Is HO₂⁺ a detectable interstellar molecule?

Although molecular oxygen, O₂, has long been thought to be present in interstellar environments, it has only been tentatively detected toward one molecular cloud. The fractional abundance of O₂ determined from these observations is well below that predicted by astrochemical models. Given the difficulty of O₂ observations from ground-based telescopes, identification of a molecule that could be used as a tracer of O₂ in interstellar environments would be quite useful. To this end, we have undertaken a collaborative examination of HO₂⁺ in an attempt to evaluate the feasibility of its detection in interstellar clouds. We have conducted high-level ab initio calculations of its structure to obtain its molecular parameters. The reaction responsible for the formation of HO ₂⁺ is nearly thermoneutral, and so a careful analysis of its thermochemistry was also required. Using the Active Thermochemical Tables approach, we have determined the most accurate values available to date for the proton affinities of O₂ and H₂, and the enthalpy, Gibbs energy, and equilibrium constant for the reaction H₃⁺ + O₂ → HO₂⁺ + H₂. We find that while this reaction is endothermic by 50 9 cm^-1 at 0 K, its equilibrium is shifted toward HO₂⁺ at the higher temperatures of hot cores. We have examined the potential formation and destruction pathways for HO₂⁺ in interstellar environments. Combining this information, we estimate the HO₂ ⁺ column density in dense clouds to be 10⁹ cm ^-2, which corresponds to line brightness temperatures of ≤0.2 mK. If our results prove correct, HO₂⁺ is clearly not a detectable interstellar molecule.