The Search for Primordial3He Using the 8.665 GHz Hyperfine Spin‐Flip Transition of3He+

Abstract

Frequency-switched observations of the 8.665 GHz hyperfine spin-flip transition of 3He+ have been made in eight Galactic H II regions in both the inner and outer Galaxy. For sources in the outer Galaxy, the observations reported here show no evidence for 3He+ to levels well below the level of detections claimed by Bania et al. using the position-switching technique. There is strong evidence that weak terrestrial interference is the explanation for the variability and relatively high level of 3He+ previously reported in several outer Galaxy sources. In the inner Galaxy, 3He+ is detected in W43, M17S, and W51 at abundance levels similar to those reported by Balser et al. with 3He+/H+~ (1-2) × 10-5. Contrary to what has been reported previously, the 3He+/H+ abundances are found to decrease rapidly with increasing distance from the Galactic center and fall below detectable levels by the Galactic radius of W49 (~8 kpc). This fall-off in second-generation 3He abundance with distance from the Galactic center is similar to what is predicted by the Pd = 0 model of Galli et al. but is at least a factor of 5 underabundant. The upper limit to the antenna temperatures obtained here for the 3He+ lines in Orion A, W49, and S209 gives an upper limit for the abundance of primordial 3He that is an order of magnitude below the upper limit suggested by Bania et al. This leads to a lower limit for the nucleon-to-photon ratio η that is at least an order of magnitude above the narrow limits set by 7Li and deuterium. Possible explanations for this are that there is a larger than expected uncertainty in determining 3He abundances or that some mechanism is destroying primordial 3He in the Galaxy over a Hubble time.