Ionization correction factors and compositions of HII regions.

The determination of ionization correction factors (ICFs) is discussed by means of a grid of new photoionization models for H II regions, mainly using the plane-parallel model stellar atmospheres of Kurucz (1979). The ionic abundances of the models are integrated either over the nebular volume or along a radius. The observations of the Orion Nebula do not lie along a single atmospheric track, which is interpreted as the effects of good spatial resolution. Observations of less well-resolved nebulae (30 Dor and NGC 604) show fairly good agreement with a Kurucz T(asterisk 3) = 42 track. Observations of NGC 5471 in M101 also suggest a track that is 'hotter' than any Kurucz atmosphere. The chemical compositions of H II regions are compatible with those found in unevolved stars.