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Astron. Astrophys. 360, 1117-1125 (2000)


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ISO-LWS observations of rotational CO lines from C-rich objects: AFGL 2688, AFGL 618 and NGC 7027 *

K. Justtanont 1, M.J. Barlow 2, A.G.G.M. Tielens 3,4, D. Hollenbach 5, W.B. Latter 6, X.-W. Liu 2, R.J. Sylvester 2, P. Cox 7, N.-Q. Rieu 8 and C.J. Skinner **

1 Stockholm Observatory, 133 36 Saltsjöbaden, Sweden
2 Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
3 Kapteyn Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
4 SRON-Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands
5 NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035, USA
6 SIRTF Science Center/IPAC, Caltech, MS 314-6, Pasadena, CA 91125, USA
7 IAS, Bat. 120, Université de Paris XI, 91405 Orsay, France
8 Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France

Received 25 February 2000 / Accepted 23 June 2000

Abstract

We present ISO-LWS full scan observations of CO rotational emission lines (J=14-13 up to J=37-36) from two C-rich post-AGB objects, AFGL 2688; AFGL 618, and one C-rich PN, NGC 7027. The presence of high rotational lines cannot be explained by a spherical, constant velocity wind during the previous AGB phase, but indicates a layer of warm, dense gas with a substantial beam filling factor ([FORMULA] sr). By simple optically thin calculations, we estimate the total mass of gas cooled through CO rotational lines to be typically [FORMULA] 0.1 [FORMULA].

We also consider the physical processes responsible for heating and cooling the warm gas in these objects. Energy sources for the gas could include FUV photons, resulting in a PhotoDissociation Region (PDR), as well as shocks driven by the interacting winds during this evolutionary phase. We have calculated detailed models for the heating and cooling in C-rich PDRs and shocks in order to explain the CO spectra observed. In both models, the gas is cooled radiatively by molecular rotational lines as well as by atomic fine structure lines. Both models can produce warm gas of [FORMULA]500 K at a density of [FORMULA] cm-3.

We conclude that the source of heating for the young PN, NGC 7027, is FUV photons from the central star. For the least evolved post-AGB star, AFGL 2688, the effective temperature is too cool to produce many FUV photons to heat the gas. However, there is evidence of a fast wind developing and hence the most likely heating mechanism for the envelope is shocks. For AFGL 618, the central star is hot enough to provide copious amounts of FUV photons but fast outflows have also been observed and both processes may be operational. However, in view of [O I ] 63 µm high resolution observations, the heating mechanism is more likely to be a PDR.

Key words: stars: circumstellar matter – stars: evolution – stars: individual: NGC 7027, AFGL 618, AFGL 2688 – stars: late-type – stars: mass-loss – infrared: stars

* 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) with the participation of ISAS and NASA.
** Deceased

Send offprint requests to: K. Justtanont (kay@astro.su.se)

© European Southern Observatory (ESO) 2000

Online publication: August 23, 2000

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