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

Abstract

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