Ion chemistry in photon-dominated regions: Examining the [HCO ⁺]/[HOC⁺]/[CO^-] chemical network

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_v in 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 O₂ leads favorably to CO ⁺.