Ultracompact HII regions in W49N at 500 AU scales: Shells, winds, and the water maser source

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 0." 045 to 0." 35 (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 ail 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 alpha = 0.3-1.1 and three are observed to be flat (S-nu proportional to nu(alpha)). Those sources with rising spectral indices (A, B1, B2, G1, and G2) also have the broadest radio recombination lines, with Delta V-FWHM > 45 km s(-1) in the H66 alpha line (De Free, 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 0." 05. 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.