Astron. Astrophys. 339, 811-821 (1998)

Molecular line observations of southern S stars

J.H. Bieging ¹, L.B.G. Knee ², W.B. Latter ³ and H. Olofsson <sup>4

1</sup> Steward Observatory, The University of Arizona, Tucson, AZ 85721, USA
² National Research Council of Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory, P.O. Box 248, Penticton, BC V2A 6K3, Canada
³ Infrared Processing and Analysis Center, Caltech, MS 100-22, Pasadena, CA 91125, USA
⁴ Stockholm Observatory, S-133 36 Saltsjöbaden, Sweden

Received 26 March 1998 / Accepted 24 August 1998

Abstract

We observed a sample of southern S stars with the SEST telescope, in the SiO (v=0, J=3-2) transition at 130.3 GHz, and the HCN (J=1-0) transition at 88.6 GHz. SiO emission was detected in all seven stars observed, while HCN was detected in two. We employed a statistical equilibrium/radiative transfer model to estimate the SiO abundance for an assumed molecular distribution. The inferred SiO abundances are consistent with formation of the molecule under thermodynamic equilibrium (TE) conditions near the stellar photosphere, for reasonable physical conditions. We also model the HCN emission by a similar analysis, and find that if HCN is produced near the stellar photosphere, our model abundances are much higher than predicted by TE chemistry, unless the gas temperature is 1300 K and the gas density n(H₂) 10¹² cm^-3. Under such conditions, condensation of silicate grains may enhance production of HCN for C/O 1. Alternatively, HCN may be formed by photochemical reactions in the outer circumstellar envelope, as has been proposed for O-rich giants.

Key words: stars: chemically peculiar circumstellar matter stars: mass-loss stars: AGB

Send offprint requests to: J. Bieging, (jbieging@as.arizona.edu)

SIMBAD Objects

Contents

• 1. Introduction
• 2. Observational results
  • 2.1. Source selection
  • 2.2. Observations and data reduction
  • 2.3. Results
• 3. Molecular abundance estimates
  • 3.1. Statistical equilibrium model
  • 3.2. SiO models
  • 3.3. HCN models
• 4. Discussion
  • 4.1. SiO abundances
  • 4.2. Composition of the dust
  • 4.3. Formation of HCN
• 5. Summary and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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