Do Water Fountain Jets Really Indicate the Onset of the Morphological Metamorphosis of Circumstellar Envelopes?

TL;DR

This study investigates water fountain sources and finds they exhibit diverse envelope morphologies, suggesting they are not necessarily at the onset of circumstellar metamorphosis, and questions the reliability of jet dynamical age as an evolutionary indicator.

Contribution

The paper demonstrates that water fountain sources can have different envelope shapes and challenges the assumption that their jets indicate the start of morphological transformation.

Findings

Some WFs have spherical envelopes like AGB stars.

Others have aspherical envelopes typical of post-AGB stars.

Jet dynamical age may not reflect the true evolutionary stage.

Abstract

The small-scale bipolar jets having short dynamical ages from "water fountain (WF)" sources are regarded as an indication of the onset of circumstellar envelope morphological metamorphosis of intermediate-mass stars. Such process usually happens at the end of the asymptotic giant branch (AGB) phase. However, recent studies found that WFs could be AGB stars or even early planetary nebulae. This fact prompted the idea that WFs may not necessarily be objects at the beginning of the morphological transition process. In the present work, we show that WFs could have different envelope morphologies by studying their spectral energy distribution profiles. Some WFs have spherical envelopes that resembles usual AGB stars, while others have aspherical envelopes which are more common to post-AGB stars. The results imply that WFs may not represent the earliest stage of the morphological metamorphosis. We further argue that the dynamical age of a WF jet, which can be calculated from maser proper motions, may not be the real age of the jet. The dynamical age cannot be used to justify the moment when the envelope begins to become aspherical, nor to tell the concrete evolutionary status of the object. A WF jet could be the innermost part of a larger well-developed jet, which is not necessarily a young jet.