CO2 formation in quiescent clouds; an experimental study of the CO + OH pathway

TL;DR

This study investigates the formation of CO2 in quiescent molecular clouds through experimental analysis of the CO + OH pathway on water and silicate surfaces, revealing low yields due to reaction barriers and competition with other surface reactions.

Contribution

It provides the first experimental and modeling insights into the CO + OH pathway for CO2 formation in cold interstellar environments, highlighting reaction barriers and rate competition.

Findings

Maximum CO2 yield is around 8% relative to CO.

Reaction rate of CO + OH is 24 times slower than O2 + H.

Competition between surface reactions limits CO2 production.

Abstract

The formation of CO2 in quiescent regions of molecular clouds is not yet fully understood, despite CO2 having an abundance of around 10-34 % H2O. We present a study of the formation of CO2 via the non-energetic route CO + OH on non-porous H2O and amorphous silicate surfaces. Our results are in the form of temperature-programmed desorption spectra of CO2 produced via two experimental routes: O2 + CO + H and O3 + CO + H. The maximum yield of CO2 is around 8 % with respect to the starting quantity of CO, suggesting a barrier to CO + OH. The rate of reaction, based on modelling results, is 24 times slower than O2 + H. Our model suggests that competition between CO2 formation via CO + OH and other surface reactions of OH is a key factor in the low yields of CO2 obtained experimentally, with relative reaction rates k(CO+H) \ll k(CO+OH) < k(H2O2+H) < k(OH+H), k(O2+H). Astrophysically, the presence of CO2 in low AV regions of molecular clouds could be explained by the reaction CO + OH occurring concurrently with the formation of H2O via the route OH + H.