| 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. |
