CHEMICAL PROCESSES IN DARK INTERSTELLAR CLOUDS - THE EFFECTS OF CNO DEPLETION

G.P. Desforets, D.R. Flower, TJ Millar

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8 Citations (Scopus)

Abstract

We have investigated the effects of depletion of the elements C, N and O on the chemical composition of dark clouds, using both isothermal and isochoric cloud models. Our work differs from previous approaches in that we have considered a much larger range of CNO depletions. We have included the chemistry of the ortho-and para-forms of H2 and the exothermic reaction between N+ and ortho-H2, which synthesizes NH3. 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(-8) and this may help to explain the puzzling observation that, in dark clouds, the column density of NH3 is always close to 10(15) cm-2.
Original languageEnglish
Pages (from-to)217-221
Number of pages5
JournalMonthly Notices of the Royal Astronomical Society
Volume253
Publication statusPublished - 1991

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