Photo-Dissociation Regions (PDRs) constitute an impressive fraction of the gas of a galaxy. Recent observations lead by the Madrid and Pan's groups have established that a very rich carbon chemistry is taking place in PDRs: the detection of benzene, the simpler aromatic species; the observation of non-polar carbon chains C3 and C4; the surprisingly high abundance and survival of polar carbon chains and small hydrocarbons, and the spatial correlation found with the polycyclic aromatic hydrocarbons (PAHs) emission.
These results suggest that very interesting processes are taking place in PDRs triggered by the onset of the UV radiation field. Several theoretical models, for instance by the team from the Observatoire de Paris, including the thermal structure, the chemistry and the radiative transfer of atomic/molecular lines have been developed in the recent years to explain the PDRs observations. These codes solve the problem under the hypothesis of local thermal and chemical equilibrium. Nevertheless, these stationary models cannot explain the most outstanding properties recently observed in the organic chemistry. Besides, "real" PDRs have velocity and density gradients as well as moving ionisation and dissociation fronts not taken into account by the static PD R models. Understanding carbon chemistry is of prime importance as it is a step forward to understand the formation of complex organic species (such as the PAHs or the fullerenes) and of prebiotic material (such as simple aminoacids and sugars), which have been detected in the ISM and may be related with the origin of life. Due to the dominant role and ubiquity of PDRs, new observational strategies and further theoretical studies including more realistic phenomena are strongly required to improve our knowledge of the physical and chemical processes taking place in the ISM.
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