3,651 research outputs found
CO ice photodesorption: A wavelength-dependent study
UV-induced photodesorption of ice is a non-thermal evaporation process that
can explain the presence of cold molecular gas in a range of interstellar
regions. Information on the average UV photodesorption yield of astrophysically
important ices exists for broadband UV lamp experiments. UV fields around
low-mass pre-main sequence stars, around shocks and in many other astrophysical
environments are however often dominated by discrete atomic and molecular
emission lines. It is therefore crucial to consider the wavelength dependence
of photodesorption yields and mechanisms. In this work, for the first time, the
wavelength-dependent photodesorption of pure CO ice is explored between 90 and
170 nm. The experiments are performed under ultra high vacuum conditions using
tunable synchrotron radiation. Ice photodesorption is simultaneously probed by
infrared absorption spectroscopy in reflection mode of the ice and by
quadrupole mass spectrometry of the gas phase. The experimental results for CO
reveal a strong wavelength dependence directly linked to the vibronic
transition strengths of CO ice, implying that photodesorption is induced by
electronic transition (DIET). The observed dependence on the ice absorption
spectra implies relatively low photodesorption yields at 121.6 nm (Ly-alpha),
where CO barely absorbs, compared to the high yields found at wavelengths
coinciding with transitions into the first electronic state of CO (singulet Pi
at 150 nm); the CO photodesorption rates depend strongly on the UV profiles
encountered in different star formation environments.Comment: 5 pages, 2 figures, published in ApJ
Model-independent constraints on spin observables
We discuss model-independent constraints on spin observables in exclusive and
inclusive reactions, with special attention to the case of photoproduction.Comment: 6 pages, 5 figures, Talk by J.-M. Richard at NSTAR 2009, IHEP,
Beijing (China), April 19-22, 2009, Proc. to appear in "Chinese Physics C
Indirect ultraviolet photodesorption from CO:N2 binary ices - an efficient grain-gas process
UV ice photodesorption is an important non-thermal desorption pathway in many
interstellar environments that has been invoked to explain observations of cold
molecules in disks, clouds and cloud cores. Systematic laboratory studies of
the photodesorption rates, between 7 and 14 eV, from CO:N2 binary ices, have
been performed at the DESIRS vacuum UV beamline of the synchrotron facility
SOLEIL. The photodesorption spectral analysis demonstrates that the
photodesorption process is indirect, i.e. the desorption is induced by a photon
absorption in sub-surface molecular layers, while only surface molecules are
actually desorbing. The photodesorption spectra of CO and N2 in binary ices
therefore depend on the absorption spectra of the dominant species in the
subsurface ice layer, which implies that the photodesorption efficiency and
energy dependence are dramatically different for mixed and layered ices
compared to pure ices. In particular, a thin (1-2 ML) N2 ice layer on top of CO
will effectively quench CO photodesorption, while enhancing N2 photodesorption
by a factors of a few (compared to the pure ices) when the ice is exposed to a
typical dark cloud UV field, which may help to explain the different
distributions of CO and N2H+ in molecular cloud cores. This indirect
photodesorption mechanism may also explain observations of small amounts of
complex organics in cold interstellar environments.Comment: 21 pages 5 figure
Production of individual marine organic aggregates using paramagnetic microspheres : a new tool for examining microbial associations with aggregates
We describe a new method to produce marine aggregates from natural organic material based on the sticking properties of transparent exopolymeric particles. Seawater samples were prescreened and ultrafiltered to concentrate the 30 kDa to 10 mu m size fraction. First, we produced small magnetizable aggregates by combining glass microfibers and paramagnetic 1-mu m beads with the organic matter present in the concentrated solution. The second step involved clustering the small aggregates into a single macro-aggregate, using a small ring-shaped magnet as an aggregation nucleus. Viral and bacterial densities, determined after dissolution of the newly formed aggregates with methanol, averaged 13.8 x 10(6) +/- 3.6 x 10(6) vir. agg.(-1) and 4.1 x 10(6) +/- 1.1 x10(6) bact. agg.(-1). Bacterial respiration and production measurements of single aggregates averaged 8.47 +/- 1.72 nmol O-2 agg.(-1) h(-1) and 1.54 +/- 0.45 ng C agg.(-1) h(-1), respectively. Particulate organic carbon and nitrogen content of the newly formed macro-aggregates averaged 31.92 +/- 2.67 mu g C agg.(-1) and 3.44 +/- 0.43 mu g N agg.(-1), respectively. This approach allows the concentration and isolation of the organic matter precursors that compose natural aggregates and provides a simple protocol for recombining those precursors into single newly formed macro-aggregates, which can then be easily manipulated for further investigation. This method is a new tool for investigations into the interactions between microorganisms and marine aggregates and their implications at the ecosystem level, but also into the interactions between aggregates and dissolved organic or inorganic substances
Quelle R&D Mener pour le DĂ©veloppement Des RĂ©seaux D'Ă©nergie De Demain ? Les Propositions de L'ancre en 2015
Feuille de route sur les rĂ©seaux Ă©lectriques et stockage Ă©laborĂ©e par le GP10 RĂ©seaux et Stockages de l'Energie de l'ANCRECette feuille de route concerne les rĂ©seaux dâĂ©nergie Ă©lectrique, de chaleur et de froid, les rĂ©seaux de gaz (hydrogĂšne, gaz naturel), leurs stockages associĂ©s, ainsi que leurs couplages Ă venir dans le cadre de la transition Ă©nergĂ©tique et des Ă©volutionsqui lâaccompagneront, que ce soit sur les modes de production dâĂ©nergie ou sur lâĂ©volution des usages.Le focus est portĂ© sur les rĂ©seaux Ă©lectriques qui seront les premiers impactĂ©s par cette transition Ă©nergĂ©tique. Hormisquelques Ă©lĂ©ments trĂšs spĂ©cifiques aux rĂ©seaux Ă©lectriques (et qui seront notĂ©s dans le texte par une couleur diffĂ©rente)il est Ă souligner que la quasi-totalitĂ© des considĂ©rations et axes de R&D Ă©voquĂ©s pour les rĂ©seauxĂ©lectriques et le dĂ©veloppement de leur « intelligence » et/ou de leur flexibilitĂ© sâappliquentĂ©galement aux autres rĂ©seaux dâĂ©nergie. Par ailleurs, si le groupe programmatique« RĂ©seaux et Stockage » de lâANCRE (GP10) sâest largement appuyĂ© sur les nombreuses feuilles de route Ă©mises tant au niveau national, dont celles de lâADEME, quâeuropĂ©en, il a Ă©galement souhaitĂ© sâen dĂ©marquer en insistantlargement et en dĂ©taillant les recherches scientifiques et technologiques Ă mener face aux verrous actuellement identifiĂ©s
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