| Description |
One of the most intriguing aspects of the evolution ofintermediate mass stars is how these simply spherical ballsof gas can evolve resulting in the formation of PNe, which display a variety of shapes far from being isotropic. This transformation takes place after the end of the AGB, when some post-AGB flows collide with the CSEs of the former AGB star. These envelopes, which are spherical and expanding at a low velocity, are accelerated and re-shaped due to the interaction of the post-AGB flows, which are much faster and highly bipolar. Later, the starbecomes much hotter, ionizing the surrounding material and forming the PN.This wind interaction is active only during a very short times, virgul 100 yr, and to gain insight into the subject we need to study objects in which this transformation is taking place, or it has happened very recently. So far, the earliest post-AGB sources we know about are the water fountains. These are OH/IR sources in which, contrarily to what happens in AGB stars, the H2O masers show very wide velocity ranges, wider than OHmasers. VLBI observations tell us that in water fountains H2Omasers trace very fast bipolar outflows, with kinematic ages as short as 40-100 yr, comparable to the expected duration of the post-AGB acceleration of the envelope.Unfortunately, the main properties of water fountain nebulae are now well known, as the masers do not provide information on the density and temperature of the gas. The usual probe, low-J transitions of CO, becomes unusable in most cases, as these sources suffer from strong interstellar contamination. We can overcome these problems with Herschel/HIFI, byobserving higher J-transitions of CO, where contamination is negligible, and the strong lines of H2O and OH available within the band. Here we propose to observe six water fountains infive frequency setups, to study the molecular envelope of thesesources by means of the observation of several lines of thesevery abundant molecules in their envelopes. |
