2008MNRAS.390.1127U

We have studied the kinematics traced by the water masers located at the centre of the planetary nebula (PN) K3-35, using data from previous Very Large Array (VLA) observations. An analysis of the spatial distribution and line-of-sight velocities of the maser spots allows us to identify typical patterns of a rotating and expanding ring in the position-velocity diagrams, according to our kinematical model. We find that the distribution of the masers is compatible with tracing a circular ring with a ≃0.021-arcsec (≃100-au) radius, observed with an inclination angle of 55° with respect to the line of sight. We derive expansion and rotation velocities of 1.4 and 3.1km/s, respectively. The orientation of the ring, projected on the plane of the sky, at positionangle (PA) ≃ 158°, is almost orthogonal to the direction of the innermost region of the jet observed in K3-35, suggesting the presence of a disc or torus that may be related to the collimation of the outflow.