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-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_{H_2}{≈ }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

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 HO masers associated with the water fountain IRAS 18113-2503. Themasers 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 HO maser region n ≈10 cm. 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.© 2018 The Author(s).GO was supported by the Xinjiang Key Laboratory of Radio Astrophysics grant 2016D03020, NSFC 11503072, the National Key R&D Program of China grant 2018YFA0404602, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, the Joint Institute of VLBI ERIC, Kagoshima University, and the MEXT Monbukagakusho scholarship. JFG, JMT, and LFM are partially supported by MINECO (Spain) grants AYA2014-57369-C3 and AYA2017-84390-C2-R (co-funded by FEDER). HI and GO were supported by the MEXTKAKENHI program (16H02167). HI, JMT and JFG were supported by the Invitation Program for Foreign Researchers of the Japan Society for Promotion of Science (JSPS grants S17115 and S14128). DT was supported by the ERC consolidator grant 614264. MAG acknowledges support from the MINECO grant AYA2014-57280-P, co-funded by FEDER. MAPT received support from the MINECO grants AYA2012-38491-C02-02 and AYA2015-63939-C2-1-P. JRR acknowledges support from the project ESP2015-65597-C4-1-R (MINECO and FEDER). LU received support from the University of Guanajuato (Mexico) grant ID CIIC 17/2018.Peer Reviewe