HAL-INSU
Not a member yet
203706 research outputs found
Sort by
Collectifs en action : récits sensibles mémoire du risque et territoire de « résistance » face au risque d’inondation dans un quartier informel de Beyrouth
No full textInternational audienceCette étude examine le fonctionnement de communautés locales dans un quartier informel de la périphérie de Beyrouth, exposé de manière récurrente aux inondations saisonnières. Dans un contexte où la gouvernance prend une forme hybride, avec un État largement absent et la coexistence d’acteurs religieux et politiques, les résidents mettent en place des stratégies collectives pour faire face à l’incertitude et construisent une mémoire du risque oscillant entre présence attentive et hésitation, parfois même oubli partiel. Cette mémoire collective du risque, fragile et mouvante, se manifeste dans des pratiques quotidiennes de vigilance et d’entraide qui tissent des liens sociaux profonds. L’hésitation, loin d’être une faiblesse, reflète les tensions entre incertitude, adaptation et résilience face à un danger perçu comme à la fois familier et menaçant. Ces collectifs ne sont pas formalisés dans des structures rigides, mais s’appuient sur des échanges informels, des mémoires partagées et des pratiques de vigilance quotidienne, tissant ainsi des liens forts malgré la précarité. Cette recherche met en lumière les formes sensibles de coopération qui permettent aux habitants de construire un rapport au territoire, qui dépasse la simple gestion du risque, intégrant des dimensions politiques, sociales et identitaires. En combinant l’ethnographie immersive, la marche exploratoire, le dessin et la photographie, cette étude met en lumière les récits, les atmosphères et les gestes qui contribuent à la résilience collective. Il invite à reconsidérer les collectifs comme des acteurs, capables d’investir les marges urbaines et d’y générer des transformations durables. L’approche combine les savoirs issus des sciences sociales et des pratiques artistiques, ouvrant une fenêtre sur des manières de penser et de représenter la relation entre mémoire, discours et territoire. Cette approche se veut pertinente dans un contexte où l’incertitude et l’instabilité définissent le quotidien
Physical interpretation of the oscillation spectrum on the RGB and AGB
No full textInternational audienceContext. The high frequency resolution of the four-year time series collected by the space-borne telescope Kepler gives us an opportunity to study the seismic mode structure of highly luminous giants in great detail. Seismic observables can be used as to infer the interior structure through comparisons with stellar models. However, we still need to extend the physical interpretation of previously observed seismic differences between hydrogen-shell burning (Red-Giant Branch; RGB) and helium-burning (red clump and Asymptotic-Giant Branch; AGB) stars towards high luminosity stages.Aims. Here we aim to investigate which physical conditions differ between H-shell and He-burning stars in the heliumsecond ionisation zone, through the signature this zone leaves in mode frequencies. In addition, we explore the sensitivity of seismic parameters to the physics implemented in models. Methods. We used a grid of stellar models with mass between 0.8 M⊙ and 2.5 M⊙ and metallicity between -1.0 dex and 0.25 dex. Transfer mechanisms are implemented such as mass loss, core and envelope overshooting, and thermohaline mixing. We infer the p-mode frequencies of the models by artificially suppressing the gravity modes in the core. Results. In accordance with observations, we find that the main stellar properties affecting the seismic observables in the models are the stellar mass and metallicity. Mass loss on the RGB and rotation-induced mixing from the main sequence to the early-AGB cause a phase difference of the helium ionisation zone glitch signature between H-shell and He-burning stars. The amplitude of the glitch signature in the local large separation, ∆ν, is correlated with the density in the helium ionisation zone, which explains the different glitch amplitudes observed between H-shell and He-burning stars. The amplitude exceeds 10% of the observed value of ∆ν in high-luminosity red giants, which makes the asymptotic expansion less accurate when ∆ν ≤ 0.5 µHz.Conclusions. An efficient mass loss on the RGB, typically encountered when M ≤ 1.5 M⊙, can explain the classification of H-shell and He-burning stars based on the p-mode pattern. When M ≥ 1.5 M⊙, efficient mixing mechanisms might leave an important detectable signature in the p-mode frequencies, permitting a potential classification of these stars
Complementary aerosol mass spectrometry elucidates sources of wintertime sub-micron particle pollution in Fairbanks, Alaska, during ALPACA 2022
