Detection of HOC⁺ toward the orion bar and M17-SW: Enhanced abundances in photon-dominated regions?

New measurements of HOC⁺ emission have been carried out toward several photon-dominated regions using the National Radio Astronomy Observatory 12 m telescope. Toward the Orion bar, the J = 1 → 0, 2 → 1, and 3 → 2 transitions of HOC⁺ at 89, 179, and 268 GHz were detected, as well as the J = 1 → 0 line in M17-SW. In conjunction with observations of HC¹⁸O⁺, [HCO⁺]/[HOC⁺] abundance ratios have been established in these objects. Toward M17-SW, a ratio of ∼2260 was found. In the Orion bar, on the other hand, this ratio was determined to be [HCO⁺]/[HOC⁺] ∼ 270, both from J = 1 → 0 and J = 2 → 1 measurements - the lowest ratio thus far found in the Galaxy, and remarkably different from the value of 1800 found in nearby OMC-1. Moreover, the fractional abundance of HOC⁺ was found to be f ≳ 7 × 10^-12 in this source, the highest value yet observed for this molecular ion. The Orion bar observations suggest that the abundance of HOC⁺ is enhanced relative to HCO⁺ because of the unique chemistry of photon-dominated regions.