Confirmation of interstellar HOC⁺: Reevaluating the [HCO⁺]/[HOC⁺] abundance ratio

The identification of interstellar HOC+ has been confirmed through new observations of this species at 3, 2, and 1.2 mm using the NRAO 12 m telescope. Toward Sgr B₂ (OH), the J = 2 → 1 and J = 3 → 2 lines of this ion at 179 and 268 GHz were detected for the first time. In addition, the J = 1 → 0 line of HOC⁺ at 89 GHz was observed toward several new sources, including Orion-KL. In conjunction with past observations of the J = 1 → 0 transition toward Sgr B2 (OH), these data clearly establish the presence of this species in the interstellar medium. The column density toward Sgr B₂ (OH) is estimated to be N_tot ~ 3 X 10¹² cm-2, which corresponds to a fractional abundance relative to H₂ of f ~ 10-12. Measurements of HCO+ were also carried out toward Sgr B₂ (OH), resulting in an abundance ratio of [HCO⁺]/[HOC⁺] ~ 140-360 at gas densities of m(H₂) > 10⁵ cm^-3. This ratio contradicts past predictions of ion-molecule models by several orders of magnitude and suggests that the reaction HOC⁺ + H₂ → HCO⁺ + H₂ is not as fast as previously thought.