DENSITY-ENHANCED GAS AND DUST SHELLS IN A NEW CHEMICAL MODEL FOR IRC+10216
A new chemical model is presented for the carbon-rich circumstellar envelope (CSE) of the asymptotic giant branch star IRC+10216. The model includes shells of matter with densities that are enhanced relative to the surrounding circumstellar medium. The chemical model uses an updated reaction network including reactions from the RATE06 database and a more detailed anion chemistry. In particular, new mechanisms are considered for the formation of CN{sup -}, C{sub 3}N{sup -}, and C{sub 2}H{sup -}, and for the reactions of hydrocarbon anions with atomic nitrogen and with the most abundant cations in the CSE. New reactions involving H{sup -} are included which result in the production of significant amounts of C{sub 2}H{sup -} and CN{sup -} in the inner envelope. The calculated radial molecular abundance profiles for the hydrocarbons C{sub 2}H, C{sub 4}H, and C{sub 6}H and the cyanopolyynes HC{sub 3}N and HC{sub 5}N show narrow peaks which are in better agreement with observations than previous models. Thus, the narrow rings observed in molecular microwave emission surrounding IRC+10216 are interpreted as arising in regions of the envelope where the gas and dust densities are greater than the surrounding circumstellar medium. Our models show that CN{sup -} and C{sub 2}H{sup -} may be detectable in IRC+10216 despite the very low theorized radiative electron attachment rates of their parent neutral species. We also show that magnesium isocyanide (MgNC) can be formed in the outer envelope through radiative association involving Mg{sup +} and the cyanopolyyne species.
- OSTI ID:
- 21300603
- Journal Information:
- Astrophysical Journal, Vol. 697, Issue 1; Other Information: DOI: 10.1088/0004-637X/697/1/68; Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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