Chemical modelling of molecular sources - I. TMC-1.
Abstract
A model aimed at understanding the chemical processes which occur in cold dark interstellar clouds is described and applied to the molecular cloud TMC-1. The hydrocarbon chemistry is driven by the C(+)-H2 radiative association reaction and includes C(+) insertion reactions, abstraction reactions with H2 and condensation reactions involving CH4 and C2H2. It is shown that the observed abundance of HC3N in TMC-1 requires cyanopolyyne formation to proceed via N atom reactions rather than CN or HCN as has previously been suggested. Quantitative agreement with the observed abundances is found to within a factor of 5 for 13 out of 17 molecules included in the scheme. Several key reactions have been identified which need to be studied in the laboratory, of which the most important is the H3(+) + N reaction. In addition, it is predicted that C3H is one of the most abundant hydrocarbon species in dark clouds and that C5H may also be detectable. The chemical model described in Part I is applied to the dark dust cloud L183 (L134N). This cloud shows similarities in density, temperature and extinction to TMC-1, but has much less abundances of several molecular species, including the cyanopolyynes, than TMC-1. It is shown that reasonable changes to the free parameters in the scheme, in particular the elemental depletions, can lead to calculated abundances which are in good agreement with the observations. The key details of the reaction scheme are discussed and some observational tests of the model are suggested.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 1984
- DOI:
- 10.1093/mnras/207.3.405
- Bibcode:
- 1984MNRAS.207..405M
- Keywords:
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- Abundance;
- Astronomical Models;
- Hydrocarbons;
- Interstellar Chemistry;
- Molecular Clouds;
- Association Reactions;
- Chemical Reactions;
- Cyano Compounds;
- Cyanoacetylene;
- Astrophysics