2015MNRAS.453.3163B

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 (µαcosδ, µδ) = (0.79±0.12, -2.41±0.14) mas/yr. 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, vrot = 22±3 km/s, 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.