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D. Fong, M. Meixner, E.C. Sutton (Univ. of Illinois), A. Castro-Carrizo, V. Bujarrabal (Observatorio Astronómico Nacional, Spain), W.B. Latter (Caltech, SIRTF Science Center), A.G.G.M. Tielens (Kapteyn Astronomical Institute, The Netherlands), D.M. Kelly (Univ. of Arizona, Steward Obs.), W.J. Welch (Univ. of California)
We present ISO LWS and SWS observations of far-infrared atomic fine structure lines of 24 evolved stars including asymptotic giant branch (AGB) stars, proto-planetary nebulae (PPNe) and planetary nebulae (PNe). The spectra include grating and Fabry-Perot measurements of the line emission from [OI], [CII], [SiI], [SiII], [SI], [FeI], [FeII], [NeII] and [NII] which trace the low-excitation atomic gas. Atomic emission was only found in those sources where Teff \geq 10000 K. Above this cutoff, the number of detectable lines and the intensity of the line emission increase as Teff increases. These trends suggest that the atomic lines originate from photodissociation regions (PDRs). In general, the kinematics of the atomic gas, derived from line fits to the Fabry-Perot data, are comparable to the molecular expansion velocities. These kinematics are expected for atomic cooling lines associated with circumstellar PDRs. A new PDR code which properly treats enhanced carbon abundances was used to model the observations. The predicted line intensities agree reasonably well with the observations. Shock models, however, do not compare well with the observed line intensities. PDR mass estimates ranging from ~0.01-0.2 M\odot were derived from the [CII] 158 \mum line emission. The atomic gas only occupies a small fraction of the total mass for young planetary nebulae, but grows significantly as they evolve. To compliment our atomic gas study we also present CO J=1-0 observations of 7 objects in our ISO sample to investigate the evolution of the molecular envelope. By combining data from the Berkeley-Illinois-Maryland-Association (BIMA) Millimeter Array and the NRAO 12m, we have constructed full synthesis data cubes for MIRA, IRC +10216, IRAS 17436+5003 (HD 161796), AFGL 2688, IRAS 22272+5435 (HD 235858), AFGL 2343 (IRAS 19114+0002) and NGC 7027. The history of the circumstellar gas is imprinted on the circumstellar envelope itself, such as the record of its molecular mass loss, and its interaction with fast winds and dissociating/ionizing photons. By imaging the morphology and the kinematic structure, we can model the mass loss history and piece together how fast winds, shocks and photodissociation/photoionization have transformed these envelopes. Our overall analysis shows that photodissociation and not shocks dominates the evolution of the circumstellar envelope by transforming the initially molecular asymptotic giant branch envelopes into the atomic gas found in proto-planetary and planetary nebulae.
This work has been partially supported by NASA JPL 961504, NASA NAG 5-3350, NSF AST 99-81363 and NSF AST 97-33697.