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

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 <sup>1,2

1</sup> 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 cm^-2). The derived excitation temperatures for the different bands are in the range K in a region which extends from . The fundamental CO₂ bending mode at 14.98 µm furthermore shows evidence for an optically thin component arising from a much cooler ( K) and more extended () layer. The strong spectral signature of ¹³CO₂ in the spectrum allows an (uncertain) determination of the ¹²C/¹³C ratio . 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)

SIMBAD Objects

Contents

• 1. Introduction
• 2. EP Aqr
• 3. ISO observations and data reduction
• 4. Modeling the emission lines
  • 4.1. Line data
  • 4.2. The model
    • 4.2.1. The background continuum
    • 4.2.2. The CO₂ layer
  • 4.3. The parameters and their influence
    • 4.3.1. The temperature T
    • 4.3.2. The column density N
    • 4.3.3. The size of the emitting region,
  • 4.4. Determining the model parameters
    • 4.4.1. Quantifying the quality of the fit
    • 4.4.2. Searching parameter space
    • 4.4.3. The model parameters and their uncertainties
• 5. Results
  • 5.1. Temperatures
  • 5.2. Column densities
  • 5.3. Isotopic ratio
• 6. Discussion
  • 6.1. Temperature structure
  • 6.2. Density structure
  • 6.3. Evolutionary status
• 7. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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