2020ApJ...894...98M

Complex organic molecules (COMs) can be produced by energetic processing of interstellar ice mantles accreted on top of dust grains. Two COMs with proposed energetic ice formation pathways are formamide and acetaldehyde. Both have been detected in solar system comets and in different circumstellar and interstellar environments. In this work, we study the NH₂CHO and CH₃CHO formation upon UV photoprocessing of CO:NH₃ and CO:CH₄ ice samples. The conversion from NH_{dot}2_ radicals to NH₂CHO is 2-16 times higher than the conversion from CH_{dot}3_ radicals to CH₃CHO under the explored experimental conditions, likely because the formation of the latter competes with the formation of larger hydrocarbons. In addition, the conversion of NH_{dot}2_ into NH₂CHO at 10 K increases with the NH₃ abundance in the ice, and also with the temperature in CO-dominated CO:NH₃ ices. This is consistent with the presence of a small NH_{dot}2_ and HCO. reorientation barrier for the formation of NH₂CHO, which is overcome with an increase in the ice temperature. The measured NH₂CHO and CH₃CHO formation efficiencies and rates are similar to those found during electron irradiation of the same ice samples under comparable conditions, suggesting that both UV photons and cosmic rays would have similar contributions to the solid-state formation of these species in space. Finally, the measured conversion yields (up to one order of magnitude higher for NH₂CHO) suggest that in circumstellar environments, where the observed NH₂CHO/CH₃CHO abundance ratio is ∼0.1, there are likely additional ice and/or gas-phase formation pathways for CH₃CHO.