Heavy-ion irradiation of interstellar carbonaceous and siliceous composite dust analogs. Cosmic ray-grain interactions at the nanometer scale

Interstellar dust made of siliceous and carbonaceous grains is exposed to cosmic ray ions in the diffuse and dense medium. This highly energetic interaction not only affects the morphology of such systems, but also plays a role in altering the surface and bulk chemistry. These two effects are both key points in understanding the evolution of the interstellar medium matter cycle throughout which we observe variations in the dust's spectroscopic signatures. The goal is to better understand, at the nanoscale, the effect of impinging ions on the physical and chemical structure of irradiated dust grains. To experimentally investigate this phenomenon, two types of dust analogs were produced in the laboratory: an organic fraction made of hydrogenated amorphous carbon (a-C:H) samples and polystyrene (PS) samples to which an inorganic component (SiO(_2)) was added. Their irradiation was then conducted at the GSI UNILAC accelerator using a 945.4 MeV 197 )Au(^ 25+ ) beam, simulating the irradiation by Galactic cosmic rays. In situ infrared spectra were acquired during irradiation to follow the sample's chemical evolution, and ex situ transmission electron microscopy (TEM) and energy-dispersive X-ray (EDX) micrographs were recorded to investigate the post-irradiation nanostructure and composition. The radiolysis cross sections of the different dust grain analogs were determined. The TEM and EDX images allowed us to constrain the size of tracks produced by the heavy ions in these matrices and visualize the irradiation effects on the morphology of the different samples. The results are extrapolated in an astrophysical context to quantify the tracks produced in interstellar dust under Galactic cosmic ray irradiation.