CHEMICAL EVOLUTION OF THE MAGELLANIC CLOUDS - ANALYTICAL MODELS

We have extended our analytical chemical evolution modelling ideas for the Galaxy to the Magellanic Clouds. Unlike previous authors (Russell & Dopita, Tsujimoto et al. and Pilyugin), we assume neither a steepen ed initial mass function nor selective galactic winds, since among the ct-particle elements only oxygen shows a large deficit relative to ir on and a similar deficit is also found in Galactic supergiants. Thus w e assume yields and time delays identical to those that we previously assumed for the solar neighbourhood. We include inflow and non-selecti ve galactic winds and consider both smooth and bursting star formation rates, the latter giving a better fit to the age-metallicity relation s. We predict essentially solar abundance ratios for primary elements and these seem to fit most of the data within their substantial scatte r. Our model for the Large Magellanic Cloud also gives a remarkably go od fit to the anomalous Galactic halo stars discovered by Nissen gz Sc huster. Our models predict current ratios of Type Ia supernova to con- collapse supernova rates enhanced by 50 and 25 per cent respectively r elative to the solar neighbourhood, in fair agreement with ratios foun d by Cappellaro et al. for Sdm-Im relative to Sbc galaxies, but these ratios are sensitive to detailed assumptions about the bursts and a st ill higher enhancement in the Large Magellanic Cloud has been deduced from X-ray studies of remnants by Hughes et al. The corresponding rati os integrated over time up to the present are slightly below 1, but th ey exceed 1 if one compares the Magellanic Clouds with the Galaxy at t imes when it had the corresponding metallicities.