2004ApJ...616..966S

HOC⁺ and CO⁺ have been detected toward two well-known photon-dominated regions (PDRs), S140 and NGC 2023, using the Arizona Radio Observatory 12 m telescope. The J=1⟶0 transition of HOC⁺ at 89 GHz and the two spin components of the N=2⟶1 line of CO⁺ near 236 GHz were observed, as well as the J=1⟶0 transitions of H¹³CO⁺ and HC¹⁸O⁺. The J=3⟶2 line of HOC⁺ at 268 GHz was also mapped across the Orion Bar. The [HCO⁺]/[HOC⁺] ratios determined in S140 and NGC 2023 were ∼12,408 and 1913, respectively, values indicative of quiescent molecular gas rather than PDR sources, where the abundance of HOC⁺ is thought to be enhanced. However, the beam in both these measurements may contain material from the adjoining molecular cloud, favoring HCO⁺. Alternatively, the [HCO⁺]/[HOC⁺] ratio may vary with A_vin PDR regions. The [CO⁺]/[HOC⁺] ratio in S140 and NGC 2023, at several positions in the Orion Bar, and in other PDRs, on the other hand, falls uniformly in the range ∼1-10. In addition, the line profiles of CO⁺ and HOC⁺ in the Orion Bar look remarkably similar. The syntheses of HOC⁺ and CO⁺ appear to be correlated in PDRs, most likely through the common precursor, C⁺. The reaction of C⁺ and H₂O is thought to preferentially create HOC⁺, as opposed to HCO⁺, and C⁺+OH or O₂leads favorably to CO⁺.