A ‘water spout’ maser jet in S235AB-MIR

We report on annual parallax and proper motion observations of H2O masers in S235AB-MIR, which is a massive young stellar object in the Perseus Arm. Using multi-epoch VLBI (very long baseline interferometry) astrometry we measured a parallax of π = 0.63 ± 0.03 mas, corresponding to a trigonometric distance of |D= 1.56+0.09-0.08| kpc, and source proper motion of (μαcos δ, μδ) = (0.79 ± 0.12, −2.41 ± 0.14) mas yr−1. Water masers trace a jet of diameter 15 au which exhibits a definite radial velocity gradient perpendicular to its axis. 3D maser kinematics were well modelled by a rotating cylinder with physical parameters: vout = 45 ± 2 km s−1, vrot = 22 ± 3 km s−1, i = 12° ± 2°, which are the outflow velocity, tangential rotation velocity and line-of-sight inclination, respectively. One maser feature exhibited steady acceleration which may be related to the jet rotation. During our 15-month VLBI programme there were three ‘maser burst’ events caught ‘in the act’ which were caused by the overlapping of masers along the line of sight.