THE INSITU FORMATION OF LARGE MOLECULES IN DENSE INTERSTELLAR CLOUDS

A simple model has been developed to estimate the abundances of large hydrocarbon molecules produced by gas-phase, ion-molecule reactions in dense interstellar clouds. Hydrocarbons through 30 carbon atoms in size are considered in the model. Molecular complexity is built up via synthetic reactions involving smaller hydrocarbon neutrals and ions. The ions produced in these reactions are neutralized via dissociative recombination reactions with electrons or destroyed via reactions with atomic oxygen. The resulting hydrocarbon neutrals are depleted by both ion-molecule reactions and neutral-neutral reactions with atomic oxygen. A quasi - time-dependent model has been utilized in which the abundances of hydrocarbons with fewer than 10 carbon atoms are fixed at values calculated in a previous model. The time dependence of the abundances of the more complex hydrocarbons is then followed. Our results show that with favorable values for important rate coefficients, it is possible to produce sizeable abundances of hydrocarbon molecules exceeding ≈20 carbon atoms in size in a time scale of ≤ 106 yr.