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 (HCO+)-O-18, [HCO+]/[HOC+] abundance ratios have been established in these objects. Toward M17-SW, a ratio of similar to 2260 was found. In the Orion bar, on the other hand, this ratio was determined to be [HCO+]/[HOC+] similar to 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 greater than or similar to 7 x 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.