title: High energy electron irradiation of interstellar carbonaceous dust analogs: Cosmic ray effects on the carriers of the 3.4 µm absorption band.
abstract: The effects of cosmic rays on the carriers of the interstellar 3.4 μm absorption band have been investigated in the laboratory. This band is attributed to stretching vibrations of CH3and CH2in carbonaceous dust. It is widely observed in the diffuse interstellar medium (ISM), but disappears in dense clouds. Destruction of CH3and CH2by cosmic rays could become relevant in dense clouds, shielded from the external ultraviolet field. For the simulations, samples of hydrogenated amorphous carbon (a-C:H) have been irradiated with 5 keV electrons. The decay of the band intensity vs electron fluence reflects a-C:H dehydrogenation, which is well described by a model assuming that H2molecules, formed by the recombination of H atoms liberated through CH bond breaking, diffuse out of the sample. The CH bond destruction rates derived from the present experiments are in good accordance with those from previous ion irradiation experiments of HAC. The experimental simplicity of electron bombardment has allowed the use of higher energy doses than in the ion experiments. The effects of cosmic rays on the aliphatic components of cosmic dust are found to be small. The estimated cosmic ray destruction times for the 3.4 μm band carriers lie in the 108yr range and cannot account for the disappearance of this band in dense clouds, which have characteristic lifetimes of 3 × 107yr. The results invite a more detailed investigation of the mechanisms of CH bond formation and breaking in the intermediate region between diffuse and dense clouds.
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain.
authors.forename: Belén
authors.initials: B
authors.lastname: Maté
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain.
authors.forename: Germán
authors.initials: G
authors.lastname: Molpeceres
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain.
authors.forename: Miguel
authors.initials: M
authors.lastname: Jiménez-Redondo
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain.
authors.forename: Isabel
authors.initials: I
authors.lastname: Tanarro
authors.affiliation: Instituto de Estructura de la Materia, IEM-CSIC, Serrano 123, 28006 Madrid, Spain.
authors.forename: Víctor J
authors.initials: VJ
authors.lastname: Herrero
keywords: astrochemistry; dust, extinction; infrared : ISM; methods: laboratory: molecular; techniques: spectroscopic