Ultra-compact HII regions embedded in infrared sources

The formation of an ionized region and its subsequent interaction with surrounding neutral material can occur in either the very early or very late stages of stellar evolution. Both newborn and old stars are often surrounded by thick dust clouds and the interaction of a newly formed compact HII region with this circumstellar material can take place either: (1) in a site of star formation where a compact HII region develops in a dark molecular cloud or (2) when a star is in transition from a red-giant to a planetary nebula and an ionized nebular shell interacts with the remnant circumstellar envelope of the red-giant progenitor. Compact HII regions embedded in dense neutral clouds can be most effectively probed by observing: (1) radio free-free emission and (2) infrared recombination line emission. (Auth.)