The Effects of Ion Irradiation on the Evolution of the Carrier of the 3.4 Micron Interstellar Absorption Band

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Abstract
Carbon grains in the interstellar medium evolve through exposure to UV photons, heat, gas, and cosmic rays. Understanding their formation, evolution, and destruction is an essential component of evaluating the composition of the dust available for newly forming planetary systems. The 3.4 μm absorption band, attributed to the aliphatic C–H stretch vibration, is a useful probe of the degree to which energetic processing affects hydrogenated carbon grains. Here we report on the effects of ion bombardment of two different kinds of nano-size hydrogenated carbon grains with different hydrogen content. Grain samples, both with and without a mantle of H2O ice, were irradiated with 30 keV He+ to simulate cosmic-ray processing in both diffuse and dense interstellar medium conditions. The ion fluences ranged between 1.5 × 1013 and 7.9 × 1015 ions cm-2. Infrared and Raman spectroscopy were used to study the effects of ion irradiation on grains. In both the dense and diffuse interstellar medium simulations, ion bombardment led to a reduction of the 3.4 μm band intensity. To discuss the effects of cosmic-ray irradiation of interstellar hydrogenated carbon materials we adopt the approximation of 1 MeV monoenergetic protons. An estimate of the C–H bond destruction cross section by 1 MeV protons was made based on experiments using 30 keV He+ ions and model calculations. In combination with results from our previous studies, which focused on UV irradiation and thermal H atom bombardment, the present results indicate that the C–H bond destruction by fast-colliding charged particles is negligible with respect to that of UV photons in the diffuse ISM. However, in dense cloud regions, cosmic-ray bombardment is the most significant C–H bond destruction mechanism when the optical depth corresponds to values of the visual extinction larger than ~5 mag. The results presented here strengthen the new interpretation of the evolution of the interstellar aliphatic component (i.e., the C–H bonds in the CH2 and CH3 groups) as evidenced by the presence of the 3.4 μm absorption band in the diffuse medium and the absence of such a signature in the dense cloud environment. The evolutionary transformation of carbon grains, induced by H atoms, UV photons, and cosmic rays, indicates that C–H bonds are readily formed, in situ, in the diffuse interstellar medium and are destroyed in the dense cloud environment.