Astronomy and Astrophysics – Astrophysics
Scientific paper
Jun 1981
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26as...44..405h&link_type=abstract
Astronomy and Astrophysics Supplement Series, vol. 44, June 1981, p. 405-435.
Astronomy and Astrophysics
Astrophysics
10
Astronomical Models, Interstellar Chemistry, Interstellar Gas, Molecular Clouds, Reaction Kinetics, Vapor Phases, Abundance, Chemical Equilibrium, Concentration (Composition), Hydrocarbons, Reaction Products
Scientific paper
The gas phase (GP) production of molecules in molecular clouds is studied by means of a numerical model containing 100 atomic and molecular species connected by some 500 reactions. This increase of the number of reactions considered also increases the scope of GP chemistry as a whole. By following the time evolution with initial parameters representing the major variety of cloud types it is found that most of the smaller molecules including CH, H2 CO, CH(+), and possibly methane can be formed in sufficient amounts by GP chemistry alone. Molecules which cannot be formed in this way are H2, NH3, and possibly CH4. Ionization, moreover, can easily be balanced by GP reactions; as long as the metals are depleted by less than a factor 10 they are responsible for this balance. In the depleted case, however, the C(+) and molecular ions such as HCO(+) and NH3(+) maintain the ionization balance. The most favourable conditions for the formation of molecules occur at the optical depth and at densities of some 1000/cu cm, conditions which are representative for typical dark clouds. In the case of small dense clouds the physical lifetime is found to be too short to reach chemical equilibrium; different observed concentrations may thus point to different ages of individual clouds.
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