We present Very Long Baseline Interferometry ( VLBI ) observations of 22 GHz H _ { 2 } O masers in the high-mass star-forming region of ( catalog W75N ) , carried out with VLBI Exploration of Radio Astrometry ( VERA ) for three-epochs in 2007 with an angular resolution of \sim 1 mas .
We detected H _ { 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 _ { 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 .
We have made elliptical fits to the VLA 2 H _ { 2 } O 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 the epochs 1999 ( \simeq 71 \pm 1 mas ) , 2005 ( \simeq 97 \pm 3 mas ) , and 2007 ( \simeq 111 \pm 1 mas ) , we estimate an average expanding velocity of \sim 5 mas yr ^ { -1 } , similar to the proper motions measured in the individual H _ { 2 } O maser features .
A kinematic age of \sim 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 between the minor and major axes of \sim 0.6 , in contrast with a most circular shape for the shell detected in 1999 and 2005 ( b/a \sim 0.9 ) .
This suggests that we are probably observing the formation of a jet-driven H _ { 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 ) .
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 .