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Seasonal dynamics of cable bacteria in an estuarine intertidal mudflat subject to green tides: Implications for the foraminiferal community and test preservation
No full textInternational audienceAs many coastal areas draining intensive agricultural activities, Ledano estuary mudflats (French Brittany) experience Ulva proliferation, causing green tides. We studied the seasonal dynamics of sulphoxidizing cable bacteria from April 2019 to July 2020 using microsensors (O 2 , pH, H 2 S). The activity of these filamentous bacteria, called electrogenic sulphur oxidation (eSOx), results in strong acidification and in pore-water CO 3 2depletion in the first few centimeters of sediment (within the suboxic zone). Living and dead benthic foraminiferal assemblages were studied in July 2020 to observe the effects of eSOx on the calcareous meiofauna and their shell preservation in the sediment. eSOx was patchy on the mudflat but persistent throughout the year. It contributed up to 45% of oxygen consumption during the algal mat decay, and exceeded 100% during the flooding period suggesting stimulation by nitrate inputs. The corrosive effect was maximal in July 2020 (ΔpH ~ 1.7, [CO]calc < 10 μM). The living foraminiferal community was sparse and nearly monospecific, dominated by the calcareous species Haynesina germanica, probably due to green tides coupled with eSOx-driven acidification. However, living specimens of H. germanica showed no signs of advanced dissolution of their shell suggesting a biological capacity to survive in such environmental acidification, potentially linked to their photosynthetic capability. In contrast, the dead assemblages displayed greater diversity despite a loss of about 20% of the calcareous shell recording with depth due to the synergetic effect of low salinity and eSOx. Overall, this study shows that green tides strongly influence cable bacteria activity and then, sedimentary biogeochemical processes in eutrophic coastal environments
Evidence of an Extended Alfvén Wing System at Enceladus: Cassini's Multi‐Instrument Observations
No full textInternational audienceWe report in situ evidence for Enceladus' Alfvén wing system and its coupling with Saturn's ionosphere, based on multi-instrument observations from the Cassini spacecraft. Analysis of 36 events, including 13 from non-flyby paths, confirms the existence of a Main Alfvén Wing (MAW) current system generated at Enceladus, and associated Reflected Alfvén Wings (RAWs) occurring both at Saturn's ionosphere and on the density gradient of Enceladus' plasma torus, extending longitudinally to at least ∼ 120°(∼2,000 moon radii) downstream of the moon. Additionally, the observations reveal the systematic existence of a filamentation process of these large-scale Alfvénic perturbations (MAW and RAWs) during their propagation at any distance from their source. These findings demonstrate a more extensive electrodynamic coupling than previously reported for Enceladus and more generally for any moon-magnetosphere interaction. Moreover, the observation of energetic electron depletions and water-group ion signatures at longitudes even further from the moon supports the interpretation of an extended and persistent interaction region. These results highlight Enceladus' role in shaping Saturn's magnetospheric environment and underscore the importance of future missions to exhaustively analyze this type of complex interaction between a moon and a planet. Plain Language Summary Saturn's small icy moon Enceladus interacts with the planet's magnetic field, generating intermittent aurora in Saturn's upper atmosphere and electromagnetic waves that travel along invisible magnetic connections between them. During its 13-year mission, the Cassini spacecraft repeatedly crossed these magnetic field lines linked to Enceladus. We used data from several Cassini instruments to study how energy and particles move between the moon and Saturn. We detected wave activity characteristic of Alfvén waves (similar to vibrations on a string), forming as Saturn's magnetic field flows past Enceladus. Due to a complex system of reflection at both Saturn's ionosphere and the boundary of Enceladus' torus, these waves were found not only near the moon but also trailing far behind it, extending more than 504,000 km (over 2,000 times the moon's radius) behind it. This is the first time that Alfvén waves have been observed to be directly linked to the charged particles associated with Enceladus. This shows that Enceladus plays a much bigger role in shaping Saturn's space environment than previously thought, and reveals how moons can influence their host planet across vast distances
Approches muséales de Byzance en France
No full textInternational audienceComment se manifeste la « présence de Byzance » (selon les termes de J.-M. Spieser) dans les pratiques curiatoriales en France depuis la grande exposition organisée par le musée du Louvre en 1993 intitulée "Byzance. L’art byzantin dans les collections publiques françaises" ? De quoi les expositions sur Byzance sont-elles le nom ? Que montre-t-on à l’occasion de ces expositions, que ne montre-t-on pas et que cherche-t-on à transmettre ? Ainsi, à partir d’un corpus d’expositions françaises organisées autour du monde byzantin, nous envisagerons trois questions particulièrement significatives et débattues : nous aborderons l’orientalisme tout d’abord, caractère lié à Byzance depuis le XVIIIe siècle. L’idée impériale sera ensuite étudiée, puisque le thème d’un empire millénaire reste un sujet de fascination ou de rejet. Enfin une place sera accordée à l’icône, typologie d’œuvre d’art longtemps éloignée des musées français et en voie de reconnaissance
Massless Representations in Conformal Space and Their de Sitter (dS) Restrictions. The Split-Octonionic Fabric of Inflationary Field Dynamics
No full textInternational audienceThe monograph develops a self-contained, mathematically rigorous framework for massless representations of the conformal group U(2,2) and their restriction to the de Sitter group Sp(2,2). It systematically constructs ladder (massless) representations, computes invariant bilinear forms and Casimir operators, and provides explicit constructions of vertex operators and two-point functions for low-helicity fields. The monograph employs a novel Clifford-split-octonion approach, embedding 8-component Majorana spinors within an alternative composition algebra, which allows for fully explicit algebraic, spinorial, and geometric constructions. The text bridges foundational representation theory, explicit computational methods, and physical applications in quantum field theory and cosmology. While targeted at researchers in mathematical physics, quantum cosmology, and quantum field theory, the exposition is pedagogically structured to guide advanced graduate students through subtle algebraic and representation-theoretic ideas, making complex constructions accessible without sacrificing rigor
