Modifications of astrophysical ices induced by cosmic rays

I. Water, carbon monoxide, and methanol mixtures

¹ Facultad de Ciencias Químicas, Universidad de Cuenca, Cuenca 010168, Ecuador
e-mail: cfmejiag@gmail.com
² Departament of Physics – CEFET/RJ, Av. Maracanã 229, Rio de Janeiro 20271-110, Brazil
³ Centre de Recherche sur les Ions, les Matériaux et la Photonique, CIMAP-CIRIL-GANIL, Normandie Université, ENSICAEN, UNICAEN, CEA, CNRS, 14000 Caen, France
⁴ Department of Physics, PUC-Rio, Rua Marquês de São Vicente 225, Rio de Janeiro, RJ 22451-900, Brazil

Received: 28 December 2023
Accepted: 17 April 2024

Abstract

Aims. Astrophysical ices on dust grain mantles in the interstellar medium (ISM) and dense circumstellar envelopes (CSEs) are continuously exposed to galactic cosmic rays (GCRs). In a laboratory setting, we studied the physical and chemical modifications of ice layers induced by energetic heavy ions as GCR analogues. The ice layers used have a molecular composition similar to that of icy grain mantles.

Methods. Mixtures of H₂O:CO:CH₃OH molecules (percentages 73:24:3, 68:30:3, and 58:38:3) were condensed on a substrate at 15 K and irradiated with 40 MeV ⁵⁸Ni¹¹⁺ ion beams. Irradiation-induced modifications were followed using the mid-infrared absorption spectroscopy technique.

Results. We observed the evolution of infrared bands of CO₂, HCO, HCOOH, CH₄, H₂CO,H₂O₂, and more complex synthesised molecules. From the molecular column densities, cross-sections and sputtering yields were determined and compared to published results of water and carbon monoxide. Analysis of the chemical modifications reveals that the precursors are easily destroyed when they are in a molecular mixture, while others are desorbed.

Conclusions. The main radiolitic modifications induced by GCR irradiations are molecular decomposition and sputtering. Extrapolation to astrophysical radiation conditions shows a strong dependence on the intensity of the GCR distributions at low energies, which allows the analysis of the ice evolution at timescales comparable to those of the ISM and CSE.