Rapidly evolving episodic outflow in IRAS 18113-2503: Clues to the ejection mechanism of the fastest water fountain

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 (similar to 150-300 km s(-1)) bipolar bow shocks on a scale of 0 ''.18 (similar to 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 similar to 840 km s(-1), a period for the ejections similar to 10 yr, with the youngest being similar to 12 yr old, and an average envelope density within the H2O maser region n(H2) approximate to 10(6) cm(-3). We hypothesize that IRAS 18113-2503 hosts a binary central star with a separation of similar to 10 au, revealing novel clues about the launching mechanisms of high-velocity collimated outflows in water fountains.