Modelling the physical and chemical evolution of PAHs andPAH-related species in astrophysical environments

Abstract

An active carbon chemistry is observed at the border of photo-dissociation regions(PDRs), involving small hydrocarbons, polycyclic aromatic hydrocarbon (PAH) macromoleculesand evaporating very small grains (VSGs). In this context, we aim at quantifying thephysical and chemical evolution of PAHs (hydrogenation and charge states, aggregation, andcomplexation with heavy atoms) as a function of the local physical conditions (radiationfield, temperature, density, abundances of atomic and molecular hydrogen, electrons andheavy atoms). We have developed a numerical model that follows the time dependency of theabundance and internal energy of each species. In this paper, we use this model tocalculate the hydrogenation and charge states of coronene C₂₄H₁₂ asan interstellar PAH prototype. We take advantage of recent results on photodissociationand reaction rates and provide guidelines for future laboratory studies. Reaction rates ofcoronene-derived radical cations with H and H₂ are found to be sufficientlyconstrained by experiments, whereas the absence of experimental data for neutral speciesis critical.