Investigating the spectroscopic, magnetic and circumstellar variability of the O9 subgiant star HD 57682

The O9IV star HD 57682, discovered to be magnetic within the context of the MiMeS survey in 2009, is one of only eight convincingly detected magnetic O-type stars. Among this select group, it stands out due to its sharp-lined photospheric spectrum. Since its discovery, the MiMeS Collaboration has continued to obtain spectroscopic and magnetic observations in order to refine our knowledge of its magnetic field strength and geometry, rotational period, and spectral properties and variability. In this paper we report new ESPaDOnS spectropolarimetric observations of HD 57682, which are combined with previously published ESPaDOnS data and archival H{\alpha} spectroscopy. This dataset is used to determine the rotational period (63.5708 \pm 0.0057 d), refine the longitudinal magnetic field variation and magnetic geometry (dipole surface field strength of 880\pm50 G and magnetic obliquity of 79\pm4\circ as measured from the magnetic longitudinal field variations, assuming an inclination of 60\circ), and examine the phase variation of various lines. In particular, we demonstrate that the H{\alpha} equivalent width undergoes a double-wave variation during a single rotation of the star, consistent with the derived magnetic geometry. We group the variable lines into two classes: those that, like H{\alpha}, exhibit non-sinusoidal variability, often with multiple maxima during the rotation cycle, and those that vary essentially sinusoidally. Based on our modelling of the H{\alpha} emission, we show that the variability is consistent with emission being generated from an optically thick, flattened distribution of magnetically-confined plasma that is roughly distributed about the magnetic equator. Finally, we discuss our findings in the magnetospheric framework proposed in our earlier study.Comment: 21 pages, 19 figures, Accepted for publication in 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

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

Echange de charge à énergie thermique entre ions de gaz rares et molécules simples

Le transfert de charge à énergie thermique entre les ions de gaz rares He+, Ne+, Ar+, Kr+ Xe+ et deux molécules simples O2 et H2O est étudié dans une cellule de Résonance Cyclotronique Ionique spécialement conçue pour permettre l'observation du spectre de fluorescence et la détermination de l'énergie cinétique des produits. Avec He+ et Ne+ le transfert de charge est dissociatif et procède vraisemblablement par un saut ďélectron à distance. Avec Ar+ et Kr+, le transfert de charge n’est pas dissociatif. Il s’accompagne de transfert de moment cinétique et ďexcitation vibrationnelle de l'ion moléculaire formé. La population des niveaux vibrationnels de l'ion n'est pas proportionnelle aux facteurs de Franck-Condon, une fraction importante de l'énergie disponible dans la réaction se retrouve sous forme ďénergie vibrationnelle

Development of a transportable FT-ICR MS associated with a glow discharge ionization source

International audienc

Pseudomonas protegens CHA0 does not increase phosphorus uptake from 33P labeled synthetic hydroxyapatite by wheat grown on calcareous soil

Soil microorganisms exuding organic acids have the potential to solubilize inorganic phosphorus (P), which could improve the P availability to plants growing on calcareous soil. The gluconic acid exuding bacteria Pseudomonas protegens CHA0 can solubilize P under glucose rich in vitro conditions, but evidence on the effectiveness in soil is lacking. This discrepancy in P solubilization between in vitro and in vivo is common for many P solubilizing bacteria. Possible causes for this discrepancy are rarely explored in soil using mechanism oriented approaches. Proposed reasons for limitation of bacterial P solubilization in soil are low persistence of the inoculant or low glucose availability in the plant rhizosphere. To test these two hypotheses we investigated the solubilization of 33P labeled synthetic hydroxyapatite (Ca33P) by the gluconic acid producing P. protegens CHA0 wild type and the mutant strain CHA1198 lacking the capacity to produce this acid, in a plant growth experiment with wheat (Triticum aestivum) and an incubation experiment. Neither in the plant growth- nor in the incubation experiment solubilization of Ca33P by strain CHA0 was detected, in spite the inoculated strain persisted in the rhizoplane of wheat and in the sterilized soil amended with glucose. No detected P solubilization in the sterilized inoculated soil suggests that glucose availability was the main limiting factor. The comparison of the results obtained from the two bacterial inoculants suggested that overall microbial activity, i.e., via protonation due to respiration, increased inorganic P mobilization. P solubilizing bacteria should be evaluated using a tracer and an appropriate bacterial control in order to reveal the mechanisms involved in increased plant available P in soil inoculated with P solubilizing bacteria