Hypercompact HII Regions: A New Stage of O-star Evolution?

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AST 0303689PI ChurchwellRecent radio observations of massive star formation regions have discovered what appears to be a new class of very compact regions of ionized hydrogen (HII) gas that have been dubbed "hypercompact HII" regions. They have several common properties that distinguish them from ultracompact HII regions and other thermal radio sources. For example they have electron densities ~100 times larger and diameters ~10 times smaller than typical ultracompact HII regions. In addition, they appear to occur in pairs or multiple systems and are located in massive star formation regions. The Principle Investigator speculates that hypercompact regions may represent the transition from the rapid accretion phase of massive protostars in hot molecular cores to the stage where accretion has terminated and an easily detectable ionized hydrogen region has been formed. In this project the PI is characterizing the properties of these hypercompact regions to determine their niche in the early evolution of massive protostars by increasing the sample size, measuring their properties, and also determining the apparent binary frequency and projected separations from high resolution Very Large Array radio continuum images. From new data plus models, the PI is obtaining the structure (morphology, range of diameters), physical properties (velocity structure and distributions of electron density and temperature), and state of evolution of hypercompact regions and hence developing a deeper understanding of massive star formation.Broader Impacts: This project is the PhD thesis topic of a member of an under-represented group in science. Some undergraduates receive their first experience with basic research from first-hand observing experience and data reduction and analysis methods by working on this project. Numerous popular talks are given each year to school groups and service groups. Benefits to society are a generally better understanding of how science works and specifically how stars are made.