The chemical composition of galactic and extragalactic H II regions

Abstract

The results of a spectrophotometric survey of 15 galactic H II regions are presented. From the observed line intensities in the spectral region lambdalambda3700--7100, electron densities, temperatures, and abundances of helium, oxygen, nitrogen, sulfur, and neon are computed. The average helium abundance in galactic H II regions agrees with the average helium abundance in galactic planetary nebulae. The oxygen and nitrogen abundances are found to correlate with the distance of the H II regions from the galactic center in much the same manner as has been proposed previously for other galaxies. The magnitude of the oxygen gradient agrees with that found from planetary nebulae and with the metal grdient found from intermediate age stars. The average abundances in this sample agree well with published abundances in the Orion Nebula.New observations of H II regions in M101 allow the estimate of abundance gradients and a comparison with abundance gradients in the Galaxy. Generally, the magnitudes of the various gradients agree, although there is some evidence for a steeper nitrogen gradient in the Galaxy. The absolute abundances in H II regions in M101 are probably not different from abundances in the galactic H II regions in this sample.