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.

ON THE MOLECULAR COMPLEXITY OF THE HOT CORES IN ORION-A - GRAIN SURFACE-CHEMISTRY AS THE LAST REFUGE OF THE SCOUNDREL

We have modeled the gas phase chemistry of warm molecular material around protostars that is seeded with evaporating grain mantles. We show that the release of simple molecules into the gas drives ion-molecule and neutral chemistries which can account for many of the complex 0-bearing and N-bearing molecules observed in hot cores. Initial grain mantle components and secondary product molecules are identified, and the observational consequences are discussed.

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.

PHOSPHORUS CHEMISTRY IN DENSE INTERSTELLAR CLOUDS

I have used recent laboratory studies on the reactions of the phosphorus hydride ions, PH(n)+ (n = 0-4) to construct a new model of phosphorus chemistry in interstellar clouds. I find that the non-detection of PN in cold, dark clouds in consistent with the chemical models only if the depletion of phosphorus in large, approximately 10(4) in TMC-1. Although the laboratory studies indicate that organo-phosphorus chains C(n)P can be formed, this large depletion precludes the detection of any phosphorus-bearing moleclues in cold clouds. However, in warm clouds associated with star formation, the depletion of phosphorus may be reduced. In this case one can reproduce the PN abundance toward Orion KL with a depletion factor of about 300. Interestingly, if the organo-phosphorus species are not destroyed by O atoms, I predict fractional abundances in Ori KL of between 10(-11) and 10(-10) for C(n)P (n = 2-4) and HCCP.