| Description |
The study of shells around AGB and post-AGB stars with Herschel/HIFI isbeing very fruitful. Observations of molecular lines have yieldedextremely rich information on these objects, particularly on their warmgas components, which are crucial to understand the nature of thenebulae and the late stellar evolution.Two guaranteed time projects, HIFISTARS and SUCCESS, have alreadyobtained most of the originally proposed observations, yielding severalhundreds of high-quality line profiles. But these results alsoindicate that some interesting observations were not proposed, becausewe could not imagine some unexpected results or because our sampleswere biased to previously well observed objects. We have realized thata small additional amount of time would significantly improve theexisting data in three aspects: A) A few interesting O-rich AGB starsin the HIFISTARS sample were not observed in the strongest lines ofH$_2^{18}$O and H$_2^{17}$O, which are in general found to be moreintense than expected. B) We detected surprisingly strong andwidespread NH$_3$ J,K=1,0-0,0 emission, but a proper analysis of theselines obviously requires data on higher-J lines. C) Our sample ofpost-AGB objects was biased towards northern sources, the best studiedones until now, but the proper exploitation of ALMA will requirehigh-frequency data of southern nebulae.We are accordingly proposing a few HIFI GT2 observations of evolvedstars that, in combination with already existing data, willsignificantly improve the reach and quality of the previously allocatedGT1. The total telescope time necessary to obtain these additionalspectra is only 12.5 hr. |
