In this work, the authors used a stability criteri on in disc galaxies to define a threshold for star formation and to de duce a self-consistent star formation rate by combining an energy equa tion for the gas with the standard equations for the chemical evolutio n of galaxies. These models reproduced the general observable features of spiral galaxies and of the solar neighbourhood, but failed to repr oduce the metallicity gradients observed in most galaxies. A biased in fall rate increased the gradient, but not sufficiently to correspond w ith some observations. If the disc is formed out of centrifugal equili brium, the resulting radial flows will also enhance the composition gr adient, as is shown in the present paper.