Physics Colloquium
Monday, March 4th, 2005, 4:00 P.M.

Chris DePree

Department of Physics, Agnes Scott College

Shells, Comets and Peanuts: What Ultracompact HII Region Morphologies Tell Us About the Formation of Massive Stars

The first surveys of ultracompact (UC) HII regions indicated that a large fraction of sources had cometary morphologies, and a small number of numerical models were developed to explain how such morphologies might arise. Ideas included density gradients in molecular material (the "champagne outflow model"), and motion of the star and associated ionized material with respect to the host molecular cloud (the "moving star bow shock" model). Subsequent observations of UC HII regions, in particular, Very Large Array imaging of the UC HII regions in the W49A and Sagittarius B2 star forming regions has indicated that in these regions, cometary morphologies are relatively rare, and in fact, shell-like morphologies dominate. We review past and current models for the development of cometary and shell morphologies in UC HII regions, and show current results from high resolution line and continuum observations of UC HII regions and hydrodynamic modeling of the evolution of these sources in high density environments.