| Description |
AGB stars evolve to form blue dwarfs in a very short time, about 1000
yr. Simultaneously, the circumstellar envelope around the AGB star,
which is spherical and expands at moderate velocity, also evolves to
form a planetary nebula (PN) around the dwarf, which usually shows
axial symmetry and fast bipolar flows. This change is due to very
strong shock interaction between the fossil AGB envelope and fast and
collimated post-AGB jets.
The nature of the gas also changes: the AGB shells are molecular and
cool, 50 - 500 K, while gas in evolved PNe is fully ionized and hot,
about 10000 K. This change is thought to proceed progressively during
the intermediate phase of protoplanetary nebula (PPN), in which an
intermediate phase of low- or intermediate-excitation atomic gas is
expected to develop, due to photodissociation (in a PDR) or to gas
heating in the shocks. This neutral or slightly ionized atomic gas can
only be observed by means of fine-structure lines of abundant
atoms. ISO detected a few nebulae in such lines, but the relatively
poor sensitivity of that instrument and the lack of spectral resolution
prevented any deep study of this component. Herschel/HIFI can
significantly improve those data, in particular yielding accurate line
profiles, from which we will be able to identify the emission from the
different nebular components. New observations will allow us to study
the properties of low-excitation atomic gas in PPNe and, in particular,
to discern the origin the molecular dissociation, in a PDR or in
shocks, from analysis of the fine-structure line profiles.
We accordingly propose observations of the CII (158 microns), CI (609,
370 mic), and OI (63 mic, using PACS) lines in a sample 13 objects,
mostly PPNe and young PNe, but also including a few red (super)giants
and more evolved PNe for comparison. |
