"Ultracompact HII Regions and the Origin of Broad Line Sources"

The Galactic survey of Wood & Churchwell (1989) was the first attempt to classify the morphologies and physical parameters of a large number of ultracompact (UC) HII regions. Additional "snapshot" Very Large Array (VLA) surveys were published in subsequent years, and the apparent prevalence of the "cometary" morphology motivated attempts to understand how cometary sources might arise and evolve. Complementary studies that made deep, multi-configuration VLA images of single sources were difficult to compare with the survey work, since the single-source studies did not produce sufficient numbers of sources. Multi-configuration, multi-frequency studies of the crowded Sgr B2 and W49A regions in the past decade have now provided a large number of sources whose morphologies and physical parameters can be compared with the survey work. I will review these two observational approaches, and highlight recent developments in the study of UC HII regions, including the discovery of broad recombination line sources, sources with rising continuum spectra, and hydrodynamic modeling of source evolution. As a recent example of observational work, I will highlight results from high-resolution VLA observations of line and continuum 7 mm emission in the Sgr B2 Main region.