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Mécanisme et quantification de la photodésorption VUV : de glaces modèles à des analogues de glaces interstellaires
No full textMore than 300 molecules, from the simplest species to complex organic compounds, have been detected in the gas phase of the interstellar medium (ISM). In cold and dense regions (T<110 K), these species condense onto dust grains, forming icy mantles that act as molecular reservoirs. Yet, a fraction of them is still observed in the gas phase. This persistence cannot be explained by thermal desorption alone and requires non-thermal mechanisms, among which ultraviolet (VUV) photon-induced desorption plays a central role. A quantitative understanding of this process is crucial for astrochemical models aiming to reproduce observations from large telescopes (ALMA, NOEMA, JWST), but it remains limited by the scarcity of laboratory data covering the relevant energies and by the complexity of interstellar ices. This thesis aims to characterize and quantify VUV photon-induced desorption, from simple model systems (CO, CO2) to mixtures representative of astrophysical ices (H2O, CO2, CO). The methodology combines (i) photon-energy-dependent studies using the VUV DESIRS beamline (7-14eV) at the SOLEIL synchrotron, and (ii) fixed-energy studies with a pulsed VUV laser source at the MONARIS laboratory (~8eV). Desorption yields are measured using quadrupole mass spectrometry, while laser-REMPI-time-of-flight coupling is employed to probe the internal and kinetic energies of desorbed molecules. The first focus is a detailed study of CO photodesorption at 15 K, with the laser tuned to bands of the A-X transition (7.8eV or 8.5 eV) of solid CO. Molecules desorb without vibrational excitation and with little rotational or translational energy (a few hundred meV). Correlations between translational and rotational energies are observed and discussed. The energy distributions are in excellent agreement with results from quantum molecular dynamics simulations, validating the proposed indirect desorption model. The second focus investigates the influence of flux and fluence on CO2 photodesorption. CO2 photodesorption follows a DIET (Desorption Induced by Electronic Transition) mechanism, independent of flux and correlated with bulk chemistry. In contrast, the photodesorption of photoproducts CO and O2 shows a strong flux dependence, reflecting surface chemistry involving the diffusion of “hot” oxygen atoms. This distinction highlights the need to couple photodesorption and photochemistry to understand the evolution of interstellar ices. At high fluxes, competing pathways (O+O→O2, O+CO→CO2) reduce the overall efficiency, emphasizing the need for caution when extrapolating to the low astrophysical flux regime. Finally, the thesis explores more realistic systems: layered ices of CO or CO2 on H2O, followed by binary and ternary mixtures of H2O:CO2:CO. Beyond the major species, the photodesorption of organic molecules (HCO, H2CO, H3CO, HCOOH) is detected; in some cases, the yields of H2CO rival those of H2O or CO2, underlining that photodesorption actively contributes to the molecular complexity of the interstellar gas. “Astrophysical” yields, integrated over different UV fields (ISRF, secondary UV), are proposed for several species, providing direct constraints for models. These results demonstrate the limitations of using yields measured from pure ices. By providing a detailed experimental characterization of VUV photodesorption yields and mechanisms in ices of increasing complexity, this thesis establishes the quantitative foundations needed to interpret observations of cold ISM regions and to understand how interstellar ices contribute to the emergence of molecular complexity in the Universe.Plus de 300 molécules, des plus simples à des molécules organiques complexes ont été détectées dans les phase gazeuses du milieu interstellaire (MIS). Dans les régions froides et denses (T<110 K), ces espèces, se condensent sur les grains de poussière et forment des manteaux de glace, véritables réservoirs moléculaires. Pourtant, une fraction d'entre elles est observée en phase gazeuse. Ce maintien ne peut s'expliquer par la seule désorption thermique et implique des mécanismes non thermiques, parmi lesquels la photodésorption induite par le rayonnement ultraviolet lointain (VUV). Comprendre quantitativement ce processus est essentiel pour pour les modèles astrochimiques qui visent à reproduire les observations de grands télescopes (ALMA, NOEMA, JWST), mais reste limité par le manque de données de laboratoire couvrant les énergies pertinentes et par la complexité des glaces interstellaires. Cette thèse vise à caractériser et quantifier la photodésorption induite par photons VUV, depuis des systèmes modèles purs (CO, CO2) jusqu'à des mélanges représentatifs des glaces astrophysiques (H2O, CO2, CO). La méthodologie combine (i) des études en fonction de l'énergie de photons avec la ligne VUV DESIRS (7-14 eV) du synchrotron SOLEIL et (ii) des études à énergie fixe avec une source laser VUV pulsée au laboratoire MONARIS (~8eV). Les rendements sont mesurés par spectrométrie de masse quadripolaire, et le couplage laser-REMPI-temps de vol permet de sonder l'énergie interne et cinétique des molécules désorbées. Un premier axe concerne l'étude détaillée de la photodésorption de CO à 15 K, avec un laser accordé sur des bandes de la transition A-X (7.8 eV ou 8.5 eV) du CO solide. Les molécules désorbent sans excitation vibrationnelle et avec peu d'énergie rotationnelle et translationnelle (quelques centaines de meV). Des corrélations observées entre les énergies de translation et de rotation sont discutées. Les distributions d'énergie sont en excellents accord avec celles issues de simulations de dynamiques moléculaires quantiques, ce qui valide le modèle de désorption indirect proposé. Le second axe étudie l'influence du flux et de la fluence sur la photodésorption de