H2O masers in a jet-driven bow shock: episodic ejection from a massive young stellar object

We report the results of VERA (VLBI Exploration of Radio Astrometry) multi-epoch VLBI (very long baseline interferometry) 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+0.12-0.11| kpc and the source systemic motion was (μαcos δ, μδ) = (−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 time-scales of ∼1000 yr.