We report on annual parallax and proper motion observations of H _ { 2 } O masers in S235AB-MIR , which is a massive young stellar object in the Perseus Arm .
Using multi-epoch VLBI astrometry we measured a parallax of \pi = 0.63 \pm 0.03 mas , corresponding to a trigonometric distance of D = 1.56 ^ { +0.09 } _ { -0.08 } kpc , and source proper motion of ( \mu _ { \alpha } \cos \delta , \mu _ { \delta } ) = ( 0.79 \pm 0.12 , -2.41 \pm 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 _ { \mathrm { out } } = 45 \pm 2 km s ^ { -1 } , v _ { \mathrm { rot } } = 22 \pm 3 km s ^ { -1 } , i = 12 \pm 2 ^ { \circ } , 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 jet rotation .
During our 15 month VLBI programme there were three ‘ maser burst ’ events caught in the act which were caused by an overlapping of masers along the line of sight .