2019MNRAS.482L..40O

Water fountains are evolved stars showing early stages of collimated mass-loss during transition from the asymptotic giant branch, providing valuable insight into the formation of asymmetric planetary nebulae. We report the results of multi-epoch VLBI observations, which determine the spatial and three-dimensional kinematic structure of H₂O masers associated with the water fountain IRAS 18113-2503. The masers trace three pairs of high-velocity (∼150-300 km s^–1) bipolar bow shocks on a scale of 0^′′_.18 (∼2000 au). The expansion velocities of the bow shocks exhibit an exponential decrease as a function of distance from the central star, which can be explained by an episodic, jet-driven outflow decelerating due to drag forces in a circumstellar envelope. Using our model, we estimate an initial ejection velocity ∼840 km s^–1, a period for the ejections ∼10 yr, with the youngest being ∼12 yr old, and an average envelope density within the H₂O maser region n_H2~10⁶ cm^–3. We hypothesize that IRAS 18113-2503 hosts a binary central star with a separation of ∼10 au, revealing novel clues about the launching mechanisms of high-velocity collimated outflows in water fountains.