Astron. Astrophys. 356, 253-266 (2000)

ISO-SWS spectra of the carbon stars TX Psc, V460 Cyg, and TT Cyg ^*

U.G. Jorgensen ¹, J. Hron ² and R. Loidl <sup>2

1</sup> Niels Bohr Institute, Astronomical Observatory, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
² Institut für Astronomie der Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria

Received 10 May 1999 / Accepted 25 January 2000

Abstract

During our open and discretionary time ISO programs we have observed a number of carbon-rich and oxygen-rich AGB stars. We present here a hydrostatic analysis of 3 carbon rich stars, TX Psc, V460 Cyg, and TT Cyg, which show only modest variability.

We identify absorption features of the molecules C₂, CN, CH, CO, CS, HCN, C₂H₂, and C₃ in the three stars. The relative intensities of the corresponding bands put strict limits on the possible values of , , and C/O. In particular the ratio of the intensity of the 3 µm band (due to HCN and C₂H₂) and the 5 µm band (due to CO and C₃) is a sensitive measure of the C/O ratio.

We show that our model atmospheres and corresponding synthetic spectra are able to reproduce the observed spectra quite accurately from the visual region through the infrared out to approximately 10 µm. Beyond 10 µm the flux in the observed spectra increases compared to the computed spectra, and the photospheric 14 µm band dominated by C₂H₂ and HCN is almost absent in the observed spectra whereas it is very strong in the synthetic ones. This discrepancy is not an artefact in the reduction or in the SWS response function, but a physical phenomenon in the stars, which does not appear in oxygen-rich giants.

We exclude a so-called "warm molecular envelope" as an explanation of the discrepancies, and suggest that the flux excess at long wavelengths can be interpreted as clumps of 500 K dense material which obscures about 10% of the photosphere. Weak spectral features seen around 13.7 µm can be understood as even cooler gas, rich in C₂H₂, above the clumps. Other possible models to explain the long wavelength discrepancy between the computed and observed spectra need to be explored too.

Key words: stars: AGB and post-AGB stars: atmospheres stars: carbon molecular processes stars: variables: general 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) and with the participation of ISAS and NASA

Send offprint requests to: uffegj@nbi.dk

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Contents

• 1. Introduction
• 2. Observations and data reduction
• 3. Model atmospheres and synthetic spectrum computation
• 4. The spectrum of TX Psc
  • 4.1. Our simulations of TX Psc spectra from the literature
  • 4.2. The uncertainty in K_p(C₃)
  • 4.3. Synthetic spectrum simulation of our own ISO observation of TX Psc
  • 4.4. The variation in between the time of our observation and other observations in the literature
  • 4.5. Contribution of individual molecules to the spectrum of TX Psc
• 5. The spectrum of V460 Cyg
  • 5.1. The fundamental parameters and the value of C/O
• 6. The spectrum of TT Cyg
  • 6.1. Molecules contributing to the synthetic spectra
• 7. The 14µm region and beyond in the spectra of V460 Cyg, TT Cyg, and TX Psc
  • 7.1. The spectral slope in the long-wavelength region
  • 7.2. The absorption in the 14 µm region
  • 7.3. Discussion of the long-wavelength discrepancy
  • 7.4. A tentative model to explain the full SWS spectral region
• 8. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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