Dynamical Models of Molecular Clouds - Nitrogen Chemistry

Details Dynamical Models of Molecular Clouds - Nitrogen Chemistry Dynamical Models of Molecular Clouds - Nitrogen Chemistry

Sep 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990mnras.246..183n&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Vol. 246, NO. 1/SEP1, P. 183, 1990

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62

Scientific paper

We explore the chemistry - including particularly that of nitrogen-bearing molecules - in a dynamical model of molecular clouds. Clumps of gas collapse to form low-mass stars whose winds ablate the clumps and partially ionize the gas. These mass-loaded winds are arrested by low-velocity shocks, and the post-shock gas accumulates to form new clumps from which new stars form. The chemistry incorporates about 1200 reactions involving 84 species, and allows for the freeze-out of heavy molecules on dust grains. The calculations show that the chemistry repeats after several dynamical cycles so that the initial abundance values are unimportant. The chemistry never attains steady state, and the abundances are always characteristic of a `young' chemistry. The growth of molecular mantles on dust grains is limited by the periodic star formation which returns mantle material to the gas and controls elemental depletion. The chemistry of nitrogen has a rich network: in this dynamic model we find that neutral exchanges of N with CH and other hydrocarbons dominate the routes to N2, HNC and HCN. Ammonia formed and retained on grains, and subsequently released in shocks, provides most of the gaseous NH3. Molecular abundances predicted by the model cannot yet be compared with detailed molecular maps of nearby clouds as these are not available. However, calculated abundances are generally comparable to typical observed values for dark clouds. Observations of clump, interclump, and interface regions of dark clouds where low-mass star formation is occurring are urgently required.

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