THE ROLE OF H+ AND H-3+ IONS IN THE DEGRADATION OF INTERSTELLAR-MOLECULES

The results of recent laboratory studies of the reactions of H+ and H-3+ with a number of molecular gases are interpreted from the viewpoint of interstellar chemistry. Many of the reactions of these ions result in the ionization and fragmentation of neutral reactant gases. Pseudo-time-dependent calculations of the chemistry in dense molecular clouds indicate that molecular abundances are reduced by the inclusion of such reactions, but generally by less than a factor of 5.

FORMALDEHYDE IN OXYGEN-RICH CIRCUMSTELLAR ENVELOPES

We test the hypothesis that methane is the source of the carbon observed in carbon-bearing molecules around oxygen-rich stars, by considering the synthesis of formaldehyde which is formed in the reaction between oxygen atoms and methyl radicals. We find that, provided that the parent methane abundance is large enough, millimetre-wave emission lines of H2CO should be detectable in such stars. We also consider the formation of other species, notably H2CN and H2CS, from methyl radicals, but conclude that they will be at least one order of magnitude less abundant than H2CO and therefore not detectable with current instrumentation.

ALTERNATIVE ROUTES TO DEUTERATION IN DARK CLOUDS

We investigate new approaches to the deuteration of C3H2, HC3N and HC5N in dark clouds, following the suggestion that protonated HC3N might form different isomers, a linear structure (HC3NH+) being the most stable. We consider the effect of linear HC3NH+ and HC5NH+ on the formation of HC3N and HC5N, and find that deuteration ratios at approximately 10 K are reduced, in the case of HC3N to values significantly below those observed, such that a deuteration mechanism other than direct deuteron transfer is probably required for cold clouds.

ON THE ORIGIN OF NH IN DIFFUSE INTERSTELLAR CLOUDS

The observation by Meyer & Roch of NH in the interstellar clouds towards zeta Per and HD 27778 cannot be explained with conventional gas-phase chemistry models. A simple non-equilibrium model for the zeta Per cloud, which incorporates the grain-surface production of NH and OH or, alternatively, NH3 and H2O, is able to reproduce the abundances of all observed species (except CH+) quite accurately. Moreover, chemical models which include grain-surface reactions can reproduce the observed abundance not only of NH but also of CN, which is efficiently formed at low temperatures, initiated by the reaction of NH with C+. Pure gas-phase models and cloud interface models, in which NH and CH+ are formed in a warm and tenuous environment, fail to explain the observed high abundance of CN. Hence the observation of NH in zeta Per and HD 27778 provides evidence for the presence of grain-surface reactions leading to molecules other than H-2. It is predicted that NH2 and NH3 should have abundances not much below that of NH if NH3 instead of NH is formed on grains. With or without surface reactions, the column densities of H2O and C2H are expected to be about 10(13) cm-2, and these molecules may be detectable in the zeta Per cloud.

THE CHEMISTRY OF PAH AND FULLERENE MOLECULES IN INTERSTELLAR CLOUDS

Recent laboratory data on the ion-neutral chemistry of PAH and fullerene ions and molecules have been incorporated into chemical kinetic models of interstellar clouds. The laboratory data show that the-second ionization potentials of many complex molecules are less than the first ionization potential of helium. Thus collisions between He+, generated by cosmic ray ionization, and PAH and fullerene neutrals produce doubly charged cations. I find that these cations, and also protonated neutrals, are abundant in dark clouds. If the recombination of electrons with doubly charged cations, which releases typically 14 eV of energy, is dissociative in nature, then PAH and fullerene species are destroyed m both diffuse and dense clouds on astronomically significant time-scales.

CHEMISTRY IN A PROTOPLANETARY NEBULA

In this paper we investigate gas-phase chemistry in the remnant 'superwind' of a carbon-rich red giant star, during its transition to a planetary nebula. The interacting stellar winds model is used. It is found that during the first few hundred years of transition, significant abundances of a few small molecules and ions (e.g. CH+, CH2+, CH3+, CH, CH2, NH) may occur in the thin, dense, shocked shell of gas predicted by thiS model, but that most molecules observed in protoplanetary nebulae will be rapidly destroyed, through photodissociation by strong UV from the central star. If dense clumps are present during transition, they may allow the gas-phase formation and/or survival of small amounts of some molecules, such as HCN, CN, C2H2, and HC3N, until about 2000 yr after termination of the superwind; and young, fully developed planetary nebulae may show observable amounts of polyatomic molecules by this means. Such clumping may explain the existence of, e.g., HCN in NGC 7027.

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

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.