Evolution of the Water Maser Expanding Shell in W75N VLA 2

We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H2O masers in the high-mass star-forming region of W75N, carried out with VLBI Exploration of Radio Astrometry (VERA) for three epochs in 2007 with an angular resolution of ~1 mas. We detected H2O maser emission toward the radio jet in VLA 1 and the expanding shell-like structure in VLA 2. The spatial distribution of the H2O masers detected with VERA and measured proper motions around VLA 1 and VLA 2 are similar to those found with previous VLBI observations in epochs 1999 and 2005, with the masers in VLA 1 mainly distributed along a linear structure parallel to the radio jet and, on the other hand, forming a shell-like structure around VLA 2. We have made elliptical fits to the VLA 2 H2O maser shell-like structure observed in the different epochs (1999, 2005, and 2007), and found that the shell is still expanding eight years after its discovery. From the difference in the size of the semi-major axes of the fitted ellipses in epochs 1999 (71 ± 1 mas), 2005 (97 ± 3 mas), and 2007 (111 ± 1 mas), we estimate an average expanding velocity of ~5 mas yr–1, similar to the proper motions measured in the individual H2O maser features. A kinematic age of ~20 yr is derived for this structure. In addition, our VERA observations indicate an increase in the ellipticity of the expanding shell around VLA 2 from epochs 1999 to 2007. In fact, the elliptical fit of the VERA data shows a ratio of the minor and major axes of ~0.6, in contrast with an almost circular shape for the shell detected in 1999 and 2005 (b/a ~ 0.9). This suggests that we are probably observing the formation of a jet-driven H2O maser structure in VLA2, evolving from a non-collimated pulsed-outflow event during the first stages of evolution of a massive young stellar object (YSO). This may support predictions made earlier by other authors on this issue, consistent with recent magnetohydrodynamical simulations. We discuss possible implications of our results in the study of the first stages of evolution of massive YSOs.