Increasing chemical complexity in PDRs

Oral presentation given at the European Week of Astronomy and Space Science (EWASS 2016) held in Athens (Greece) on July 4-8th, 2016. S8 - Interstellar dust and gas coupling.We have investigated the presence of complex organic molecules (COMs) in dense photo-dissociation regions (PDRs). These environments have been traditionally considered too harsh to maintain a complex chemistry and thus significant abundances of polyatomic organic species (excluding PAHs). We have used the IRAM-30m millimetre-wave telescope and carried out two line surveys, complemented with maps, of the Orion Bar and Horsehead edges. These nearly edge-on PDRs provide excellent templates to study the molecular content and spatial stratification in strongly and mildly UV-irradiated material (G0>104 for the Bar, G0~102 for the Horsehead). In these PDRs, the UV-driven chemistry can be investigated for different gas and dust temperatures (Tgas~150K and Tdust~75K for the Bar, and Tgas~60K and Tdust~30K for the Horsehead). Warm grains at the edge of the Orion Bar should be mostly ice mantle-free grains. The dust temperature in the Horsehead PDR, however, is significantly lower than the sublimation temperature of abundant interstellar ices (water, methanol, etc.). The study of these two prototypical PDRs provide a unique opportunity to discuss the role of pure warm gas-phase reactions and ice-mantle photo-processing in the formation of COMs. Our observations show the detection of hundreds of lines from COMs in both PDRs. Surprisingly, the abundance of several COMs is enhanced in the UV-illuminated material. The presence of COMs in the interstellar medium is thus more widespread that initially expected. COM formation reflects the complicate interplay between gas and grain surface chemistry. However, the specific formation pathways are not fully clear yet. More theoretical work and laboratory experiments, constraining formation routes and reaction rates at different temperatures, as well as photo-desorption yields and chemical desorption rates are clearly needed