CO2. La photodésorption de CO2 suit un mécanisme DIET (Desorption Induced by Electronic Transition), indépendante du flux et corrélée à la chimie du volume. En revanche, la photodésorption de CO et O2 photoproduits dépend fortement du flux et traduit une chimie de surface impliquant la diffusion d'atomes d'oxygène « chauds ». Cette distinction souligne l'importance de coupler photodésorption et photochimie pour comprendre l'évolution des glaces interstellaires. À haut flux, des voies concurrentes (O+O→O2, O+CO→CO2) limitent l'efficacité globale, appelant à une extrapolation prudente vers les faibles flux astrophysiques. Enfin, la thèse explore des systèmes plus réalistes : systèmes en couches de CO ou CO2 sur H2O puis mélanges binaires et ternaires H2O:CO2:CO. Outre les espèces majeures, la photodésorption d'espèces organiques (HCO, H2CO, H3CO, HCOOH) est mise en évidence ; dans certains cas, les rendements de H2CO rivalisent avec ceux de H2O ou CO2, soulignant que la photodésorption contribue activement à la complexité moléculaire du gaz interstellaire. Des rendements « astrophysiques », intégrés sur différents champs UV (ISRF, UV secondaire), sont proposés pour plusieurs espèces, fournissant des contraintes directes aux modèles. Ces résultats démontrent l'impossibilité d'utiliser des rendements mesurés sur des glaces pures. En fournissant une caractérisation expérimentale détaillée des rendements et mécanismes de photodésorption VUV dans des glaces de complexité croissante, cette thèse établit des bases quantitatives indispensables pour interpréter les observations des régions froides du MIS et comprendre comment les glaces interstellaires participent à l'émergence de la complexité moléculaire dans l'Univers
The Composite Spectrum of QSO Absorption Line Systems in DESI DR2
No full textInternational audienceWe present details regarding the construction of a composite spectrum of quasar (QSO) absorption line systems. In this composite spectrum we identify more than 70 absorption lines, and observe oxygen and hydrogen emission features at a higher signal-to-noise ratio than in any previous study. As the light from a distant quasar travels towards an observer, it may interact with the circumgalactic medium environment of an intervening galaxy, forming absorption lines. In order to maximize the signal of these absorption lines, we have selected a sample of 238,838 quasar spectra from the second data release of the Dark Energy Spectroscopic Instrument (DESI), each identified to have absorption lines resulting from such an interaction. By stacking these spectra in the restframe of the absorption, and calculating a median composite spectrum, we are able to isolate and enhance these absorption lines. We provide a full atlas of all detected absorption and emission lines as well as their fit centroids and equivalent width values. This atlas should aid in future studies investigating the compositions and physical conditions of these absorbers
Identification of natural and anthropogenic sources of geogenic radionuclides by using a multi-isotope tracing approach
No full textInternational audienceEnvironmental monitoring programs of nuclear legacy sites,formerly producing U-enriched materials, are mainly based onthe interpretation of radionuclide concentrations in soils andsediments sampled downstream from these sites. However, thevariations of these concentrations can be very low in upstreamand downstream sampling locations. In addition, the differentgeological origins of soils and sediments can lead to differentgeochemical background values, which can be attributedwrongly to anthropogenic sources. It is therefore important to usegeochemical tracers for accurately deciphering natural andanthropogenic radionuclide sources. This information isimportant for a better understanding of transport mechanisms ofthese substances in the critical zone and improving the evaluationof site management strategies.The methodology proposed aims to provides tools fordiscriminating anthropogenic radioactive sources linked toformer U mining activities, with regard to those originating fromthe natural weathering and erosion of rocks. This approach wasperformed according to a multi-isotope tracing approach,including the determination of (226Ra/228Ra) and (234U/238U)Activity Ratios (ARs) combined with the knowledge of the other238U decay series nuclide disequilibria and Pb stable isotoperatios.Applied to a former U-mining-influenced watershed and lake,these isotopic tools reveal the existence of anthropogenicsignatures in surface environments, several kilometersdownstream from the mining source, several decades after theend of site rehabilitation operations. The integration of theseresults into the chronology of downstream lake sediment depositsshowed that even if U contents remained twice the localgeochemical background, these deposits sampled in a lakerecorded that a part of U has been brought as dissolved speciesfrom the site into the sedimentary column. Moreover, theanalyses of older lake sediment deposits indicated a clearsignature of radiogenic Pb (i. e. Pb from U ores) associated tosignificant 230Th activities, indicating the past transport of Udecay series-enriched particles. Overall, this study emphasizesthe high potential of multi-isotope tracing approaches fordocumenting the origin of geogenic radionuclides in the criticalzone. These results yield further mechanistic explanations for theobserved evolution of U concentration and speciation previouslyreported from mineralogical and spectroscopic analyses of theselake sediments
A new magnetic observatory in La Réunion Island – meeting data quality requirements in a volcanic island setting
