| Description |
We propose to carry out a comprehensive study of circumstellar
envelopes (CSEs) around evolved stars, as a HIFI GTKP. The main
scientific aims are to gain deeper insight into the structure,
thermodynamics, kinematics and chemistry of CSEs and into the
mass-loss history of evolved stars, thereby advancing our
understanding on the final stages of stellar evolution of the majority
of stars and their resultant impact on the interstellar medium and the
cosmic cycle.
We will use the unique capabilities of HIFI to probe the inner regions
of CSEs, by means of spectrally-resolved observations of the principal
cooling transitions: thermal transitions of H2O, and high-excitation
rotational transitions of CO and HCN. Such observations provide unique
information about the shells where the gas temperatures are 100 - 2000
K and the material is being accelerated.
We propose to observe a set of 38 objects, chosen to sample the
parameter space of r^ance for evolved stars and their circumstellar
material. For every star, 3 lines of both 12CO and 13CO will be
observed to unravel the mass-loss history. To further unveil the
inner structure in the CSEs, observations of different H2O lines are
crucial, since H2O is a key molecule to understand the chemistry, the
thermodynamics and the dynamics of the inner CSE. We propose to
observe a carefully-selected list of H2O transitions covering
different excitation degrees, including para- and ortho-H2O lines and
a few isotopic lines. The high spectral resolution observations will
allow us to determine the velocity field in the warm acceleration
regions. With HCN being a major coolant in C-rich CSEs, 2
high-excitation lines will be observed in C-rich AGBs to probe the
temperature structure. To further study the oxygen chemistry, we will
obtain OH and OI lines from PACS GT data.
Finally, we will address the puzzling question of the origin of water
vapor observed previously in the extreme carbon star IRC+10216, and
survey 9 additional C-rich stars for evidence of water. |
