The influence of cosmic rays on the 3.4 microns interstellar absorption band
Résumé
A 3.4 μm absorption band, assigned to aliphatic C-H stretching modes of hydrogenated amorphous carbons (a-C:H), is widely observed in diffuse interstellar medium (ISM), but the only absorption occurring in this wavelength range in dense clouds is a large featureless band at 3.47 μm whose origin and attribution is still debated. Cosmic ray bombardment is one of the interstellar processes destroying the 3.4 μm feature. With the present work, we aim at carefully experimentally investigating the effects of such cosmic rays on the interstellar 3.4 μm absorption band carriers. Samples of carbonaceous interstellar analogs (a-C:H and soot) have been irradiated by swift ions in the MeV range, and monitored by infrared spectroscopy. These ion irradiations result in an effective dehydrogenation and chemical bonding modification of the samples, seen in particular through the decreasing absorbance of the 3.4 μm feature with the fluence (cf. figure). These experimental results allow a proper destruction cross-section determination over a large range of energies. Combined to an ISM cosmic ray distribution, we discuss the implications in different astrophysical environments.