Is Ho⁺₂a Detectable Interstellar Molecule?

Although molecular oxygen, O 2 , has long been thought to be present in interstellar environments, it has only been tentatively detected toward one molecular cloud. The fractional abundance of O 2 determined from these observations is well below that predicted by astrochemical models. Given the difficulty of O 2 observations from ground-based telescopes, identification of a molecule that could be used as a tracer of O 2 in interstellar environments would be quite useful. To this end, we have undertaken a collaborative examination of HO + 2 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 O 2 and H 2 , and the enthalpy, Gibbs energy, and equilibrium constant for the reaction H + 3 + O 2 → HO + 2 + H 2 . We find that while this reaction is endothermic by 50 ± 9 cm −1 at 0 K, its equilibrium is shifted toward HO + 2 at the higher temperatures of hot cores. We have examined the potential formation and destruction pathways for HO + 2 in interstellar environments. Combining this information, we estimate the HO + 2 column density in dense clouds to be ∼10 9 cm −2 , which corresponds to line brightness temperatures of ⩽0.2 mK. If our results prove correct, HO + 2 is clearly not a detectable interstellar molecule.