Multifrequency, High-Resolution Radio Recombination Line Observations of the Massive Star-forming Region W49A

We have carried out a multifrequency, multiconfiguration VLA observational program of the massive Galactic star-forming region W49A (D = 11.4 kpc). Radio continuum observations were carried out at 0.7, 1.3, and 3.6 cm. In the highest sensitivity 3.6 cm images, we have detected 45 distinct continuum sources (located within a ~4' [13 pc] diameter region) exhibiting a wide variety of sizes and morphological types. Several sources have rising continuum spectra (α > 0.6) up to the shortest wavelengths observed (7 mm). We observe a larger fraction of sources with shell morphologies (11%) than were observed in the Galactic plane survey of Wood & Churchwell. A correlation between the rms electron density and the radius of the ultracompact H II regions is observed for the spherical, unresolved, and cometary sources in W49A. We have observed W49A in the H92α, He92α, H66α, and H52α recombination lines. We detect hydrogen line emission from 39 of the 45 continuum sources observed at 3.6 cm (H92α). We derive LTE electron temperatures from the H92α, H66α, and H52α lines to have mean values of T*e=6700±300 K, 10,400 ± 600 K, and 9100 ± 600 K, respectively. The mean singly ionized helium abundance in the 13 sources where He92α emission is detected is ⟨Y+⟩ = 11 ± 1%. Several of the most compact continuum sources in W49A have broad radio recombination lines (ΔVH52α > 45 km s-1), including W49A/B and the western portion of W49A/G (which contains a large number of high-velocity H2O masers). The high-frequency radio recombination line velocities are utilized to discriminate between various models that propose to explain the origin of the central “ring” of H II regions.

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