Solving parametric linear matrix inequalities
No full textWe consider linear matrices A = A_0 + x_1A_1 + ... + x_nA_n, where the A_i's are m×m symmetric matrices with entries in a ring R. When R = ℝ, the feasibility problem consists in deciding whether the linear matrix inequality (LMI) A ⪰ 0 is feasible, i.e., the x_i's can be instantiated to obtain a positive semidefinite matrix. When R = ℚ[y_1, ..., y_t], the problem asks for a formula on the parameters y_1, ..., y_t, which describes the values of the parameters for which the specialized LMI is feasible. This problem can be solved using general quantifier elimination algorithms, with a complexity that is exponential in n. In this work, we leverage the LMI structure of the problem to design an algorithm that computes a formula Φ describing a dense subset of the feasible region of parameters. The complexity of this algorithm is exponential in n, m and t but becomes polynomial in n when m and t are fixed. We apply the algorithm to a parametric sum-of-squares problem and to the convergence analyses of certain first-order optimization methods, which are both known to be equivalent to the feasibility of certain parametric LMIs, hence demonstrating its practical interest
Water Production of Interstellar Comet 3I/ATLAS from SOHO/SWAN Observations after Perihelion
No full textInternational audienceThe Solar Wind Anisotropies all-sky hydrogen Lyman-alpha camera on the Solar and Heliosphere Observatory observed the hydrogen coma of interstellar comet 3I/ATLAS, also called C/2025 N1 (ATLAS), beginning on 2025 November 6, 9 days after perihelion. Water production rates were calculated from each image of 3I/ATLAS using the methodology of J. T. T. Mäkinen and M. R. Combi, and fluorescence rates and g-factors were calculated using the daily solar Lyman-alpha fluxes from the LASP database ( https://lasp.colorado.edu/lisird/data ) corrected for solar rotation and for the comet’s heliocentric velocity. The method has been used for over 90 comet apparitions. A water production rate of 3.17 × 10 29 s −1 was found on November 6 when the comet was at a heliocentric distance of 1.40 au and at a sufficient solar elongation angle. It decreased over time after that, down to 1–2 × 10 28 s −1 around 40 days postperihelion (December 9)
Search for dimuon resonance in the 35 to 75 GeV mass range using 140 fb of 13 TeV collisions with the ATLAS detector
No full textInternational audienceA model-independent search for low-mass resonances decaying into pairs of oppositely charged muons is presented. The analysis uses proton-proton collision data corresponding to an integrated luminosity of 140 fb, recorded by the ATLAS detector at the Large Hadron Collider between 2015 and 2018. The search targets hypothetical dimuon resonances in the invariant mass range from 35 GeV to 75 GeV. The modelling of this mass region is particularly challenging for conventional analytic background parameterisations. To address this, a Gaussian process regression technique is used to model the background. The dimuon mass spectrum is analysed for potential signals, and no statistically significant excess is observed. Upper limits at the 95% confidence level are set on the fiducial production cross-section of new resonances decaying promptly into muons, ranging from 20 fb to 110 fb, depending on the resonance mass. These results are further interpreted in the context of dark-photon and dark-matter-mediator models, leading to new constraints on their parameter spaces
Electronic structure origins of radical character in triangular fused acenes: sextet stabilization vs. antiaromaticity release
No full textInternational audienceOpen-shell hydrocarbons are of great interest in molecular materials, yet their electronic structures remain challenging to describe. Here we investigate triangular acenes, formed by fusing three identical linear acenes through cyclobutadiene linkers into a threefold symmetric framework. Using density functional and multiconfigurational methods, we show that triangular acenes display a stronger radical character than their linear counterparts, which increases with molecular size. Analysis of singlet–triplet gaps, unpaired electron numbers, and NICS(1) aromaticity indices reveals that this behavior arises from two cooperative effects: Clar's sextet stabilization and the release of cyclobutadiene antiaromaticity. Fractional occupation densities further indicate a redistribution of unpaired electrons from cyclobutadiene units in smaller molecules to the acene cores in larger systems. These results establish triangular acenes as a distinct class of multiradicaloid hydrocarbons, offering new insights for the design of open-shell π-conjugated materials
Eustache Deschamps : facettes d’un poète du XIVe siècle
No full textInternational audienceLes actes de la journée d'études "Eustache Deschamps : facettes d’un poète du XIVe siècle", organisée le 18 novembre 2022 à l'Université Grenoble Alpes dans le cadre de la préparation au concours d'agrégation de lettres modernes, proposent d'explorer l'œuvre d'Eustache Deschamps qui se caractérise par une extrême variété de tons, de propos et de formes. Les contributions de Camille Brouzes, Clotilde Dauphant, Corinne Denoyelle, Joëlle Ducos, Thierry Lassabatère réunies et introduites par Fleur Vigneron s'intéressent aux diverses facettes de l'auteur médiéval dont les poèmes convoquent des points de vue, des modes d’écriture et des thématiques multiples. L'ouvrage s'ouvre sur une introduction consacrée à "l'intérêt lexical" de la poésie d'Eustache Deschamps, puis propose une exploration "des arts et des savoirs" qui innervent l'oeuvre du poète et s'intéresse notamment à sa manière de "compter" dans toute la polysémie du verbe, une réflexion qui se prolonge par des problématiques temporelles – "temps passés, temps présents, temps futurs" et "jeunesse et vieillesse" –, avant de se clore sur une analyse des "poèmes dialogués"