Formation of NH 2 CHO and CH 3 CHO upon UV Photoprocessing of Interstellar Ice Analogs

Abstract

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 NH 2 CHO and CH 3 CHO formation upon UV photoprocessing of CO:NH 3 and CO:CH 4 ice samples. The conversion from radicals to NH 2 CHO is 2–16 times higher than the conversion from radicals to CH 3 CHO under the explored experimental conditions, likely because the formation of the latter competes with the formation of larger hydrocarbons. In addition, the conversion of into NH 2 CHO at 10 K increases with the NH 3 abundance in the ice, and also with the temperature in CO-dominated CO:NH 3 ices. This is consistent with the presence of a small and HCO. reorientation barrier for the formation of NH 2 CHO, which is overcome with an increase in the ice temperature. The measured NH 2 CHO and CH 3 CHO 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 NH 2 CHO) suggest that in circumstellar environments, where the observed NH 2 CHO/CH 3 CHO abundance ratio is ∼0.1, there are likely additional ice and/or gas-phase formation pathways for CH 3 CHO.