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Poster contributions

Comparison of desorption energies and behavior of two isomers, DME and EtOH ices, on different analogues of interstellar dust grains
H. Mokrane, H. Chaabouni, M. Accolla, E. Matar, E. Congiu, F. Dulieu and J.L. Lemaire Université de Cergy-Pontoise & Observatoire de Paris, LERMA, UMR 8112 du CNRS, 95000 Cergy-Pontoise, France
(Observatoire de Paris et Université de Cergy-Pontoise)
The surface chemistry of CO, involving successive reactions network with O, H and traces of C and N, leads through different chemical pathways to the formation of many hydrogenated and complex compounds. These species are detected around many luminous proto-stars, the so-called hot cores that are usually surrounded by regions of warm (>100K) and dense (>106 cm-3) material. It is now well accepted that the light species (H2O, CO2, NH3, HCOOH...) are formed during the cold cloud phase (<50K) and are evaporating or sublimating during the early phase of star formation when ice covered grains are heated in the 50-150K range. But when heavier molecules are formed is still subject to discussion. Among these heavy molecules are two important isomeric compounds, CH3OCH3 (Di Methyl Ether or DME) and CH3CH2OH (Ethanol or EtOH) which are presenting very different abundances. Is this due to their respective formation mechanisms or to their desorption energies and behavior? We have studied the desorption of these two molecular ices using the FORMOLISM experimental setup at Cergy and performing TPD (thermally programmed desorption) experiments. We have measured their desorption energies and have compared their behavior on different analogues of interstellar dust grains (silicates and different morphologies of water ice). Our measurements confirm theoretical calculations. They also could explain some observational results in hot cores.

This work is funded in part by the ANR (Agence Nationale de la Recherche) under contract 07-BLAN-0129, the CR IdF (Conseil Régional d'Ile de France) under the SESAME contract I-07-597R and the CGVO95 (Conseil Général du Val d'Oise).



Morphology changes of interstellar water ice analogues after hydrogen atom exposure
Mario Accolla a,b, Emanuele Congiu a, François Dulieu a, Henda Chaabouni a, Elie Matar a, Hakima Mokrane a, Giulio Manicò b, Valerio Pirronello b & Jean-Louis Lemaire. a Université de Cergy-Pontoise and Observatoire de Paris, LERMA, UMR 8112 du CNRS, 95000 Cergy-Pontoise, France b Università di Catania, DMFCI, 95125 Catania, Sicily, Italy
(Observatoire de Paris et Université de Cergy-Pontoise)
Experiments on interstellar water ice analogues show changes in the ice morphology as a consequence of atomic hydrogen exposure. Our work gives evidence that a thin highly porous ice is gradually changed into a more compact structure, with increasing H-atom fluence.

Spectroscopic observations of cold and dense clouds show the presence of ``dirty ice'' mantles on dust grains, mainly composed by water molecules. These ices are enriched by the presence of other simple species that are either formed by surface reactions or accreted from the gas phase. While there is quite a general consensus that interstellar water ice is mainly amorphous, its morphology still remains poorly known. Morphology is important due to its influence both on the catalytic efficiency of grain surfaces and on the release to the grain of the fraction of the formation energy of species, as shown by laboratory simulations of molecular hydrogen formation1. Ice porosity may be identified through the weak infrared absorption features ($\sim$ 2.7 $\sim$m) showing the presence of dangling bonds on the pore surface. To our knowledge, there has been to date no detection of such absorptions in the infrared spectra of interstellar ices, perhaps suggesting that they may have a compact nature2. It has been already investigated that interstellar porous ice may be compacted by the transient heating of stellar radiation3 and cosmic ray bombardment4.

We report in a poster our experimental work, performed using FORMOLISM (the experimental apparatus at the Cergy-Pontoise University), that shows relevant changes in the ice morphology following atomic hydrogen irradiation. In particular, it is shown that a thin highly porous ice film is gradually changed into a more compact structure. This is probably due to the transient heating caused by the energy released to the ice during H2 formation. Such a process may also produce in the interstellar space compact amorphous ice mantles concurrently with the other envisaged processes.

Reference: 1 L. Hornekar, A. Baurichter, V. V. Petrunin, D. Field & A. C. Luntz, 2003, Science, 302, 1943; 2 J. V. Keane, A. C. A. Boogert, A. G. G. M. Tielens, P. Ehrenfreund, & W. A. Schutte, 2001a, A&A, 75, L43; 3 D. Chakarov & B. Kasemo, 1998, Phys. Rev. Lett, 81, 5181, 4 M. E. Palumbo, 2005, J. Phys. Conf. Ser., 6, 211.

This work is funded in part by the ANR (Agence Nationale de la Recherche) under contract 07-BLAN-0129, the CR IdF (Conseil Régional d'Ile de France) under the SESAME contract I-07-597R and the CGVO95 (Conseil Général du Val d'Oise).



Observations of atomic metals in the circumstellar envelope of the carbon-rich dusty star IRC+10216
Mauron N., and Huggins, P. (Graal, NYU)
(Graal, CNRS et UM2 Montpellier)
We have detected gas-phase atomic metals in the envelope of IRC+10216, the archetype dusty carbon-rich AGB star. We found absorption lines of neutral species, Na, K, Ca, Fe and Cr, as well as CaII. These lines are seen in absorption on the VLT optical spectrum of a background star at $\sim$ 1400 pc away from us and located at 35 arcsec from the center of the IRC+10216 nebula. This line of sight is close enough to the center to display some molecular species like C2, and to permit metals not to be too much ionized as in the diffuse ISM. Despite the fact that the background star has a G-type spectrum, it is possible to extract the circumstellar absorption lines with the use of a template solar spectrum. Analysis of these lines and a model of photoionization in the expanding envelope provide the column densities of the metals in the gas phase. It is found that the known metal bearing molecules (like NaCN, NaCl) are not abundant enough to be at the origin of these atomic metals that we detect in the gas. It is the first time that gaseous Fe is detected in a cool AGB dusty envelope. Its strong depletion (-2.24 in log) means that nearly all iron returns to the ISM under the form of dust.




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suivant: Session PNP monter: Session PCMI-PNPS-PNP précédent: Session PCMI-PNPS-PNP
Samuel Boissier 2010-06-16