Formation of unsaturated hydrocarbons in interstellar ice analogues by cosmic rays

Abstract

The formation of C=C and C≡C bonds from the processing of pure c-C₆H₁₂ (cyclohexane) and mixed H₂O:NH₃:c-C₆H₁₂ (1:0.3:0.7) ices by highly charged and energetic ions (219-MeV ¹⁶O^{7 +} and 632-MeV ⁵⁸Ni^{24 +}) is studied. The experiments simulate the physical chemistry induced by medium-mass and heavy-ion cosmic rays in interstellar ice analogues. The measurements were performed inside a high vacuum chamber at the heavy-ion accelerator Grand Accelératéur National d'Ions Lourds (GANIL) in Caen, France. The gas samples were deposited on to a polished CsI substrate previously cooled to 13 K. In situ analysis was performed by a Fourier transform infrared spectrometry at different ion fluences. Dissociation cross-section of cyclohexane and its half-life in astrophysical environments were determined. A comparison between spectra of bombarded ices and young stellar sources indicates that the initial composition of grains in these environments 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 products, 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 production of unsaturated hydrocarbons and possibly aromatic rings (via dehydrogenation processes) in interstellar ices induced by cosmic ray bombardment.