Evolution of the Water Maser Expanding Shell in W75N VLA 2
  We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz
H$_2$O masers in the high-mass star-forming region of \objectname{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. 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.
