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
(~150-300 km/s) 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,
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) ~ 10^6 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.
