Ultracompact HII Regions: Their Morphologies and Spectral Energy Distributions

Abstract

Ultracompact (UC) HII regions are the sites of recent O and B star formation. The structure of the circumstellar ionized gas is determined by the interaction of stellar gravitation, radiation, particle winds, and magnetic fields with the local interstellar medium. Several UC HII regions have been mapped with high spatial resolution at radio wavelengths by Felli, Johnston, and Churchwell (1981), Colley (1980), Dreher and Welch (1981), Ho and Haschick (1981), Dreher et al. (1984), Felli, Churchwell, and Massi (1984), Turner and Matthews (1984), Garay, Moran, and Reid (1985), and Garay, Rodriguez, and van Gorkom (1986). These observations show that UC HII regions have emission measures > 107 pc cm−6 and diameters which range from ~ 0.05 to 0.2 pc. Their radio brightness distributions are typically that of a shell or arc with very steep gradients along their outer boundaries, indicating that they are generally ionization bounded by dense molecular gas and dust. They are frequently associated with older, more extended, and less dense HII regions. In several cases they appear to be located near H+ — H2 interface zones formed by a diffuse HII region and a large molecular cloud (Felli, Churchwell, and Massi 1984). Many, if not all, are deeply embedded in molecular clouds, so they have been observed only in the infrared and radio wavelength domains.