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 GHz 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_iot ∼ (7 × 10¹⁰)-(3 × 10¹²) cm^-2, which correspond to fractional abundances of f∼(2 × 10^-13)-(3 × 10^-12) relative to H₂. Observations of the J = 1 → 0 transition of HC¹⁸O⁺ at 85 GHz have also been carried out, resulting in [HCO⁺]/[HOC⁺] ratios of 3606000. These values are in good agreement with recent modeling by Herbst & Woon and confirm that the reaction of HOC⁺ with H₂ 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 ≲, 50 mK) at 179 GHz, proving that measurements near the atmospheric water line at 183 GHz are not as difficult as previously thought.