view Abstract Citations (215) References (48) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Ion-Molecule Reactions, Molecule Formation, and Hydrogen-Isotope Exchange in Dense Interstellar Clouds Watson, William D. Abstract The influence of (positive) ion-molecule reactions on the formation and dissociation of molecules in dense (particle density > 1 0 cm -3) interstellar clouds is investigated. Unlike reactions between neutral molecules, no activation energy barrier inhibits these ion-molecule reactions at low interstellar gas temperatures. Based on an ionization rate due only to the observed high- energy cosmic rays, certain chemical processes are shown to occur on a time scale comparable with or faster than other relevant time scales. These processes result in the following: (a) An appreciable abundance of atoms (especially H +, H, C +, and 0) will be maintained in the gas. (b) The abundance of complex (positive) molecular ions will be about 0.1 to 1 percent of the abundance of complex molecules (NH3, HCN, H2CO, etc.). (c) Because of the large abundance of H2 and CO, essentially every ionization leads to the formation of HCO + which occupies a cornerstone position in the ion-molecule chemistry. Its predicted abundance is consistent for its identification with the unidentified X-ogen line. Ion-molecule reactions may, however, produce comparable abundances of other candidate molecules (C2H, HNC). (d) Exchange of hydrogen and deuterium in ion-molecule reactions provides a mechanism in some molecules for strong chemical fractionation in favor of deuterium. In the formation of DCN, the reaction of H2CN + + HD HDCN + + H2 is expected to be in thermodynamic equilibrium and is the chief factor in determining the abundance of DCN. Based on the observation (DCN/HCN) ratio, our best estimate for the cosmic abundance of deuterium is D/H <% 10- , independent of the exact rate of ionization. Uncertainties in this D/H are large. Subject headings: abundances, nebular - molecules, interstellar - nebulae Publication: The Astrophysical Journal Pub Date: February 1974 DOI: 10.1086/152681 Bibcode: 1974ApJ...188...35W full text sources ADS |