H2O masers in a jet-driven bowshock: Episodic ejection from a massive
  young stellar object
  We report the results of VERA multi-epoch VLBI 22 GHz water maser
observations of S255IR-SMA1, a massive young stellar object located in the S255
star forming region. By annual parallax the source distance was measured as D =
1.78 +-0.12 kpc and the source systemic motion was (u alpha cos d, u d) =
(-0.13 +- 0.20, -0.06 +- 0.27) mas yr-1. Masers appear to trace a U-shaped bow
shock whose morphology and proper motions are well reproduced by a jet-driven
outflow model with a jet radius of about 6 AU. The maser data, in the context
of other works in the literature, reveal ejections from S255IR-SMA1 to be
episodic, operating on timescales of ~1000 years.
