Low-excitation atomic gas around evolved stars I. ISO observations of C-rich nebulae

We present ISO LWS and SWS spectra of far-infrared (FIR) atomic fine struct ure lines in 12 carbon-rich evolved stars including asymptotic giant branch (AGB) stars, proto-planetary nebulae (PPNe) and planetary nebulae (PNe). T he spectra include grating and Fabry-Perot measurements of the line emissio n of [O I], [CII], [Si I], [Si II], [S I], [Fe I], [Fe II], [Ne II] and [N II]. Only 5 out of our 12 object sample have been detected in at least orle of these FIR lines. When we include the 12 oxygen-rich evolved stars from Castro-Carrizo et al. (2001, Paper II), we find that atomic line emission i s observed only in those sources in which the central stars T-eff greater t han or equal to 10 000 K. Above this cutoff. the number of detectable lines : and the intensity of the line emission increase as T-eff increases. These trends suggest that the atomic lines originate from photodissociation regi ons (PDRs). In general, the kinematics of the atomic gas, derived horn line fits to the Fabry-Perot data. are comparable to the molecular expansion ve locities. These kinematics are expected for atomic cooling lines associated with circumstellar PDRs. AFGL 618, however. appears exceptional With dual velocity components: a narrow component (<20 km s(-1)) that may be associat ed with a PDR, and a broad component (<similar to>66 km s(-1)) that may be produced in post-shocked, accelerated gas. A new PDR code which properly tr eats enhanced carbon abundances was used to model the observations of our c arbon-rich objects. The predicted line intensities agree reasonably well wi th the observations. Shock models, however, do not compare well with the ob served line intensities. PDR mass estimates ranging from similar to0.01-0.2 M. were derived from the [C II] 158 mum line emission. The atomic gas: con stitutes only a small fraction of the total mass for young planetary nebula e, but its importance grows significantly as the nebulae evolve. Our overal l analysis shows that photodissociation, and not shocks, dominates the evol ution of the circumstellar envelope by transforming the initially molecular asymptotic giant branch envelopes into the atomic gas found in proto-plane tary and planetary nebulae.