A survey of water maser emission toward ultracompact HII regions

Abstract

In this paper we present high resolution images and spectra toward 21 H2O maser sources in the vicinity of ultracompact (UC) HII regions. This survey provides the basis for future studies with milli-arcsecond resolution, utilizing very long baseline interferometry (VLBI) techniques. Emission from the 616-523 masing transition of interstellar H2O 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 0.1 pc 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. A comparison with maps in the NH3 inversion transitions shows that in at least 7 cases, the water masers are associated with hot (T > 100 K), dense (n(H2) ≈ 107 cm-3) molecular clumps. For the UC HII regions G5.89-0.38 and G45.07+0.13 we find spatial and velocity correspondence between water masers and outflowing molecular gas. It is thus likely, that for these sources the H2O masers are taking part in the bipolar outflow.