Astron. Astrophys. 324, 237-248 (1997)

Chemistry in oxygen-rich circumstellar envelopes

K. Willacy and T.J. Millar

Department of Physics, UMIST, PO Box 88, Manchester, UK

Received 9 December 1996 / Accepted 27 January 1997

Abstract

We have considered the chemistry occuring in the circumstellar envelope surrounding an oxygen-rich AGB star and have specifically modelled 4 sources; R Dor, TX Cam, OH231.8+4.2 and IK Tau. Methane has been assumed to be a parent molecule and the resulting carbon chemistry is investigated. We find that carbon chain molecules up to C₂ H₄ can be abundant as can CH₃ CN and CH₃ OH. Our model extends previous work by including the chemistry of silicon, chlorine and phosphorus. The presence of CH₄ as a parent and hence its daughter species CH₃ and CH leads to other carbon-bearing species such as H₂ CS, SiCH₂, H₂ CN and CCl.

Sulphur is an important source of electrons in some regions of the envelope. Its main effect is on the abundance of CH and molecules formed from this radical e.g. CH₃ OH.

Where possible we have compared our results to observations and in general there is good agreement. A notable exception is the case of OCS in OH231.8+4.2 whose calculated abundance is roughly 10⁴ times lower than observed.

Key words: stars: abundances stars: circumstellar matter stars: late type

Send offprint requests to: T. J. Millar

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Contents

• 1. Introduction
• 2. The model
• 3. The chemistry of the circumstellar envelope
  • 3.1. Initiating the chemistry
  • 3.2. Carbon chain chemistry
    • 3.2.1. The inner region
    • 3.2.2. The peak region
    • 3.2.3. The outer region
  • 3.3. Other carbon-bearing molecules
  • 3.4. Nitrogen chemistry
    • 3.4.1. The inner region
    • 3.4.2. The peak region
    • 3.4.3. The outer region
  • 3.5. Silicon chemistry
    • 3.5.1. Initiating the chemistry
    • 3.5.2. Silicon-carbon compounds
    • 3.5.3. Other silicon compounds
  • 3.6. Sulphur chemistry
    • 3.6.1. The peak region
    • 3.6.2. Other sulphur-bearing molecules
    • 3.6.3. The effect of the sulphur chemistry on other molecules
  • 3.7. Chlorine chemistry
  • 3.8. Phosphorus chemistry
• 4. A comparison with observations
  • 4.1. Is the presence of CH₄ as a parent molecule compatible with the observations?
• 5. Conclusions
• References

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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