The role of PAHs in the physics of the interstellar medium

Institut d’Astrophysique Spatiale, UMR 8617, University Paris-Sud 11, Bât. 121, 91405 Orsay Cedex, France

Abstract

In the interstellar medium (ISM), PAHs are abundant and also carry most of the dust surface. They are thus privileged sites for surface reactions such as the formation of H₂. In regions penetrated by UV photons, PAHs loose electrons by the photoelectric effect and efficiently heat the gas. In more shielded regions, PAH recombine and may carry an important fraction of the cloud electronic charge which plays an important role in the gas dynamics and chemistry. We review here processes involving PAHs which control key aspects of the physics of the ISM. We also discuss the corresponding observational constraints. Most of these processes involve a detailed knowledge of the charge of PAHs and we therefore review current models in this area. We argue that more laboratory measurements of the rate of electronic capture on large PAH cations are needed.