Hypercompact HII regions

The physical parameters of HII regions span orders of magnitude in scale. The classes most closely linked to star formation are the smallest, densest, and, presumably, youngest stages: compact, ultracompact, and hypercompact HII regions.Although hypercompact HII regions have been known for over ten years, until recently only a very small number of these regions were known. Moreover, it is only in the past several years that these regions have come to be recognized as a distinct class of HII region and that attempts have been made to understand their place in the scheme of massive star formation.Here we present a summary of the observational studies to date. We give special emphasis to radio continuum studies, which indicate density gradients within the ionized gas, and to radio spectral line studies, which show unusually broad recombination line profiles. Possible interpretations of these aspects of hypercompact HII regions are discussed, and their implications for the interpretation of hypercompact HII regions as an evolutionary stage in the high-mass star formation process.