Hot Cores in W49N and the Timescale for Hot Core Evolution

We present subarcsecond resolution observations of the star-forming region W49N made with the Berkeley-Illinois-Maryland Association array at 1.4 mm wavelength in the continuum and CH 3 CN J = 12-11 lines. The continuum image, at a resolution of 0 18 (2000 AU), shows many of the previously identified ultracompact H II regions and at least one new source whose steep spectrum to short wavelengths indicates dust emission. This compact dust concentration also shows the strongest CH 3 CN emission in the region. Additional peaks of CH 3 CN emission likely mark hot cores produced by other deeply embedded young massive stars that may be precursors to O-type stars like those that power the ultracompact H II regions. The number ratio of hot cores to ultracompact H II regions found in W49N provides a measure of the relative timescales of these early evolutionary phases. Although the samples are small, the source counts suggest that the hot core lifetime is most likely shorter than, but comparable to, the ultracompact H II region lifetime.