2021MNRAS.500.1188L

Methanol, which is one of the most abundant organic molecules in the interstellar medium, plays an important role in the complex grain surface chemistry that is believed to be a source of many organic compounds. Under energetic processing such as ultraviolet (UV) photons or cosmic rays, methanol may decompose into CH₄, CO₂, CO, HCO, H₂CO, CH₃O and CH₂OH, which in turn lead to complex organic molecules such as CH₃OCHO, CHOCH₂OH and HOCH₂CH₂OH through radical recombination reactions. However, although molecular oxygen and its detection, abundance and role in the interstellar medium have been the subject of many debates, few experiments on the oxidation of organic compounds have been carried out under interstellar conditions. The present study shows the behaviour of solid methanol when treated by UV light and thermal processing in oxygen-rich environments. Methanol has been irradiated in the absence and presence of O₂ at different concentrations in order to study how oxidized complex organic molecules may form and also to investigate the O-insertion reaction in the C-H bound to form methanediol HOCH₂OH through a CH₃OH + O(¹D) solid-state reaction. The adding of O₂ in the thermal and photochemical reaction of solid methanol leads to the formation of O₃, H₂O and HO₂, in addition to three main organics, HCOOH, CHOCHO and HOCH₂OH. We show that in an O₂-rich environment, species such as CO, CH₄, HCO, CH₃OH and CHOCH₂OH are oxidized into CO₂, CH₃OH, HC(O)OO, HOCH₂OH and CHOCHO, respectively, while HCOOH might be formed through the H₂CO + O(³P) - (OH + HCO)_cage - HCOOH hydrogen-abstraction reaction.