2016MNRAS.460.1839C

We measured the trigonometric parallax of the H_2O maser source in the M17 cluster-forming region to be 0.491±0.041 mas, corresponding to a distance of 2.04^{+0.16}_{-0.17} kpc with 8 per cent accuracy. This result is consistent with the Very Long Baseline Array (VLBA) previous parallax measurement of 12.2 GHz CH_3OH masers in M17-UC1. We performed the first measurement of the relative proper motions of the H_2O maser feature 2, and its morphology traces an expanding arcuate shell with a mean motion of ∼ 19.4 km s^-1. The redshifted maser component also shows expanding motion. Fitting a uniformly expanding spherical flow model, we estimate the most probable position and motion of the driving source and we propose that the motion of the masers traces an expanding bubble from a central source, with a dynamical time-scale of ∼ 12.5 yr. This is the first evidence that the maser 2 cluster is driven internally. There is no known counterpart Two-Micron All-Sky Survey (2MASS), Wide-Field Infrared Survey Explorer (WISE) or radio source around this H_2O maser. However, the Spitzer point source catalogue has a source that is spatially coincident with the position of the H_2O masers. This was detected only in the Spitzer 4.5- and 5.8-µm bands (with magnitudes 9.97±0.12 and 7.82±0.10 mag, respectively). The non-detection at other wavelengths could indicate that the source is still at a very early formative stage, and/or very deeply embedded and obscured in its formative environment.