MULTIFREQUENCY, HIGH-RESOLUTION RADIO RECOMBINATION LINE OBSERVATIONSOF THE MASSIVE STAR-FORMING REGION W49A

We have carried out a multifrequency, multiconfiguration VLA observati onal 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 similar to 4' [13 pc] di ameter region) exhibiting a wide variety of sizes and morphological ty pes. Several sources have rising continuum spectra (alpha > 0.6) up to the shortest wavelengths observed (7 mm). We observe a larger fractio n of sources with shell morphologies (11%) than were observed in the G alactic plane survey of Wood & Churchwell. A correlation between the r ms 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 H92 alpha, He92 alpha, H66 alpha, and H5 2 alpha recombination lines. We detect hydrogen line emission from 39 of the 45 continuum sources observed at 3.6 cm (H92 alpha). We derive LTE electron temperatures from the H92 alpha, H66 alpha, and H52 alpha 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 abun dance in the 13 sources where He92 alpha emission is detected is [Y+] = 11 +/- 1%. Several of the most compact continuum sources in W49A hav e broad radio recombination lines (Delta V-H5.2 alpha > 45 km s(-1)), including W49A/B and the western portion of W49A/G (which contains a l arge number of high-velocity H2O masers). The high-frequency radio rec ombination line velocities are utilized to discriminate between variou s models that propose to explain the origin of the central ''ring'' of H II regions.