Chemical composition of Galactic H II regions.

A review of the chemical compositions of M8, M17 and M42 (Orion) is presented. Observational evidence on the presence of spatial temperature variations inside H II regions is presented; these variations can be explained by photoionization models of nebulae with uniform distributions of dust and chemical composition. Possible causes of these temperature variations are analyzed. Abundance tables with probable errors (1σ) for Orion, M8 and M17 are presented. These abundances are compared with those of the Sun and B stars of the solar neighborhood. The abundances of Galactic H II regions are compared with those of extragalactic H II regions; the enrichment of the carbon abundance and the ΔY/ΔZ value are briefly discussed.