• Bujarrabal, Valentin

Abstract

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. <P />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. <P />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....

Topics

• impedance spectroscopy
• phase
• chemical ionisation