IS HO+2A DETECTABLE INTERSTELLAR MOLECULE?

Abstract

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 HO + 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 + 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 H + +O 2 → HO + +H 2. We find that while this reaction is endothermic by 50 ± 9c m −1 at 0 K, its equilibrium is shifted toward HO + at the higher temperatures of hot cores. We have examined the potential formation and destruction pathways for HO + in interstellar environments. Combining this information, we estimate the HO + column density in dense clouds to be ∼10 9 cm −2 , which corresponds to line brightness temperatures of0.2 mK. If our results prove correct, HO + is clearly not a detectable interstellar molecule.