Hydrogenation of CO on interstellar dust: what is the role of water molecules?

Abstract

We performed several co-injection experiments varying the concentration of water molecules within a binary mixture (CO/H₂O) co-condensed with (H/H₂) onto a cold mirror maintained at 10 K. We performed the [CO + H] reaction within a H₂O-rich and a N₂-rich environment and confirmed that the gradual replacement of CO molecules by water molecules greatly enhances the yields of H₂CO and CH₃OH, the main products of CO hydrogenation at 10 K. This trend is observed until the concentration of CO molecules becomes too low and significantly favors the side reaction [H + H] recombination over CO hydrogenation. Irradiation of (CO/H₂O) mixed ices with cold H-atoms highlights the action of water molecules, expressed in the spectra as a decrease in the intensity of water-ice's infrared signatures. The catalytic effect observed can be interpreted by physical, chemical, and energetic considerations. This work is inclined toward extending previously reported results with different experimental conditions.