Ion irradiation of carbonaceous interstellar analogues

Abstract

Context. A 3.4 mu m absorption band (around 2900 cm(-1)), assigned toaliphatic C-H stretching modes of hydrogenated amorphous carbons (a-C:H), is widely observed in the diffuse interstellar medium, butdisappears or is modified in dense clouds. This spectral differencebetween different phases of the interstellar medium reflects theprocessing of dust in different environments. Cosmic ray bombardment isone of the interstellar processes that make carbonaceous dust evolve.Aims. We investigate the effects of cosmic rays on the interstellar 3.4mu m absorption band carriers.Methods. Samples of carbonaceous interstellar analogues (a-C: H andsoot) were irradiated at room temperature by swift ions with energy inthe MeV range (from 0.2 to 160 MeV). The dehydrogenation and chemicalbonding modifications that occurred during irradiation were studied withIR spectroscopy.Results. For all samples and all ions/energies used, we observed adecrease of the aliphatic C-H absorption bands intensity with the ionfluence. This evolution agrees with a model that describes the hydrogenloss as caused by the molecular recombination of two free H atomscreated by the breaking of C-H bonds by the impinging ions. Thecorresponding destruction cross section and asymptotic hydrogen contentare obtained for each experiment and their behaviour over a large rangeof ion stopping powers are inferred. Using elemental abundances andenergy distributions of galactic cosmic rays, we investigated theimplications of these results in different astrophysical environments.The results are compared to the processing by UV photons and H atoms indifferent regions of the interstellar medium.Conclusions. The destruction of aliphatic C-H bonds by cosmic raysoccurs in characteristic times of a few 10(8) years, and it appears thateven at longer time scales, cosmic rays alone cannot explain theobserved disappearance of this spectral signature in dense regions. Indiffuse interstellar medium, the formation by atomic hydrogen prevailsover the destruction by UV photons (destruction by cosmic rays isnegligible in these regions). Only the cosmic rays can penetrate intodense clouds and process the corresponding dust. However, they are notefficient enough to completely dehydrogenate the 3.4 mu m carriersduring the cloud lifetime. This interstellar component should bedestroyed in interfaces between diffuse and dense interstellar regionswhere photons still penetrate but hydrogen is in molecular form.