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

¹ Université de Toulouse, UPS, CESR, 9 Av. du Colonel Roche, 31028 Toulouse Cedex 4, France
² CNRS, UMR5187, 31028 Toulouse, France

Abstract

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