Influence of Interstellar Icy Mantle Composition on Acetaldehyde and Formamide Formation Induced by Low-Energy Cosmic Rays

Icy grain mantles play a crucial role in the chemistry of star-forming regions, allowing reactions that contribute to the formation of astrophysically complex organic molecules (COMs), including acetaldehyde (CH3CHO) and formamide (NH2CHO). Both compounds are also revealed in cometary comae. Laboratory experiments allowed us to reveal the formation of acetaldehyde and formamide in ices exposed to energetic sources, suggesting that the exposure of icy mantles and surfaces to Galactic cosmic rays (GCR) and solar charged particles triggers the formation of COMs. In this article, we provide data on the relevance of the initial ice composition on the formation and abundance of acetaldehyde and formamide. Furthermore, we aim to understand the time scales required in young star-forming regions to form both compounds through the ion-bombardment of icy grain mantles. We simulated the exposure of icy grain mantles to energetic charged particles in the laboratory. We produced a large set of icy mixtures comprising various molecules detected toward the solid phase of star-forming regions, including H2O, CO, CH3OH, CH4, NH3, and N2, and we exposed them to 200 keV H+ at 17 K simulating low-energy GCR. We followed the chemical evolution of ices by means of infrared spectroscopy. In the spectra, we looked for absorption features attributed to acetaldehyde (7.40 μm) and formamide (7.20 μm). We also provide estimations of their abundance as a function of the dose in the various investigated mixtures. The analysis revealed that the ice composition plays a primary role in both the destruction of the deposited species and the formation of new compounds. Water–methanol–ammonia mixtures are the most efficient in the formation of both COMs. The time scales required for their formation fall in the typical lifetimes of pre- and protostellar phases and strongly depend on the cosmic-ray ionization rate taken into account. We show that ices exposed to energetic ions can form both acetaldehyde and formamide. Abundances are found to be strongly related to the initial ice composition. We thus suggest that the bombardment of icy grain mantles by GCR is a relevant player in the formation of both compounds in the lifetime of pre- and protostellar objects.