Abundances of the Heavy Elements in the Magellanic Clouds. III. Interpretation of Results

This paper presents chemical and structural evolutionary models for the Magellanic Clouds assuming bimodal star formation and gas infall. The models are discussed in relation to the observed chemical abundances of the Clouds and are compared with our own Galaxy. The detailed abundances derived from previous work are investigated for any obvious trends with metallicity or differences compared with the Galaxy. Considering all the data, conclusions are drawn on the possible star formation histories of the Magellanic Clouds and the implications for our own Galaxy. The following conclusions were reached in this work. The interstellar medium (ISM) of the LMC has a mean metallicity 0.2 dex lower than the local Galactic ISM, and the metallicity of the SMC is 0.6 dex lower. However, the interstellar media of both the Magellanic Clouds and the Galaxy have significantly nonsolar elemental ratios. This is most evident when considering the lightest and the heaviest of the elements. The s-process appears to have been less effective at forming the heavy neutron- capture elements in the Magellanic Clouds than in the Galaxy. The r-process, on the other hand, appears to have been more effective. Of the light elements, carbon appears to be of normal abundance in the Cloud stars relative to the iron abundance, which is in contrast to the carbon abundances in the H II regions, which appear to be anomalously low. The overall distribution of the lightest elements in the Clouds shows a gradual falling off in the abundances, relative to the Sun, as the atomic number decreases. This is seen not to be the case when the abundances are compared with the local ISM, and suggests that the present ISM in the vicinity of the Sun is material that has fallen in from farther out.