Other
Scientific paper
Nov 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991mnras.253..217p&link_type=abstract
Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711), vol. 253, Nov. 15, 1991, p. 217-221. Research supported by S
Other
18
Chemical Composition, Interstellar Chemistry, Interstellar Matter, Molecular Clouds, Carbon, Depletion, Isochoric Processes, Isothermal Processes, Nitrogen, Oxygen
Scientific paper
The effects of depletion of the elements C, N and O on the chemical composition of dark clouds have been investigated using both isothermal and isochoric cloud models. The chemistry of the ortho- and para-forms of H2 and the exothermic reaction between N(+) and ortho-H2, which synthesizes NH3 are considered. In the isothermal models, the ortho:para ratio is very small at large depletions, but NH3 formation is still efficient owing to reactions between He(+) and CN or HCN. In the isochoric models, the equilibrium temperature of the gas is larger, and a thermal ortho:para ratio, which is large enough to drive NH3 formation, results. In all cases, the fractional abundance of NH3 is close to 10 exp -8, and this may help to explain the puzzling observation that, in dark clouds, the column density of NH3 is always close to 10 exp 15 per sq cm.
Flower David R.
Millar Thomas J.
Pineau des Forêts Guillaume
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