No full textInternational audienceAbstract. A new magnetic observatory has been set on La Réunion Island in the Indian Ocean through a collaboration between the “Institut de physique du globe de Paris” (IPGP) local volcano observatory (Observatoire Volcanologique du Piton de la Fournaise – OVPF) and its magnetic observatory service. This observatory is isolated and serves to monitor the evolution of the Earth's magnetic field in that region. It is also particularly useful for large-scale modelling of the core field and other contributions to the geomagnetic field. Three-component vector magnetic field data are continuously collected at 1 Hz using a fluxgate, while scalar data are collected at 0.2 Hz with a proton magnetometer. The data are transmitted every 5 min to IPGP main site and made immediately available to the scientific community (see http://www.bcmt.fr, last access: 22 August 2025). Due to the strong magnetic field generated by the surrounding volcanic rocks, the differences between the magnetic field strengths as recorded by the proton magnetometer and the strengths calculated from the recorded vector field values vary by more than ∼2 nT during a day. To circumvent this difficulty, constant offset values of −2400, 280, and −20 nT are added to the X, Y, and Z magnetic field components respectively, prior to the data distribution. We show that this approach efficiently reduces the differences between measured and calculated magnetic field strengths inside a day. Calibrated observatory data have been calculated over the year 2023, and, although the baseline values present variations up to 70 nT throughout that year, the derived data meet the quality required for an intermagnet observatory. A Fourier analysis of the data shows that these are not contaminated by significant noise even if peaks at 0.2 Hz indicate small cross-talk between vector and scalar instruments
Return period of non-concurrent climate compound events: a non parametric bivariate Generalized Pareto approach
No full textInternational audienceCompound events (CEs), commonly defined as the "combination of multiple drivers and/or hazards that contributes to societal or environmental risk", often result in amplified impacts compared to individual hazards. In order to estimate the return period of bivariate CEs, a novel non-parametric approach employing bivariate Generalized Pareto distributions (bi-GPD) is proposed and compared to a copula-based approach. Special attention is given to account for temporal dependencies and non-concurrent compound events. The latter are defined as excess of variables over a threshold at a relatively close time. The return period of such bivariate events is carefully defined and closed-form expressions are obtained for both approaches. Simulations reveal the bi-GPD approach is effective in case of positive asymptotic dependence and should be avoided in case of asymptotic independence. The novel approach is then applied to ERA5 reanalysis data to analyze two types of compound events: a spatial CE with simultaneous floods due to accumulated precipitations across two large watersheds in France and a preconditioned CE consisting of devastating floods triggered by antecedent precipitatio
The DESI DR1 Peculiar Velocity Survey: growth rate measurements from galaxy and momentum correlation functions
No full textInternational audienceJoint analysis of the local peculiar velocity and galaxy density fields offers a promising route to testing cosmological models of gravity. We present a measurement of the normalised growth rate of structure, , from the two-point correlations of velocity and density tracers from the DESI DR1 Peculiar Velocity and Bright Galaxy Surveys, the largest catalogues of their kind assembled to date. We fit the two-point correlation measurements with non-linear correlation function models, constructed from density and momentum power spectra generated using 1-loop Eulerian perturbation theory, and validate our methodology using representative mock catalogues. We find , consistent to within with accompanying analyses of the same datasets using power spectrum and maximum-likelihood fields methods. Combining these growth rate results from different methods including appropriate correlations, we find a consensus determination , consistent with predictions from \textit{Planck}CDM cosmology. Jointly fitting to this consensus low-redshift growth rate and the DESI DR1 full-shape clustering dataset, we measure gravitational growth index , consistent with the prediction of general relativity
The SewerNet domain ontology: on clarifying and harmonising terminology
No full textInternational audienceWastewater network management is being digitised and integrated across municipalities. A shared understanding and terminology of the systems is important both for modern urban water management and modelling climate-induced stressors on the network. Ontologies are a well-known mechanism to record the shared understanding. While several ontologies exist that focus on the water, and several exist on service infrastructure, there is a gap in the ontology landscape about wastewater services infrastructure. We are currently developing an ontology about wastewater networks and a first version is publicly available at http://sewernet.hsm.umontpellier.fr/. Key aims for the use of the ontology in our project are data integration, ontology-based query answering the detect incoherent data in wastewater network databases, and document annotation, but, it being a domain ontology, SewerNet is usable also for other types of ontology-mediated information systems. In this talk, we focus on interesting discrepancies and lack of clarity in non-ontological resources we used for the development, such as the INSPIRE EU directive and the RAEPA geostandard, that needed to be harmonised in the ontology. Examples include pipe versus conduit, disambiguation between maintenance plans and individual repair actions, precision/uncertainty in the measurements, circulation mode. The use of a foundational ontology (DOLCE) to assist structuring content was perceived beneficial, as well as the ontological questions to align to the DOLCE entities, which helped probing the nature of the entity and elucidate assumptions about terms.
