| Description |
AGB stars evolve to form blue dwarfs in a very short time, about 1000yr. Simultaneously, the circumstellar envelope around the AGB star,which is spherical and expands at moderate velocity, also evolves toform a planetary nebula (PN) around the dwarf, which usually showsaxial symmetry and fast bipolar flows. This change is due to verystrong shock interaction between the fossil AGB envelope and fast andcollimated post-AGB jets.The nature of the gas also changes: the AGB shells are molecular andcool, 50 - 500 K, while gas in evolved PNe is fully ionized and hot,about 10000 K. This change is thought to proceed progressively duringthe intermediate phase of protoplanetary nebula (PPN), in which anintermediate phase of low- or intermediate-excitation atomic gas isexpected to develop, due to photodissociation (in a PDR) or to gasheating in the shocks. This neutral or slightly ionized atomic gas canonly be observed by means of fine-structure lines of abundantatoms. ISO detected a few nebulae in such lines, but the relativelypoor sensitivity of that instrument and the lack of spectral resolutionprevented any deep study of this component. Herschel/HIFI cansignificantly improve those data, in particular yielding accurate lineprofiles, from which we will be able to identify the emission from thedifferent nebular components. New observations will allow us to studythe properties of low-excitation atomic gas in PPNe and, in particular,to discern the origin the molecular dissociation, in a PDR or inshocks, 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)giantsand more evolved PNe for comparison. |
