COMPETING MECHANISMS OF MOLECULAR HYDROGEN FORMATION IN CONDITIONS RELEVANT TO THE INTERSTELLAR MEDIUM

International audienceThe most efficient mechanism of the formation of molecular hydrogen in the current universe is by association of hydrogen atoms on the surface of interstellar dust grains. The details of the processes of its formation and release from the grain are of great importance in the physical and chemical evolution of the space environmentswhere it takes place. Themain puzzle is still the fate of the 4.5 eV released in H2 formation and whether it goes into internal energy (rovibrational excitation), translational kinetic energy, or heating of the grain. The modality of the release of this energy affects the dynamics of the ISM and its evolution toward star formation.We present results of the detection of the rovibrational states of the just-formed H2 as it leaves the surface of a silicate.We find that rovibrationally excited molecules are ejected into the gas phase immediately after formation over a much wider range of grain temperatures than anticipated. Our results can be explained by the presence of twomechanisms ofmolecule formation that operate in partially overlapping ranges of grain temperature. A preliminary analysis of the relative importance of these two mechanisms is given. These unexpected findings, which will be complemented with experiments on the influence of factors such as silicate morphology, should be of great interest to the astrophysics and astrochemistry communities

Water formation at low temperatures by surface O2 hydrogenation I: characterization of ice penetration

Water is the main component of interstellar ice mantles, is abundant in the solar system and is a crucial ingredient for life. The formation of this molecule in the interstellar medium cannot be explained by gas-phase chemistry only and its surface hydrogenation formation routes at low temperatures (O, O2, O3 channels) are still unclear and most likely incomplete. In a previous paper we discussed an unexpected zeroth-order H2O production behavior in O2 ice hydrogenation experiments compared to the first-order H2CO and CH3OH production behavior found in former studies on hydrogenation of CO ice. In this paper we experimentally investigate in detail how the structure of O2 ice leads to this rare behavior in reaction order and production yield. In our experiments H atoms are added to a thick O2 ice under fully controlled conditions, while the changes are followed by means of reflection absorption infrared spectroscopy (RAIRS). The H-atom penetration mechanism is systematically studied by varying the temperature, thickness and structure of the O2 ice. We conclude that the competition between reaction and diffusion of the H atoms into the O2 ice explains the unexpected H2O and H2O2 formation behavior. In addition, we show that the proposed O2 hydrogenation scheme is incomplete, suggesting that additional surface reactions should be considered. Indeed, the detection of newly formed O3 in the ice upon H-atom exposure proves that the O2 channel is not an isolated route. Furthermore, the addition of H2 molecules is found not to have a measurable effect on the O2 reaction channel.Comment: 1 page, 1 figur

[Structure industrielle et prise de contrôle stratégique sur un marché différencié]

Cahier du Laboratoire d'Econométrie ; 502 ; Diffusion du document : Ecole Polytechnique Laboratoire d'Econométrie 1 rue Descartes 75005 Paris (FRA) ; Cote de localisation : PAR. GIR. 1999/P1127 502Les auteurs mettent en évidence les enjeux stratégiques liés à la détention d'un portefeuille de marques sur un marché différencié. Ils analysent les politiques optimales d'absorption d'une entreprise multiproduits et montrent comment une telle stratégie obéit au principe de connexification maximale des parts de marché. Il apparaît que la stratégie de conquête de nouveaux segments de marché est toujours dominée par la stratégie qui consiste à renforcer le pouvoir de marché exercé sur les segments déjà contrôlés. Ils examinent, en fonction de la structure industrielle initiale, les conditions pour lesquelles l'acquisition d'un concurrent est de type "amical" ou "inamical"

[Le comportement de la firme multi-produits sur un marché différencié]

Cahier du LORIA ; 2002-09 Diffusion du document : INRA Etablissement d'Economie et Sociologie rurales 65 boulevard de Brandebourg 94205 Ivry Cedex (FRA) 2002-09Les auteurs proposent une généralisation du modèle circulaire de différenciation des produits en considérant une firme multi-produits concurrencée par un grand nombre de firmes mono-produit. Ils montrent l'existence d'un équilibre en prix sur le marché, qu'ils caractérisent dans le cas de coûts de transport quadratiques pour les consommateurs. Ils exposent les propriétés de cet équilibre en termes de parts de marché et de profits obtenus par les entreprises. La firme multi-produits utilise ses niches de marché pour mettre en place une tarification asymétrique permettant à une partie de sa ligne d'être protégée de la concurrence et d'extraire le maximum de surplus aux consommateurs