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 H 13 CO + and HC 18 O + . 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 H 2 O is thought to preferentially create HOC + , as opposed to HCO + , and C + + OH or O 2 leads favorably to CO + .