New observations of the [HCO+]/[HOC+] ratio in dense molecular clouds

New measurements of the millimeter-wave transitions of HOC+ toward dense molecular clouds have been carried out using the NRAO 12 m telescope. The J = 1 --> 0 transition of HOC+ at 89 GHz has been detected for the first time toward DR21 (OH:), W51M, W3 (OH), NGC 7538, Orion (3N 1E), NGC 2024, and G34.3. The J = 2 --> 1 and J = 3 --> 2 lines at 179 GHz and 268 GH2 have also been observed in W51 and Orion KL. In conjunction with past observations of HOC+ toward Orion KL and SgrB2 (OH) by Ziurys & Apponi, these data confirm the presence of this ion in the interstellar medium. The column densities of HOC+ were found to be N-tot similar to (7 x 10(10))-(3 x 10(12)) cm(-1) which correspond to fractional abundances of f similar to(2 x 10(-13))-(3 x 10(-12)) relative to H-2. Observations of the J = 1 --> 0 transition of (HCO+)-O-18 at 85 GHz have also been carried out, resulting in [HCO+]/[HOC+] ratios of 360-6000. These values are in good agreement with recent modeling by Herbst & Woon and confirm that the reaction of HOC+ with H-2 indeed has a substantial barrier at low (T < 50 K) temperatures. The widespread abundance of HOC+ additionally suggests that there is not a strong preference for synthesis of this ion in photon-dominated regions, although there may be a small enhancement toward such gas. Finally, confirmation of interstellar HOC+ has been accomplished through observations of weak emission (T-R less than or similar to 50 mK) at 179 GHz, proving that measurements near the atmospheric water line at 183 GHz are not as difficult as previously thought.