Helium Enrichment and Carbon Ionization in the W3 Core

We present high-sensitivity images of the H92α, He92α, and C92α recombination line emission from H II regions in the Galactic radio source W3. Our 8.3 GHz VLA observations have 4'' resolution and sufficient sensitivity to image the carbon recombination line in four of the eight H II regions in the complex. Portions of W3A, the largest and probably most evolved of the H II regions, have a helium abundance as high as 20% by number (nearly twice the cosmic abundance) which confirms previous observations made with lower signal-to-noise ratios. W3A also shows large velocity gradients in the H92α line; we conclude that the gradient is probably caused by the evaporation of the dense molecular core associated with IRS 5. Near the edges of W3B and W3C, strong C+ emission is found where no hydrogen or helium line emission is detected. This C+ emission comes from the interface zone between the H II region and the surrouding molecular cloud. We show that the C+ velocity is more typical of the surrounding molecular cloud while the H+ and He+ velocities trace the ionized gas. A H92α velocity gradient is used as a probe of the gasdynamics in W3B; a model is proposed which explains the gradient. Despite the fact that W3C and W3D are separate H II regions, they apparently share a common C+ region; the C92α line intensity is up to 70% that of the H92α line. Three independent methods provide an electron temperature in the range of Te = 8000-9000 K for W3A; a wider range of values is derived for W3B and W3D. W3B shows signs (10 σ) of a decreasing radial electron temperature gradient.