2008MNRAS.390..523M

We have made multi-epoch very long baseline interferometer (VLBI) observations of H₂ O maser emission in the massive star-forming region IRAS 06061+2151 with the Japanese VLBI network (JVN) from 2005 May to 2007 October. The detected maser features are distributed within a 1x1 arcsec² area (2000x2000 au² at the source position) around the ultracompact HII region seen in radio continuum emission. The bipolar morphology and expanding motion traced through their relative proper motions indicate that they are excited by an energetic bipolar outflow. Our three-dimensional model fitting has shown that the maser kinematical structure in IRAS 06061+2151 can be explained by a biconical outflow with a large opening angle (>50°). The position angle of the flow major axis coincides very well with that of the large-scale jet seen in 2.1 µm hydrogen emission. This maser geometry indicates the existence of dual structures composed of a collimated jet and a less collimated massive molecular flow. We have also detected a large velocity gradient in the southern maser group. This can be explained by a very small (on a scale of several tens of astronomical units) and clumpy (the density contrast by an order of magnitude or more) structure of the parental cloud. Such a structure may be formed by strong instability of the shock front or splitting of the high density core.