Confirmation of Interstellar HOC[TSUP]+[/TSUP]: Reevaluating the [HCO[TSUP]+[/TSUP]]/[HOC[TSUP]+[/TSUP]] Abundance Ratio

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Abstract
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 Ntot ~ 3 × 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 n(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.