VLA high-sensitivity 4He imaging of Galactic HII regions

We have carried out 8.3 GHz hydrogen and helium radio recombination line and continuum observations of three Galactic H II regions with the NRAO 140 Foot Telescope and the Very Large Array. These observations measure the spatial variations in the singly ionized helium-to-hydrogen abundance ratio, He-4(+)/H+ (y(+)), and probe for any localized enhancements in y(+). The three observed regions (W43, NGC 6334A, and K3-50) are morphologically complex, and high-resolution, wide field observations are crucial to measure the helium abundances on scales that would be associated with individual massive stars or clusters of stars. The nearly constant y(+) value across the W43 source ([y(+)](W43) = 7.7% +/- 1%) indicates that any ionization correction over the spatial scales investigated is small. Such ionization corrections are important for calculating total abundances of the light element He-3, which is measured in the Galaxy via a singly ionized hyperfine transition. Localized enhancements in the y(+) value have been detected in both NGC 6334A (y(+) = 20% +/- 4%) and K3-50A (y(+) = 24% +/- 5%). The He-4 abundance results are discussed along with other measurements of the physical properties of these H II regions, such as the kinematics and excitation.