Kinematics of H2O masers in high-mass star forming regions

Abstract

We have conducted multi-epoch EVN observations of the 22.2 GHz water masers towards four high-mass star forming regions (Sh 2-255 IR, IRAS 23139+5939, WB89-234, and OMC2). The (three) observing epochs span a time range of 6 months. In each region, the water maser emission likely originates close (within a few hundreds of AU) to a forming high-mass YSO. Several maser features (~10) have been detected for each source and, for those features persistent over the three epochs, proper motions have been derived. The amplitudes of the proper motions are found to be larger than the range of variation of the line-of-sight velocities and in each of the observed sources the proper motion orientation seems to indicate an expansion motion. The gas kinematics traced by the 22.2 GHz water masers is compatible with the shock-excited nature of water maser emission. Three different kinematic models (a spherical expanding shell, a Keplerian rotating disk, and a conical outflow) were fitted to the 3-dimensional velocity field of the detected maser features. The results of these fits, together with the comparison of the VLBI maps with the highest-resolution images of the sources in several thermal tracers, suggest that the water maser features are most likely tracing the inner portion of the molecular outflows detected at much larger-scales.