Speaker: José María Torrelles (Institut de Ciències de l'Espai (CSIC), Barcelona)
Location: van de Hulst Auditorium
Water fountains (WF) 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. In this talk, we introduce the results of our multi-epoch VLBI observations published in Orosz et al. (2019; MNRAS, 482, L45), which determine the spatial and three-dimensional kinematic structure of H2O masers associated with the fastest 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 arcsec (~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. We estimate an initial ejection velocity of ~840 km/s, and a period for the ejections of 10-20 yr, with the youngest being ~12 yr old. 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 WFs. We also note that these short-lived, episodic ejections observed in the WF mimic what we also observe in some protostars. We will also present some preliminary results of our monitoring program we are currently doing on this WF.