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 pi = 0.63 +/- 0.03 mas, corresponding to a trigonometric distance of D = 1.56(-0.08)(+0.09) kpc, and source proper motion of (mu(alpha)cos delta, mu(delta)) = (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: v(out) = 45 +/- 2 km s(-1), v(rot) = 22 +/- 3 km s(-1), i = 12 degrees +/- 2 degrees, 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.