Laboratory Studies of Polycyclic Aromatic Hydrocarbons: the Search for Interstellar Candidates

International audiencePolycyclic Aromatic Hydrocarbons (PAHs) are considered as a major constituent of interstellar dust. They have been proposed as the carriers of the Aromatic Infrared Bands (AIBs) observed in emission in the mid-IR. They likely have a significant contribution to various features of the extinction curve such as the 220 nm bump, the far-UV rise and the diffuse interstellar bands. Emission bands are also expected in the far-IR, which are better fingerprints of molecular identity than the AIBs. They will be searched for with the Herschel Space Observatory. Rotational emission is also expected in the mm range for those molecules which carry significant dipole moments. Despite spectroscopic studies in the laboratory, no individual PAH species could be identified. This emphasises the need for an investigation on where interstellar PAHs come from and how they evolve due to environmental conditions: ionisation and dissociation upon UV irradiation, interactions with electrons, gas and dust. Insights into this question have recently been obtained from the analysis of the AIB spectra in different environments including mild UV-excited photodissociation regions, planetary nebulae and protoplanetary disks. PAH species are found -(i)- to be produced by destruction of very small grains (VSGs), -(ii)- to have an evolving charge state: neutral or cationic, and -(iii)- to undergo severe destruction in highly UV-irradiated environments, only the largest species being able to survive. There is also evidence for PAH species to contribute to the depletion of heavy atoms from the gas phase, in particular Si and Fe. This paper illustrates how laboratory work can be inspired from observations. In particular there is a need for understanding the chemical properties of PAHs and PAH-related species, including VSGs, in physical conditions that mimic those found in interstellar space. This motivates a joint effort between astrophysicists, physicists and chemists. Such interdisciplinary studies are currently performed, taking advantage of the PIRENEA set-up, a cold ion trap dedicated to astrochemistry