A survey of chemical compositions of H II regions in the Magellanic Clouds

Relative line intensities have been measured for six H II regions in the LMC and 17 in the SMC using the IDS and IPCS on the Anglo-Australian Telescope. The results, together with similar measurements by other authors, are analysed to derive electron temperatures and densities and abundances of nitrogen, oxygen, neon, sulphur, chlorine and argon. The resulting abundances of oxygen, nitrogen and neon agree with those previously derived by other authors; the present work extends these results to additional H II regions, especially in the SMC, where oxygen is deficient by a factor of 4 relative to the Orion Nebula, while nitrogen is deficient relative to oxygen by a factor of about 4 in both Clouds. Sulphur, chlorine and argon have the same (approximately solar) abundances relative to oxygen as in Orion. A spatial abundance gradient, analogous to those found in spiral galaxies, is small or absent in the Large Cloud and is conspicuously absent in the Small Cloud. Average abundances in the two Clouds are similar to those found in M33 and M101 in zones having the same ratio of mass (or surface density) of gas relative to total mass; both are consistent with the simple galactic-enrichment model provided that the yield is small, about 0.003, if temperature fluctuations are assumed to be negligible. (author)