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

摘要

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 mu m 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.