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Astron. Astrophys. 360, 562-574 (2000)

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CO2 emission in EP Aqr: Probing the extended atmosphere *

J. Cami 1,2, I. Yamamura 1, T. de Jong 3,1, A.G.G.M. Tielens 2,4, K. Justtanont 5 and L.B.F.M. Waters 1,2

1 Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 SRON-Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands
3 SRON, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4 Kapteyn Institute, P.O. Box 800, 9700 AV Groningen, The Netherlands
5 Stockholm Observatory, 13336 Saltsjöbaden, Sweden

Received 24 February 2000 / Accepted 7 June 2000

Abstract

We present an analysis of the ISO/SWS full resolution scan between 12.5 and 16.5 µm of the O-rich AGB star EP Aqr, exhibiting a number of strong CO2 emission bands. We have developed a simple LTE model to calculate theoretical CO2 spectra assuming a single-layer slab geometry and compared the SWS observations to this model in order to infer the physical properties of the extended atmosphere. The single layer slab model is able to reproduce the individual band profiles quite well with optically thick bands (column densities typically 10[FORMULA] cm-2). The derived excitation temperatures for the different bands are in the range [FORMULA] K in a region which extends from [FORMULA]. The fundamental CO2 bending mode at 14.98 µm furthermore shows evidence for an optically thin component arising from a much cooler ([FORMULA] K) and more extended ([FORMULA]) layer. The strong spectral signature of 13CO2 in the spectrum allows an (uncertain) determination of the 12C/13C ratio [FORMULA]. The parameters derived for the CO2 bands allow us to estimate the local temperature and density structure of the extended atmosphere. We find that the derived local gas temperatures are somewhat lower than predicted by hydrodynamical model calculations whereas the local gas densities are in good agreement with these models when using CO2 abundances derived from chemical network calculations. The CO2 layer extends from close to the stellar photosphere to the inner part of the dust forming region which makes it a unique new probe of the whole extended atmosphere and the region where dust formation takes place.

Key words: stars: AGB and post-AGB – stars: circumstellar matter – stars: individual: EP Aqr – stars: late-type – stars: winds, outflows – 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. The SWS is a joint project of SRON and MPE.

Send offprint requests to: J. Cami, (cami@astro.rug.nl)

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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