Ultracompact HII regions associated with massive star formation

This dissertation establishes the physical properties of ultracompact (UC) H sc II regions and investigates their interaction with the interstellar medium. UC H sc II regions are small photoionized nebulae produced by massive stars embedded in clouds of molecular gas and dust. New observations have been made using the Very Large Array (VLA) at cm wavelengths and single dish telescopes at mm wavelengths. Data has also been collected from the Infrared Astronomical Satellite (IRAS) and from the literature. It is shown that UC H sc II regions have electron densities ;The VLA was used at 2 and 6 cm to identify 75 UC H sc II regions. Radio continuum brightness distributions with 0.4;At high angular resolution five different morphologies were seen: spherical or unresolved (43%), cometary (20%), core-halo (16%), shell (4%) and irregular or multiply peaked (17%). Cometary UC H sc II regions may be produced by the bow shock of a star moving supersonically through the molecular gas. The total number of UC H sc II regions found is inconsistent with a UC H sc II region lifetime of ;It is shown that UC H sc II regions have a characteristic FIR flux density distribution that is very different from typical entries in the IRAS Point Source Catalog. A two-color selection criterion based on the known UC H sc II regions is developed and when applied to the entire PSC a total of 1708 embedded OB star candidates are found. This represents a direct measure of the population and distribution of galactic OB stars embedded in molecular clouds. A comparison with optically visible O-stars in the solar neighborhood shows that 10 to 20% of all O-stars are embedded in molecular clouds, implying that 10 to 20% of a massive star's main sequence lifetime is spent inside a molecular cloud. The current rate of massive star formation in the Galaxy is...