3He in the Milky Way Interstellar Medium: Abundance Determinations

We are making precise determinations of the abundance of the light isotope of helium, ³He. The ³He abundance in Milky Way sources provides strong constraints for theories of stellar evolution, chemical evolution, and cosmology. Here we report on new ³He⁺/H⁺ abundance determinations for a sample of 21 Galactic H II regions observed for the past 14 yr with the NRAO 140 Foot telescope. The abundances are based on improved ³He⁺ observations, as well as new models for source density structure. The models are constrained by the observed ³He⁺ emission, the strength of the H and ⁴He recombination lines, and the continuum emission at 3.5 cm wavelength. Density structure serves only to increase the actual ³He⁺/H⁺ abundance ratio over that derived using a uniform density sphere assumption. The density structure models allow one to identify sources that are ``simple'' in that their structure corrections are negligible. These objects provide a pool of H II regions that can in principle yield accurate ³He⁺/H⁺ abundances. For the seven simple H II regions in our sample there is no evidence for any systematic trend of the ³He⁺/H⁺ abundance with any other parameter. For these sources, we derive an average abundance ratio by number of ³He⁺/H⁺=(1.6+/-0.5)×10^-5 (statistical error).