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[53.13] VLA ⁴He Imaging in Galactic HII Regions

D. S. Balser (NRAO), W. M. Goss (NRAO), C. G. De Pree (Agnes Scott College)

Standard Big Bang Nucleosynthesis predicts that greater than 90% of the observed ⁴He in the Galaxy should have been produced during the era of primordial nucleosynthesis. Observations of the primordial ⁴He/H abundance ratio from metal poor galaxies vary from 0.0764 -- 0.0803 by number. Additional production of ⁴He comes from the evolution of stars. ⁴He cannot be directly observed in the photosphere and the best current estimates for the Sun are based on models. Measurements of ⁴He in Galactic HII regions are made via optical and radio recombination lines. Making precise measurements of the ⁴He/H abundance ratio are difficult. One major problem is estimating the fraction of neutral helium within the observed HII region. Current estimates of the present day ⁴He/H abundance ratio range from 0.08 -- 0.10 with no observable gradient in the Galaxy.

High resolution VLA observations have been made of the H92\alpha, He92\alpha, and C92\alpha radio recombination line and continuum emission towards three Galactic HII regions: W43, NGC6334A, and K3-50. Information about the kinematics, electron temperatures, densities, and excitation is determined for each object. The variations of the ⁴He/H abundance ratio are probed across each nebula. The ionization structure and local enhancements of ⁴He are discussed.