Maser Bowshocks in a Water Fountain Outflow: A Case of IRAS 18043-2116

© Ann Njeri Ross Burns Gabor Orosz

The transition of AGBs into Planetary Nebulae (PN) represents the last stage of the stellar evolution. However, there is a significant change in the morphology from a very symmetrical AGB to a largely asymmetrical PN. Water Fountains (WF) are considered to be an intermediate stage between AGB and PN. They show highly collimated, high-velocity (>100 km/s) outflows traced by 22 GHz H2O masers. The spatial distributions and kinematics of the jets of WFs therefore offer important clues in these morphological changes.

IRAS 18043-2116 is a young post-AGB star that exhibits highly collimated high velocity H20 maser jets and is classified as a WF. Analysis from 3 epochs of VLBA observations showed 194 km/s jets and the maser features clearly show a bowshock shape (figure 1). The proper motions of the maser features were used to calculate the systemic motion of the source (Delta_R.A., Delta_Dec = -0.23, -0.983) mas/yr and the average internal motions reveal jet expansion of 1.76 mas/yr (vectors in figure 1).

Figures 2 and 3 show the transverse and longitudinal motions of the maser features in the WF fitted to the models of Ostriker & Lee (2001). The fitting clearly shows that the outflow in our source is jet driven rather than wind driven. This is the first evidence so far of a jet driven outflow with symmetric bowshocks in a water fountain obtained using VLBI observations.