| Description |
The study of shells around AGB and post-AGB stars with Herschel-HIFI is being very fruitful. Observations of molecular lines have yielded extremely rich information on these objects, particularly on their warm gas components, which are crucial to understand the nature of the nebulae and the late stellar evolution. Two guaranteed time projects, HIFISTARS and SUCCESS, have already obtained most of the originally proposed observations, yielding several hundreds of high-quality line profiles. But these results also indicate that some interesting observations were not proposed, because we could not imagine some unexpected results or because our samples were biased to previously well observed objects. We have realized that a small additional amount of time would significantly improve the existing data in three aspects: A) A few interesting O-rich AGB stars in the HIFISTARS sample were not observed in the strongest lines of H _2^{18} O and H _2^{17} O, which are in general found to be more intense than expected. B) We detected surprisingly strong and widespread NH _3 J,K=1,0-0,0 emission, but a proper analysis of these lines obviously requires data on higher-J lines. C) Our sample of post-AGB objects was biased towards northern sources, the best studied ones until now, but the proper exploitation of ALMA will require high-frequency data of southern nebulae. We are accordingly proposing a few HIFI GT2 observations of evolved stars that, in combination with already existing data, will significantly improve the reach and quality of the previously allocated GT1. The total telescope time necessary to obtain these additional spectra is only 12.5 hr. |
