EVN observations of H2O masers towards the high-mass young stellar object in AFGL 5142

Abstract

We have conducted multi-epoch EVN observations of the 22.2 GHz water masers towards the high-mass young stellar object in AFGL 5142. With four observing epochs, spanning a time of ~1 year, 12 distinct maser features have been detected, 7 of which were detected in more than one epoch. The positions and velocities of the VLBI features agree well with those of the emission centers previously identified by means of VLA observations. For a few features, persistent over three or four epochs, accurate values of the proper motions are derived. The observed proper motions have an amplitude of 15–20 km s-1, significantly larger than the range of variation of the line-of-sight velocities (±4 km s-1 around the systemic velocity). On the basis of their spatial distribution, the observed maser features can be divided into two groups. A model fit to the positions and velocities of the maser features of Group I, detected in the same region (within ~500 mas) where the massive YSO should be located, demonstrates that these might arise on the surface of a nearly edge-on Keplerian disk, rotating around a massive young stellar object. The maser features of Group II, found at large distances from the YSO (≥1''), have positions and line-of-sight velocities in agreement with the blue-shifted lobe of a large-scale molecular outflow (traced by the HCO+ and SiO emission), and might result from the interaction between the gas flowing away from the young stellar object and the ambient gas of the progenitor molecular core.