Ultracompact HiiRegions in W49N at 500 AU Scales: Shells, Winds, and the Water Maser Source

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Abstract
We have observed the ionized gas in the star-forming region W49N with the National Radio Astronomy Observatory Very Large Array (VLA) at 13 mm and 7 mm, and with the Berkeley Illinois Maryland Association (BIMA) Array at 3.3 mm. These observations vary in resolution from 0045 to 035 (500 AU to 4000 AU at a distance of 11.4 kpc). In addition, we have used the VLA to observe water maser emission towards the bright W49N:G sources over a wide velocity range from -435 to 435 km s-1. The high-resolution continuum observations reveal the morphologies in the ultracompact sources; most of the sources at 0.045'' resolution appear to have shell or ring morphologies. The 3.3 mm emission observed with the BIMA array is dominated by free-free emission in all of the compact sources. There is no evidence for any spectral breaks corresponding to the emergence of a dust component. Of the seven bright sources in W49N for which multifrequency flux densities have been measured, four are observed to have rising spectral indices, with values ranging from α = 0.3-1.1 and three are observed to be flat (Sν ∝ να). Those sources with rising spectral indices (A, B1, B2, G1, and G2) also have the broadest radio recombination lines, with ΔVFWHM > 45 km s-1 in the H66α line (De Pree, Mehringer, & Goss). High-resolution 1.3 cm continuum images made at the same time as the water maser observations have been used to align the maser positions with the high-resolution 7 mm continuum to within 005. The maser positions are closely associated with the G1/G2 sources. The outflow traced by the water masers (Gwinn, Moran & Reid) appears to be centered within 0.2'' of the G2 peak, the brightest continuum source in the region, but it remains unclear whether this source drives the outflow.