The Partially Ionized Medium Adjacent to HII Regions
Recombination line emission from a partially ionized medium has been detected toward approximately fifteen galactic HII regions. The carbon line is the strongest and is the best studied, but lines from elements heavier than carbon and from hydrogen have also been observed. Therefore, I want to include in my review the characteristics of the more general partially ionized medium (PIM) which is adjacent to HII regions, and I will concentrate on developments after about 1974 when Dupree (1974) published a compilation of carbon line data and presented an interpretation. The components of the PIM are not only the C+ region, but also what I will term Z+ regions where elements heavier than carbon are ionized, and what I will call an H° region where hydrogen is partially ionized (i.e. relative ionization of 10-4 - 10-3). Many properties are shared by the three types of region, and there is probably some spatial coincidence.
KeywordsMolecular Cloud Recombination Line Hydrogen Line Ionization Front Carbon Line
Unable to display preview. Download preview PDF.