Green Bank Telescope Observations of ³He⁺: H II Regions
Files
First author draft
Date
2018-12
Authors
Balser, Dana S.
Bania, Thomas M.
Version
First author draft
OA Version
Citation
D.S. Balser, T.M. Bania. 2018. "Green Bank Telescope Observations of 3-He+: H II Regions" The Astronomical Journal, Volume 156, Issue 6, pp.280-280. https://doi.org/10.3847/1538-3881/aaeb2b
Abstract
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⁺ 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) × 10⁻⁵ kpc⁻¹, consistent with stellar evolution models including thermohaline mixing that predict a small net contribution of ^3He from solar mass stars.