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S. Kurtz (Univ. Nacional Autonoma de Mexico), P. Carral (Univ. de Guanajuato), P. Hofner (Univ. of Puerto Rico and Arecibo Observatory), D. S. Shepherd (California Institute of Technology), L.F. Rodriguez (Univ. Nacional Autonoma de Mexico)
We present molecular line, millimeter recombination line, and radio continuum observations of two massive star forming regions made with the OVRO Millimeter Array and the NRAO Very Large Array.
The ultracompact HII regions G12.21-0.10 and G75.78-0.34 show evidence for nearby warm, high density molecular gas and very high density ionized gas, respectively. In both cases the highest density gas is coincident with water maser emission. Such high density gas may indicate the presence of hot molecular cores or ``super ultracompact'' HII regions. We discuss the results in terms of the hot core and ultracompact HII region phases of massive star formation and consider the possible transition from one phase to the other.