2003A&A...406..899F

We have carried out a survey of reactive ions (CO⁺, HOC⁺, HC¹⁸O⁺, SO⁺) and cyclopropenylidene (C₃H₂) in three prototypical photodissociation regions (PDRs), the reflection nebula NGC 7023, the Orion Bar and the planetary nebula (PN) NGC 7027. The reactive ion CO⁺ has been detected in all the targets with fractional abundances ranging from ∼10^–11 to ∼a few 10^–9. Its spatial distribution in NGC 7023 and the Orion Bar show that CO⁺ arises in the innermost part (A_v<2mag) of the PDR. In NGC 7027, the CO⁺ lines shows an expansion velocity higher than that of the CO lines. This high expansion velocity is consistent with the CO⁺ emission arising in the high velocity layer of neutral gas which is being accelerated by the ionized gas. Photochemistry determines the chemical composition of this layer. The reactive ions HOC⁺ and SO⁺ have been detected in NGC 7023 and the Orion Bar. In both sources, the fractional abundance of HOC⁺ is enhanced by a factor of ∼10 towards the PDRs, with typical abundances, X_HOC+=0.7-3x10^–11. This enhancement produces a decrease of the [HCO⁺]/[HOC⁺] abundance ratio towards the PDR. In fact, we have derived [HCO⁺]/[HOC⁺]∼50-120 in NGC 7023, which is the lowest ratio measured thus far. HOC⁺ and SO⁺ have not been detected in NGC 7027. Interestingly, this is the source with the highest CO⁺ abundance, X_CO+=5x10^–9. This lack of detection is interpreted as due to the peculiar chemistry of C-rich PNs, in which the abundance of oxygenated molecules, in particular H₂O, is low. We have detected cyclopropenylidene (C₃H₂) in the three target PDRs. Similarly to the reactive ions, the abundance of C₃H₂ in NGC 7023 and the Orion Bar is a factor of 10-100 higher in the PDRs than in the foreground molecular cloud with peak values ranging from 10^–10 to 10^–9. In NGC 7027, we measured the maximum C₃H₂ abundance with a value of ∼10^–8. Similarly to the case of CO⁺, the high expansion velocities of the C₃H₂ lines in NGC 7027 suggests that its emission arises in the neutral gas which is being accelerated by the ionized gas. Photodestruction of Polycyclic Aromatic Hydrocarbons (PAHs) is proposed to explain the enhanced C₃H₂ abundance in these PDRs.