No full textInternational audienceFairbanks, Alaska, is a subarctic city that frequently suffers from non-attainment of national air quality standards in the wintertime due to the coincidence of weak atmospheric dispersion and increased local emissions. However, significant uncertainties exist about aerosol sources, formation, and chemical processes during cold winter periods. We aim to determine the composition, size, and concentrations of atmospheric sub-micron non-refractory particulate matter (NR-PM1) and quantify their sources in the urban centre of Fairbanks. As part of the Alaskan Layered Pollution and Chemical Analysis (ALPACA) campaign, we deployed a Chemical Analysis of Aerosol Online (CHARON) inlet coupled with a proton transfer reaction – time of flight mass spectrometer (PTR-ToF MS) and an Aerodyne high-resolution aerosol mass spectrometer (AMS) to measure organic aerosol (OA) and NR-PM1, respectively. We used positive matrix factorisation (PMF) for source identification. PTRCHARON factorisation delineated four residential heating sources, including wood and oil combustion, that contribute 47 ± 20 % and 16 ± 9 % of OACHARON, on average, respectively. In contrast, only a single biomass burning-related factor was identified by AMS for both OA and NR-PM1, but it provided information on two additional factors that were rich in sulphur and nitrate. These results demonstrate that PTRCHARON can generate robust quantitative information with enhanced resolution of organic aerosol sources. When combined with suitable complementary instruments like the AMS, such evidence-based insights into the sources of sub-micron aerosol pollution can assist environmental regulators and citizen efforts for the improvement in air quality in Fairbanks and in the wider Arctic winter
The nature of soil blue carbon varies across mangrove geomorphic settings
No full textInternational audienceMangrove forests store significant amounts of blue carbon, mainly as soil organic matter. Insights into mangrove organic matter are limited, despite its importance for understanding blue carbon accumulation and vulnerability to global change. Here, we quantified soil organic matter preserved through chemical recalcitrance and association with the mineral phase, as key factors influencing blue carbon persistence. We found that the nature of the soil organic matter varied with mangrove geomorphic settings. Delta settings were dominated by presumably the most persistent soil organic matter associated with minerals, while open coast karstic settings contained mostly particulate soil organic matter, likely preserved due to low microbial activity. Across mangrove soil depths, there was little difference in soil organic matter pools. The soil organic matter pool across mangroves’ geomorphic settings exhibited greater variation than that observed across all terrestrial biomes. These findings underscore the need to tailor mangrove conservation and restoration to geomorphic settings
Pulsed heating atomic layer deposition of TiO2 thin films on soft substrates
No full textInternational audienceOne of the strengths of the atomic layer deposition (ALD) technique is its ability to produce very thin and uniform deposits conform to the morphology of the substrates even in the case of high roughness or porosity. The range of potential substrates to support ALD films continues to expand. Among them, porous, flexible, organic and natural substrates are attracting increasing interest. Even though the deposition temperature is low in thermal ALD (the ALD window is commonly in between 150 and 400 °C for most metal-oxide deposits), the risk of degradation cannot be excluded for sensitive substrates. To reduce this risk, it would be interesting to limit heating on short periods of time in ALD cycles. Moreover, the energy cost of the production process would also be reduced. To date, several approaches have been proposed to provide additional energy in short periods oftime during an ALD process such as photo-assisted ALD using a pulse light source [1], atomic layer annealing (ALA) using in-situ plasma bombardment [2] and in-situ rapid thermal annealing introducing a heat pulse after a deposition cycle [3]. The goal was mainly to improve the quality of the ALD films by reducing the presence of defects and promoting the compound stoichiometry and the atomic arrangement in crystalline layers. In previous works [4,5], we reported the ALD synthesis of inverse opal TiO2 films using a template of polystyrene micro-beads deposited on a silicon substrate. A post-annealing treatment was used to remove the template to obtain inverse opal layers. The interest of these kind of films for photocatalysis and sensor applications was also shown. However, the possibility of reducing the size of PS beads, in order to increase the specific surface area, has