ULTRACOMPACT H II REGIONS IN W49N AT 500 AU SCALES: SHELLS, WINDS, AND THE WATER MASER SOURCE

摘要

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 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.l and three are observed to be flat (S_v∝ v~α). Those sources with rising spectral indices (A, B1, B2, G1, and G2) also have the broadest radio recombination lines, with ΔV_fwhm > 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 0"05. The maser positions are closely associated with the Gl/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 contin- uum source in the region, but it remains unclear whether this source drives the outflow.