IS HO2+ A DETECTABLE INTERSTELLAR MOLECULE?

Susanna L. Widicus Weaver

doi:10.1088/0004-637X/697/1/601

2009-05-04

Susanna L. Widicus Weaver *

David E. Woon

Branko Ruščić

Benjamin J. McCall

DOI：10.1088/0004-637X/697/1/601

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摘要

En 中文

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 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 HO2+ 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 -> HO2+ + H-2. 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 similar to 10(9) 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.

Keyword:

ISM: abundances

ISM: molecules