Water maser features in ultracompact HII regions

This survey provides the basis for future studies with milli-arcsecond resolution, utilizing very long baseline interferometry (VLBI) techniques. Emission from the 6_16_-5_23_ masing transition of interstellar H_2_O is observed in the close vicinity of UC HII regions with a median angular distance of 2.9" and a median linear projected distance of 0.1pc from the continuum peak.

We find that for UC HII regions with cometary morphology the water maser emission is located in front of the cometary arc whereas for non-cometary UC HII regions the water masers are often observed projected onto the contours of the ionized gas. Due to the large median distance of the water masers from the I-front of the UC HII region, it is unlikely, that the water masers are formed in the shocked layer of warm molecular gas in the interface between the ionized gas of the UC HII region and surrounding molecular gas which is predicted by the Bow Shock theory of UC HII regions.