Optical emission from shock waves. III. Abundances in supernova remnants.

The diagnostic techniques described by Dopita have been applied to observations of many supernova remnants in our Oalaxy and the Magellanic Clouds to yield abundances of oxygen, nitrogen, and sulfur. A strong gradient in chemical evolution is found, increasing from the SMC through the LMC to the Galaxy, consistent with theories of enrichment. An upper limit of 4 x 10- is placed on the pregalactic oxygen abundance, and no strong evidence is found to indicate galactic evolutionary dependence of enrichment processes. Evidence is presented to indicate that two objects observed could be fragments of the original supernova preferentially thrown off in a certain direction during the explosive event. Subject headings: galaxies: Magellanic Clouds - nebulae: abundances - nebulae: supernova remnants - shock waves