run up against the problem of their softening, even at low ALD temperatures (90-100 °C). The aim of this work is to explore the possibility of using pulsed heating to preserve the morphology of the template, and more broadly to use soft thermal-sensitive substrates. In particular, we herein report the ALD synthesis of TiO2 films on flat substrates and PS micro-bead templates (Fig.1) using pulsed heating induced by short pulses of power supplied to infrared lamps. Several examples of TiO2 films deposited on soft substrates (polymer foams and natural material fibers) will also be presented. SEM observations, together with XPS and micro-Raman analysis, were carried out to study the morphology, stoichiometry and structure of the deposits
Deep learning based detection and classification of small bright sources on ISS images from the Cassini mission and application to Saturn's outer magnetosphere
No full textInternational audienceRecent advancements in artificial intelligence, particularly in deep neural networks, have opened new horizons for space exploration, especially in the field of astrometry. In this study, we address the large-scale categorization of bright astronomical sources in the Cassini Imaging Science Subsystem (ISS) image dataset. Specifically, we train the state-of-the-art YOLO (You Only Look Once) network to detect and classify small sources such as stars, satellites and cosmic rays in ISS images. To compensate for the lack of labeled data, we first develop an automated ground truth generation process using a custom detection and classification algorithm based on traditional image processing. The conducted quantitative and qualitative analyses of the network's predictions demonstrate that it can reliably identify key features for the detection and classification of bright sources. Moreover, it is able to correct some errors in the automatically generated ground truth data such as missed detections or misclassifications. Finally, we analyze the temporal variations of detected cosmic rays in Saturn's outer magnetosphere and compare them with Neutron monitor and sunspot number count rates. The good agreement between the expected cosmic ray behavior outside Saturn's magnetosphere and the particles detected by our network supports the validity of our approach. This result highlights the potential of our method to support deeper scientific investigations aiming for a better understanding of the Saturnian system
Tracer Dispersion by Surfzone Eddies: Assessing the Impact of Undertow Vertical Shear
No full textInternational audienceAbstract The nearshore region, which includes both the surf zone and the inner shelf, is a highly dynamic environment where eddies and rip currents of different scales coexist and interact. Understanding transport and mixing in this interface between land and sea is essential for many reasons, not least to better predict the fate of pollutants, fish larvae, and sediments. Previous modeling studies on the transport of passive tracers have primarily used depth-averaged Boussinesq models. Although these models have provided valuable information, they seem to generally underestimate mixing in the surf zone, while they overestimate it on the inner shelf. Coastal and Regional Ocean Community (CROCO), a 3D free-surface, wave-resolving model, is used here to study the effect of vertical dimension on surfzone eddies and nearshore tracer dispersion. The model is applied to both a directional wave basin experiment and a large-scale dye release experiment conducted in 2009 at Imperial Beach, California. By comparing simulations with and without vertical shear in surfzone currents, we isolate various mixing processes, such as shear dispersion, flash rips, and mini-rips, recently identified as resulting from undertow vertical shear instability. Quantifying each dispersion process in terms of bulk diffusivity reveals the important role of vertical shear in weakening flash rips (through a reduction in inverse kinetic energy cascade) and hence tracer dispersion offshore, while increasing mixing in the surf zone due to mini-rips. This study suggests potential improvements for the parameterization of coarser or simpler models to obtain a more accurate representation of nearshore transport and dispersion mechanisms. Significance Statement The purpose of this study is to investigate the dispersion of tracers (e.g., contaminants, larvae, sediments) in the nearshore. We combined laboratory and field experiments with a 3D wave-resolving model, resulting in a novel view of the physical processes involved, to explore the effect of undertow-driven vertical shear. Two main results emerge. First, vertical shear reduces stirring by transient rip currents between the surf zone and the inner shelf waters. Second, mini-rips (a process resulting from vertical shear instability) appear to be the dominant mixing process in the surf zone itself. These new results on turbulence and mixing in the nearshore zone call for a reexamination of dispersion rates, critical environmental factors, and parameterizations in coarser models
