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

TL;DR

This study uses multi-epoch VLBI observations to analyze the structure and kinematics of water masers in IRAS 18113-2503, revealing episodic, high-velocity outflows and suggesting a binary star system as the outflow driver.

Contribution

It provides the first detailed 3D kinematic model of water fountain IRAS 18113-2503, showing episodic jet-driven outflows decelerating in a circumstellar envelope.

Findings

High-velocity bipolar bow shocks traced by H2O masers.

Episodic ejections approximately every 10 years.

Estimated initial ejection velocity of about 840 km/s.

Abstract

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.