Evolution of flood generating processes under climate change in France
No full textInternational audienceAbstract. The impact of climate change on floods varies across regions, and observed trends in flood characteristics are often explained by differential changes in the processes that cause flooding. This study explores changes in flood magnitude and flood-generating processes under different climate change scenarios for a large number of basins in France. It is based on an unprecedented exercise to model the impacts of climate change on hydrology, using a semi-distributed model (GRSD) applied to 3727 basins with 22 Euro-CORDEX bias-corrected climate projections using two greenhouse gas emission scenarios (RCP4.5 and RCP8.5). Annual maxima of daily simulated streamflow were extracted for the period 1975–2100, resulting in a set of over 10 million flood events, and a trend analysis was carried out on both flood magnitudes and flood generating processes. Increasing trends in flood magnitudes are only found in the northern regions of France, although multi-model convergence rarely exceeds 60 %. The highest increases are observed for the 20 year floods and under the RCP8.5 scenario. A classification of floods according to their generating process revealed that floods linked to soil saturation represent more than half of all floods in France. The relative change in the importance of the different flood-generating processes is not spatially homogeneous and varies by region. The proportion of floods linked to soil saturation excess is increasing in the temperate and continental climate zones in the Northeast, while decreasing in the southern Mediterranean regions. In these Mediterranean regions, the proportion of floods linked to infiltration excess related to extreme rainfall is increasing. Both the frequency and magnitude of floods linked to snowmelt processes are decreasing in mountainous areas. On the contrary, the most extreme floods associated with rainfall on dry soils tend to increase, in line with the increase of rainfall intensity. Overall, trends in antecedent soil moisture conditions are as important as trends in intense rainfall to explain flood hazard trends in the different climate projections. This study shows how important it is to decipher the changes in the different flood generating processes in order to better understand their evolution in different hydroclimatic regions
Real-time phase control methods for cold-atom interferometry
No full textWe present two methods to achieve real-time inertial phase compensation in atom interferom-eters. Both methods, based on jumps of the position of the retroreflection mirror or frequenciesof Raman lasers, demonstrate similar state-of-the-art performance on our cold atom gyroscope,comparable to that of the reference method based on optical phase jumps. These alternative ap-proaches broaden the scope of applications for real-time inertial phase compensation methods inatomic interferometers, particularly for space applications
The Galaxy Activity, Torus, and Outflow Survey (GATOS). VII. The 20–214 μm Imaging Atlas of Active Galactic Nuclei Using SOFIA
No full textInternational audienceWe present a 19.7–214 μ m imaging atlas of local (4–181 Mpc; median 43 Mpc) active galactic nuclei (AGN) observed with FORCAST and HAWC+ on board the SOFIA telescope with angular resolutions ~3 ″ –20 ″ . This atlas comprises 22 Seyferts (17 Type 2 and five Type 1) with a total of 69 images, 41 of which have not been previously published. The AGN span a range of luminosities of log 10 ( L bol [ erg s - 1 ] ) = [ 42 , 46 ] with a median of log 10 ( L bol [ erg s − 1 ] ) = 44.1 ± 1.0 . We provide the total fluxes of our sample using aperture photometry for point-source objects and a 2D Gaussian fitting for objects with extended host galaxy emission, which was used to estimate the unresolved nuclear component. Most galaxies in our sample are pointlike sources; however, four sources (Centaurus A, Circinus, NGC 1068, and NGC 4388) show extended emission in all wavelengths. The 30–40 μ m extended emission in NGC 4388 is coincident with the narrow-line region at PA ~ 50°, while the dusty extension at longer wavelengths arises from the host galaxy at PA ~ 90°. Our new observations allow us to construct the best-sampled parsec-scales (spectral energy distributions, SEDs) available between 30 and 500 μ m for a sample of nearby AGN. We estimate that the average peak wavelength of the nuclear SEDs is ~40 μ m in νF ν , which we associate with an unresolved extended dusty region heated by the AGN