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 (HCO+)-C-13 and (HCO+)-O-18. 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 similar to12,408 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 similar to1-10. 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 O-2 leads favorably to CO+.