The evolution of massive bipolar jets and the birth of HII regions.

Observations suggest that outflows of neutral gas are among the first visible signs of the birth of a new star. Stars destined to be of spectral type O or B on the main sequence eventually emit large quantities of ionizing ultra-violet radiation which produce HII regions. The powerful neutral wind associated with the bipolar outflow phase of pre-main sequence stellar evolution produces cavities in the gas surrounding a newborn star and regulates the escape of ionizing radiation from the vicinity of the star. The birth and early evolution of HII regions is the final phase in the development of bipolar, molecular outflows surrounding massive stars.