The formation of carbon dioxide in molecular cores by a non-energetic route

The formation of CO₂ in quiescent molecular cores has long been of interest to astrochemists as CO₂ is one of the most abundant solid phase molecules present in the interstellar medium. Previous studies have concentrated, for the most part, on formation mechanisms involving high energy particle or UV bombardment of ices, to mimic the influence of cosmic rays on solid phase species in the outer, lower density regions of molecular clouds. However, condensed phase CO₂ is also observed in the inner, denser regions of clouds, where less UV radiation penetrates. To date, very few studies have been made of CO₂ formation in the absence of energetic particles. Low-energy routes to the formation of CO₂ are crucial to explain the high abundances of this molecule observed in quiescent regions. We discuss recent experimental results of a study of the low-energy reaction CO + OH. A simple model, developed to describe the kinetics of the system, suggests that the various reactions of the OH radical are key in characterising the production of CO₂ and other species. Our results indicate that some CO₂ forms concurrently with H₂O in molecular clouds, in line with both previous observations and theory. The results of this research are published in Noble et al. (2011).