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

Abstract

The small-scale bipolar jets having short dynamical ages from “water fountain (WF)” sources are suggested to be indicating the onset of morphological metamorphosis of circumstellar envelopes 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 contradicts to the belief that WFs are late AGB or early post-AGB stars where the deviation from spherical symmetry of the envelopes has just begun. In the present work, we aim to show that WFs do not share the same evolutionary status by studying their spectral energy distribution (SED) profiles. In particular, we found that WFs could be associated with spherical or aspherical envelopes, according to the fitting results from the radiative transfer code DUSTY. Those WFs with spherical envelopes could be AGB or very early post-AGB stars, while others with aspherical structures are very likely more evolved. These results imply that WF jets may not represent the earliest stage of the change in envelope morphology. The dynamical ages of WF jets cannot be used to justify the moment when the envelopes begin to become aspherical, nor to tell the concrete evolutionary statuses of the objects. A WF jet could be the innermost part of a larger well-developed jet, which is not necessarily a young jet.