Volume 459, Number 1, November III 2006
|Page(s)||147 - 159|
|Section||Interstellar and circumstellar matter|
|Published online||12 September 2006|
Nature and evolution of the dominant carbonaceous matter in interplanetary dust particles: effects of irradiation and identification with a type of amorphous carbon
Institut d'Astrophysique Spatiale, UMR 8617, Bât. 121, Université Paris XI, 91405 Orsay, France
2 Centro de Astrobiología, INTA-CSIC, Carretera de Ajalvir, km. 4, Torrejón de Ardoz, 28850 Madrid, Spain e-mail: email@example.com
3 Department of Astronomy, Box 351580, University of Washington, Seattle, WA 98195, USA
4 Institute of Astronomy, National Central University, No. 300, Jhongda Rd, Jhongli City, Taoyuan County 32049, Taiwan
5 Laboratoire de Sciences de la Terre, ENS Lyon, 46 allée d`Italie, 69007 Lyon, France
6 CSNSM, IN2P3/CNRS, Campus Paris XI, 91405 Orsay, France
Accepted: 28 June 2006
Aims.Interplanetary dust particle (IDP) matter probably evolved under irradiation in the interstellar medium (ISM) and the solar nebula. Currently IDPs are exposed to irradiation in the Solar System. Here the effects of UV and proton processing on IDP matter are studied experimentally. The structure and chemical composition of the bulk of carbon matter in IDPs is characterized.
Methods.Several IDPs were further irradiated in the laboratory using ultraviolet (UV) photons and protons in order to study the effects of such processing. By means of infrared and Raman spectroscopy, IDPs were also compared to different materials that serve as analogs of carbon grains in the dense and diffuse ISM.
Results.The carbonaceous fraction of IDPs is dehydrogenated by exposure to hard UV photons or 1 MeV protons. On the other hand, proton irradiation at lower energies (20 keV) leads to an efficient hydrogenation of the carbonaceous IDP matter. The dominant type of carbon in IDPs, observed with Raman and infrared spectroscopy, is found to be either a form of amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H), depending on the IDP, consisting of aromatic units with an average domain size of 1.35 nm (5-6 rings in diameter), linked by aliphatic chains.
Conclusions.The D- and 15N-enrichments associated to an aliphatic component in some IDPs are probably the result of chemical reactions at cold temperatures. It is proposed that the amorphous carbon in IDPs was formed by energetic processing (UV photons and cosmic rays) of icy grains, maybe during the dense cloud stage, and more likely on the surface of the disk during the T Tauri phase of our Sun. This would explain the isotopic anomalies and morphology of IDPs. Partial annealing, 300-400 °C, is required to convert an organic residue from ice photoprocessing into the amorphous carbon with low heteroatom content found in IDPs. Such annealing might have occurred as the particles approached the Sun and/or during atmospheric entry heating.
Key words: infrared: ISM / infrared: solar system / methods: laboratory / molecular processes
© ESO, 2006
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