1999ApJ...519L..89A

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≳7x10^–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.