Systematic Variation of the Helium Ionization in Galactic HII Regions

Abstract

The existence of a gradient of the ionized helium abundance as a function of the galactic radius was first noted by Churchwell et al. (1974). This result has recently been confirmed by the much more complete survey of giant HII Regions presented by Churchwell et al. (1978). In detail, the ratio He⁺ /H⁺ is found to increase from ~0.02 near the galactic center passing through ~0.06 at 5 Kpc to reach a maximum of ~0.09 near 9 Kpc and then to decrease for larger radii. This behavior can be interpreted as reflecting a general decrease of the He abundance with the galactic radius (similar to that also found for planetary nebulae; Torres-Peimbert and Peimbert, 1977) plus a reduction of the ionization in the inner parts of the Galaxy. The crucial aspect is that the observations