Modelling CO formation in the turbulent interstellar medium

We present results from high-resolution three-dimensional simulations of turbulent interstellar gas that self-consistently follow its coupled thermal, chemical and dynamical evolution, with a particular focus on the formation and destruction of H2 and CO. We quantify the formation timescales for H2 and CO in physical conditions corresponding to those found in nearby giant molecular clouds, and show that both species form rapidly, with chemical timescales that are comparable to the dynamical timescale of the gas. We also investigate the spatial distributions of H2 and CO, and how they relate to the underlying gas distribution. We show that H2 is a good tracer of the gas distribution, but that the relationship between CO abundance and gas density is more complex. The CO abundance is not well-correlated with either the gas number density n or the visual extinction A_V: both have a large influence on the CO abundance, but the inhomogeneous nature of the density field produced by the turbulence means that n and A_V are only poorly correlated. There is a large scatter in A_V, and hence CO abundance, for gas with any particular density, and similarly a large scatter in density and CO abundance for gas with any particular visual extinction. This will have important consequences for the interpretation of the CO emission observed from real molecular clouds. Finally, we also examine the temperature structure of the simulated gas. We show that the molecular gas is not isothermal. Most of it has a temperature in the range of 10--20 K, but there is also a significant fraction of warmer gas, located in low-extinction regions where photoelectric heating remains effective.Comment: 37 pages, 15 figures; minor revisions, matches version accepted by MNRA

Benthic and Hyporheic Macroinvertebrate Distribution Within the Heads and Tails of Riffles During Baseflow Conditions

The distribution of lotic fauna is widely acknowledged to be patchy reflecting the interaction between biotic and abiotic factors. In an in-situ field study, the distribution of benthic and hyporheic invertebrates in the heads (downwelling) and tails (upwelling) of riffles were examined during stable baseflow conditions. Riffle heads were found to contain a greater proportion of interstitial fine sediment than riffle tails. Significant differences in the composition of benthic communities were associated with the amount of fine sediment. Riffle tail habitats supported a greater abundance and diversity of invertebrates sensitive to fine sediment such as EPT taxa. Shredder feeding taxa were more abundant in riffle heads suggesting greater availability of organic matter. In contrast, no significant differences in the hyporheic community were recorded between riffle heads and tails. We hypothesise that clogging of hyporheic interstices with fine sediments may have resulted in the homogenization of the invertebrate community by limiting faunal movement into the hyporheic zone at both the riffle head and tail. The results suggest that vertical hydrological exchange significantly influences the distribution of fine sediment and macroinvertebrate communities at the riffle scale

Synthese de metalloporphyrines fonctionnelles en vue d'applications en hematologie et imagerie medicale

Available from INIST (FR), Document Supply Service, under shelf-number : TD 82006 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueSIGLEFRFranc

Étude des radicaux formés en phase gazeuse par impact d’électrons de faible énergie dans les hydrocarbures

Les radicaux formés au cours de la radiolyse du néopentane en phase gazeuse par des électrons lents (5-100 eV) ont été étudiés par RPE.Deux radicaux ont été observés : le tert-butyle t-Ċ4H9 et le néopentyle Ċ5H11. L’étude de la concentration des radicaux en fonction de l’énergie des électrons a permis de déterminer les potentiels d’apparition des radicaux et a montré que des radicaux étaient formés en dessous du potentiel d’ionisation du néopentane. L’étude en fonction de la pression a permis de déterminer l’ordre global des réactions de formation des radicaux. A partir de ces résultats, nous avons essayé de déterminer les réactions conduisant à la formation des radicaux observés : décomposition de molécules excitées et réactions ioniques