Green Bank Telescope Observations of 3He+: H II Regions

During the era of primordial nucleosynthesis, the light elements {}²{{H}}, {}³{He}, {}⁴{He}, and {}⁷{Li} were produced in significant amounts, and these abundances have since been modified primarily by stars. Observations of {}³{He}}⁺ in H II regions located throughout the Milky Way disk reveal very little variation in the {}³{He}/{{H}} abundance ratio—the “{}³{He} Plateau”—indicating that the net effect of {}³{He} production in stars is negligible. This is in contrast to much higher {}³{He}/{{H}} abundance ratios found in some planetary nebulae. This discrepancy is known as the “{}³{He} Problem”. Stellar evolution models that include thermohaline mixing can resolve the {}³{He} Problem by drastically reducing the net {}³{He} production in most stars. These models predict a small negative {}³{He}/{{H}} abundance gradient across the Galactic disk. Here we use the Green Bank Telescope to observe {}³{He}}⁺ in five H II regions with high accuracy to confirm the predictions of stellar and Galactic chemical evolution models that include thermohaline mixing. We detect {}³{He}}⁺ in all the sources and derive the {}³{He}}⁺/{{{H}}}⁺ abundance ratio using model H II regions and the numerical radiative transfer code NEBULA. The over 35 radio recombination lines (RRLs) that are simultaneously observed, together with the {}³{He}}⁺ transition provide stringent constraints for these models. We apply an ionization correction using observations of {}⁴{He} RRLs. We determine a {}³{He}/{{H}} abundance gradient as a function of Galactocentric radius of -(0.116 ± 0.022) × {10}^-5 kpc^-1, consistent with stellar evolution models including thermohaline mixing that predict a small net contribution of {}³{He} from solar mass stars.