H₂ formation on PAHs in photodissociation regions

Abstract

Aims. Molecular hydrogen is the most abundant molecule in the Universe. It is thought that a large portion of H₂ forms by association of hydrogen atoms to polycyclic aromatic hydrocarbons (PAHs). We model the influence of PAHs on total H₂ formation rates in photodissociation regions (PDRs) and assess the effect of these formation rates on the total cloud structure. Methods. We set up a chemical kinetic model at steady state in a PDR environment and included radiative transfer to calculate the chemistry at different depths in the PDR. This model includes known dust grain chemistry for the formation of H₂ and a H₂ formation mechanism on PAHs. Since H₂ formation on PAHs is impeded by thermal barriers, this pathway is only efficient at higher temperatures (T> 200 K). At these temperatures the conventional route of H₂ formation via H atoms physisorbed on dust grains is no longer feasible, so the PAH mechanism enlarges the region where H₂ formation is possible. Results. We find that PAHs have a significant influence on the structure of PDRs. The extinction at which the transition from atomic to molecular hydrogen occurs strongly depends on the presence of PAHs, especially for PDRs with a strong external radiation field. A sharp spatial transition between fully dehydrogenated PAHs on the outside of the cloud and normally hydrogenated PAHs on the inside is found. As a proof of concept, we use coronene to show that H₂ forms very efficiently on PAHs, and that this process can reproduce the high H₂ formation rates derived in several PDRs.