Better out than in: faecal matrix inhibits establishment success after waterfowl endozoochory
No full textInternational audienceAbstract Background and Aims Many plant species undergo long-distance dispersal through migratory waterbirds. However, there is little information about the effectiveness of this dispersal, especially regarding the chances of plant establishment and the impact of gut passage or the faecal matrix on plant germination, growth and reproductive investment in seeds. Methods In a greenhouse experiment, we addressed these questions using an annual mudflat species, Juncus bufonius (Juncaceae), and a perennial emergent aquatic species, Eleocharis palustris (Cyperaceae), whose seeds are dispersed by many waterbird species in Europe. We planted seeds directly in soil or within mallard faeces placed on soil, using both control seeds and seeds that survived gut passage through mallards. Over the following 11 weeks, we quantified germination and plant performance. Key Results Gut passage reduced germination time of J. bufonius when there was no faecal matrix, and it increased asymptotic height of E. palustris. Presence of the faecal matrix hindered germinability, plant growth and final biomass for both species, along with total seed production for J. bufonius. Presence of the faecal matrix slowed down germination in E. palustris, but had the opposite effect for J. bufonius. It was also associated with greater relative investment in seeds in J. bufonius (more seeds per unit biomass), probably as a consequence of later germination. In both species, earlier germination increased final biomass (and seed production in J. bufonius). Conclusions Our results support the importance of waterbird endozoochory in plant dispersal but suggest that it might be more effective when faeces disintegrate, such as when egested into water or disaggregated on land (e.g. by insects). Previous studies with other plants have recorded accelerated germination following waterbird gut passage, and our results show that this can benefit plant fitness
Euclid preparation. Predicting star-forming galaxy scaling relations with the spectral stacking code SpectraPyle
No full textInternational audienceWe introduce SpectraPyle, a versatile spectral stacking pipeline developed for the Euclid mission's NISP spectroscopic surveys, aimed at extracting faint emission lines and spectral features from large galaxy samples in the Wide and Deep Surveys. Designed for computational efficiency and flexible configuration, SpectraPyle supports the processing of extensive datasets critical to Euclid's non-cosmological science goals. We validate the pipeline using simulated spectra processed to match Euclid's expected final data quality. Stacking enables robust recovery of key emission lines, including Halpha, Hbeta, [O III], and [N II], below individual detection limits. However, the measurement of galaxy properties such as star formation rate, dust attenuation, and gas-phase metallicity are biased at stellar mass below log10(M*/Msol) ~ 9 due to the flux-limited nature of Euclid spectroscopic samples, which cannot be overcome by stacking. The SFR-stellar mass relation of the parent sample is recovered reliably only in the Deep survey for log10(M*/Msol) > 10, whereas the metallicity-mass relation is recovered more accurately over a wider mass range. These limitations are caused by the increased fraction of redshift measurement errors at lower masses and fluxes. We examine the impact of residual redshift contaminants that arises from misidentified emission lines and noise spikes, on stacked spectra. Even after stringent quality selections, low-level contamination (< 6%) has minimal impact on line fluxes due to the systematically weaker emission of contaminants. Percentile-based analysis of stacked spectra provides a sensitive diagnostic for detecting contamination via coherent spurious features at characteristic wavelengths. While our simulations include most instrumental effects, real Euclid data will require further refinement of contamination mitigation strategies
LOFAR constraints on the repetition & environments of CHIME FRBs
No full textInternational audienceThe behaviour of fast radio bursts (FRBs) at radio frequencies -0.9α_s>0.55α_{s}\sim101<z<3$. Such detections will provide robust constraints on cosmological parameters due to their clean environments. Our results guide future low-frequency FRB searches by showing how even non-detections can place meaningful constraints on the repetition rates and circumburst environments of FRBs