Formation of unsaturated hydrocarbons in interstellar ice analogues by cosmic rays

The formation of C=C and C≡C bonds from the processing of pure c-C6H12 (cyclo- hexane) and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ices by highly-charged, and energetic ions (219 MeV 16 O 7+ and 632 MeV 58 Ni 24+ ) is studied. The experiments simulate the physical chemistry induced by medium-mass and heavy-ion cosmic rays in interstellar ices analogs. The measurements were performed inside a high vacuum chamber at the heavy-ion acceler- ator GANIL (Grand Accelerateur National d'Ions Lourds) i n Caen, France. The gas samples were deposited onto a polished CsI substrate previously cooled to 13 K. In-situ analysis was performed by a Fourier transform infrared (FTIR) spectrometry at different ion fluences. Dis- sociation cross section of cyclohexane and its half-life in astrophysical environments were determined. A comparison between spectra of bombarded ices and young stellar sources in- dicates that the initial composition of grains in theses env ironments should contain a mixture of H2O, NH3, CO (or CO2), simple alkanes, and CH3OH. Several species containing double or triple bounds were identified in the radiochemical produc ts, such as hexene, cyclohexene, benzene, OCN − , CO, CO2, as well as several aliphatic and aromatic alkenes and alkynes. The results suggest an alternative scenario for the product ion of unsaturated hydrocarbons and possibly aromatic rings (via dehydrogenation processes) in interstellar ices induced by cosmic ray bombardment.