* Astronomy

Title: Evolution of the Water Maser Expanding Shell in W75N VLA 2
Authors: Jeong-Sook Kim (1,2), Soon-Wook Kim (2,), Tomoharu Kurayama (3,4), Mareki Honma (5), Tesuo Sasao (6), Gabriele Surcis (7), Jorge Canto (8), Jose M. Torrelles (9), Sang Joon Kim (1) ((1) Kyunghee University, (2) Korea Astronomy and Space Science Institute, (3) Kagoshima University, (4) Teikyo University of Science, (5) National Astronomical Observatory of Japan, (6) Yaeyama Star Club, (7) Joint Institute for VLBI in Europe, (8) Instituto de Astronomia (UNAM), (9) Instituto de Ciencias del Espacio (CSIC)-UB/IEEC, Universitat de Barcelona)

We present Very Long Baseline Interferometry (VLBI) observations of 22 GHz H_2O 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_2O 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_2O 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 ±1 mas), 2005 (\simeq 97 ±3 mas), and 2007 (\simeq 111 ±1 mas), we estimate an average expanding velocity of ~ 5 mas yr^{-1}, similar to the proper motions measured in the individual H_2O 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 between the minor and major axes of ~ 0.6, in contrast with a most 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 H_2O 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.

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