Studies of astrophysical ices by heavy cosmic ray: radiolysis of methanol

Full text. The chemical and physical effects induced by fast heavy ions irradiation on frozen pure methanol (CH3OH) at 15 K are studied. These energetic ions can simulate electronic energy transfer processes produced by cosmic ray irradiation of interstellar ices, comets, and icy solar system bodies. Measurements were performed in CIMAP's Casimir set-up at the medium energy beam-line of the heavy ion accelerator GANIL (Grand Accelerateur National d'Ions Lourds), Caen/France. The analysis was done by infrared (IR) spectroscopy, before and after irradiation by 16 MeV 16O5+, 220 MeV 16O7+, 606 MeV 65Zn20+ and 774 MeV 86Kr+ ion beams. Integrated values of the absorbance of methanol bands are determined. The induced CH3OH dissociation gives rise to the formation of molecular species such as H2CO, CH2OH, CH4, CO, CO2, H CO and HCOOCH3. Their formation and dissociation cross sections are determined. H2CO, CH4 and CO are the most abundant molecules for all beams. The analysis of the carbon/oxygen budget of products and of CH3OH radiolysis shows that the decrease of the column density of carbon atoms contained in the methanol in a good approximation corresponds to the sum of the column densities of the newly formed species. The destruction cross section of CH3OH follows a power law as a function of the electronic stopping power. The G values for CH3OH destruction by fast heavy ion irradiation are found to be considerably larger than those for protons or helium. As astrophysical implication, the current results allow to estimate chemical reaction rates in ices covering interstellar grains