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

Fong, D. and Meixner, M. and Castro-Carrizo, A. and Bujarrabal, V. and Latter, W. B. and Tielens, A. G. G. M. and Kelly, D. M. and Sutton, E. C. (2001) Low-excitation atomic gas around evolved stars. I. ISO observations of C-rich nebulae. Astronomy and Astrophysics, 367 (2). pp. 652-673. ISSN 0004-6361. doi:10.1051/0004-6361:20000434.

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We present ISO LWS and SWS spectra of far-infrared (FIR) atomic fine structure lines in 12 carbon-rich 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 of [OI], [CII], [SiI], [SiII], [SI], [FeI], [FeII], [NeII] and [NII]. Only 5 out of our 12 object sample have been detected in at least one of these FIR lines. When we include the 12 oxygen-rich evolved stars from Castro-Carrizo et al. ([CITE], Paper II), we find that atomic line emission is observed only in those sources in which the central star's T_(eff) ≥ 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 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. AFGL 618, however, appears exceptional with dual velocity components: a narrow component (<20 km s^(-1)) that may be associated with a PDR, and a broad component (∼66 km s^(-1)) that may be produced in post-shocked, accelerated gas. A new PDR code which properly treats enhanced carbon abundances was used to model the observations of our carbon-rich objects. 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⊙ were derived from the [CII] 158 μm line emission. The atomic gas constitutes only a small fraction of the total mass for young planetary nebulae, but its importance grows significantly as the nebulae evolve. 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.

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Additional Information:© 2001 ESO. Article published by EDP Sciences. Received 11 July 2000; Accepted 24 November 2000; Published online 15 February 2001. Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA. This work has been financially supported by NASA JPL 961504 and NASA NAG 5-3350. Meixner has been additionally supported by NSF AST-97-33697. Castro-Carrizo and Bujarrabal has been supported by Spanish DGES, under project PB96-0104. Kelly has also been supported by USAF Contract No. F19628-93-K-0011. Latter and Tielens acknowledge additional support from NASA grant 399-20-61 from the Long Term Space Astrophysics Program. We thank A. Glassgold, M. Haas, J. Lacy, Q. Nguyen-Rieu, and C. Skinner for discussions on the observations. The ISO Spectral Analysis Package (ISAP) is a joint development by the LWS and SWS Instrument Teams and Data Centers. Contributing institutes are CESR, IAS, IPAC, MPE, RAL and SRON. OSIA is a joint development of the SWS consortium. Contributing institutes are SRON, MPE, KUL and the ESA Astrophysics Division.
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NASANAG 5-3350
Direção-Geral do Ensino Superior (DGES)PB96-0104
Air Force Office of Scientific Research (AFOSR)F19628-93-K-0011
Subject Keywords:atomic data - stars: AGB and post-AGB - stars: circumstellar matter - stars: mass-loss - ISM: planetary nebulae
Issue or Number:2
Record Number:CaltechAUTHORS:20180228-074207253
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Official Citation:Low-excitation atomic gas around evolved stars - I. ISO observations of C-rich nebulae. D. Fong, M. Meixner, A. Castro-Carrizo, V. Bujarrabal, W. B. Latter, A. G. G. M. Tielens, D. M. Kelly and E. C. Sutton. A&A, 367 2 (2001) 652-673. DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:84983
Deposited By: Tony Diaz
Deposited On:28 Feb 2018 20:39
Last Modified:15 Nov 2021 20:24

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