2018AJ....156..280B

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