EVOLUTION OF THE WATER MASER EXPANDING SHELL IN W75N VLA 2

We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H[sub 2]O 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 H[sub 2]O maser emission toward the radio jet in VLA 1 and the expanding shell-like structure in VLA 2. The spatial distribution of the H[sub 2]O 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. This suggests that we are probably observing the formation of a jet-driven H[sub 2]O 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). We discuss possible implications of our results in the study of the first stages of evolution of massive YSOs.