Evidence supporting the kinematic interpretation of water maser proper motions

We have analyzed multi-epoch 22 GHz water maser observations performed with the Very Long Baseline Array (VLBA) towards the high-mass star forming region (SFR) G24.78+0.08. The spatial structure of the water maser integrated intensity has been obtained at three different epochs and found to maintain a very remarkable persistent morphology over the three epochs. Evidence of systematic (expanding) motions for the whole structure traced by the maser emission is also reported. In addition, we have obtained, from previously reported data, the integrated emission of a cluster of water masers spread over $\simeq$ 10 mas within the expanding shell of $\simeq$0.16'' size around the young stellar object W75 N (B)-VLA 2. As in G24.78+0.8, we also find that the morphology of the integrated intensity of the water masers of this cluster persists over different observed epochs. These results strongly support the interpretation that the measured proper motions of the water masers are due to real physical motions of distinct blobs of maser-emitting gas, rather than to other effects, such as a travelling background illuminating wave or turbulence in the circumstellar medium. This result is crucial in astrophysical applications of maser proper-motion measurements, including distance determinations and studies of circumstellar gas kinematics in SFRs and late-type stars.