SPATIALLY RESOLVED l-C₃H⁺ EMISSION IN THE HORSEHEAD PHOTODISSOCIATION REGION: FURTHER EVIDENCE FOR A TOP-DOWN HYDROCARBON CHEMISTRY

Small hydrocarbons, such as C₂H, C₃H, and C₃H₂ are more abundant in photo-dissociation regions (PDRs) than expected based on gas-phase chemical models. To explore the hydrocarbon chemistry further, we observed a key intermediate species, the hydrocarbon ion l-C₃H⁺, in the Horsehead PDR with the Plateau de Bure Interferometer at high-angular resolution (6″). We compare with previous observations of C₂H and c-C₃H₂ at similar angular resolution and new gas-phase chemical model predictions to constrain the dominant formation mechanisms of small hydrocarbons in low-UV flux PDRs. We find that at the peak of the HCO emission (PDR position), the measured l-C₃H⁺, C₂H, and c-C₃H₂ abundances are consistent with current gas-phase model predictions. However, in the first PDR layers, at the 7.7 μm polycyclic aromatic hydrocarbon band emission peak, which are more exposed to the radiation field and where the density is lower, the C₂H and c-C₃H₂ abundances are underestimated by an order of magnitude. At this position, the l-C₃H⁺ abundance is also underpredicted by the model but only by a factor of a few. In addition, contrary to the model predictions, l-C₃H⁺ peaks further out in the PDR than the other hydrocarbons, C₂H and c-C₃H₂. This cannot be explained by an excitation effect. Current gas-phase photochemical models thus cannot explain the observed abundances of hydrocarbons, in particular, in the first PDR layers. Our observations are consistent with a top-down hydrocarbon chemistry, in which large polyatomic molecules or small carbonaceous grains are photo-destroyed into smaller hydrocarbon molecules/precursors