Poster

Effects of Cosmic Rays on the 3.4 mm Interstellar Absorption Band

^1,2Marie Godard, ³Géraldine Féraud, ⁴Marin Chabot, ³Thomas Pino, ²Rosario Brunetto, ⁵Yvain Carpentier, ⁶Jean Duprat, ⁶Cécile Engrand, ³Philippe Bréchignac, ²Louis d'Hendecourt, ²Emmanuel Dartois (¹NASA Goddard Space Flight Center, ²Institut d'Astrophysique Spatiale, ³Institut de Sciences Molecular d'Orsay, ⁴Institut de Physique Nuclear d'Orsay, ⁵Laboratoire de Physique des Lasers, Atomes et Molecules, ⁶Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse)

An important component of the interstellar carbonaceous dust is observed through the 3.4 mm absorption band (stretching modes of the C-H bonds in methyl (-CH3) and methylene (-CH2-) groups), ubiquitous along sight lines that sample the diffuse ISM of the Milky Way and other galaxies. The corresponding bending modes (6.85 and 7.25 mm) are also observed. These interstellar signatures of aliphatic hydrocarbon dust are well reproduced by hydrogenated amorphous carbons (a-C:H or HAC) that can be synthesized in laboratory to produce interstellar analogues. One of the puzzling observations concerning this carbonaceous dust component is the fact that the 3.4 mm feature is widely detected in the diffuse ISM, but is not observed in dense clouds. Considering the rapid cycling of dust between these two interstellar phases, it implies an efficient mechanism for the destruction/modification of aliphatic hydrocarbon dust.

As energetic particles are known to dehydrogenate aliphatic materials, the exposition of interstellar dust to cosmic rays could play a role in the disappearance of the 3.4 mm feature in dense clouds. To investigate the evolution of the carbonaceous dust due to cosmic ray exposure, we have performed experiments of carbonaceous interstellar analogues irradiation by swift ions. Samples of a-C:H and soot have been exposed to different ions (from hydrogen to iodine) and swift energies (between 0.2 and 160 MeV). This allows to reproduce a wide range of conditions comparable to the distribution of elements and energies of interstellar cosmic rays. The induced dehydrogenation and chemical bonding modifications were studied with in situ infrared spectroscopy. The decreasing absorbance of the aliphatic C-H features with the fluence has been carefully analysed with a model describing the mechanisms of hydrogen loss in the material. The results are compared to the processing by UV photons and H atoms in the different interstellar regions.

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