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ALMA millimetre-wavelength imaging of HD 138965: new constraints on the debris dust composition and presence of planetary companions
No full textInternational audienceHD 138965 is a young A type star and member of the nearby young Argus association. This star is surrounded by a broad, bright debris disc with two temperature components that was spatially resolved at f ar-infrared w avelengths by Herschel . Here, we present Atacama Large Millimiter/Submillimeter Array (ALMA) millimetre-wavelength imaging of the cool outer belt. These reveal its radial extent to be 150 + 10 -7 au with a width ( σ ) of 49 + 7 -6 au ( R/R = 0 . 77), at a moderate inclination of 49 .• 9 + 3 . 3 -3 . 7 . Due to the limited angular resolution, signal-to-noise and inclination we have no constraint on the disc's vertical scale height. We modelled the disc emission with both gravitational and radiation forces acting on the dust grains. As the inner belt has not been spatially resolved, we fixed its radius and width prior to modelling the outer belt. We find astronomical silicate is the best fit for the dust composition. Ho we ver, we could not reject possible scenarios where there are at least 10 per cent water-ice inclusions. Combining the spatially resolved imaging by ALMA with non-detection at optical wavelengths by HST , we obtain a limit on the scattering albedo ω ≤ 0 . 09 for the debris dust in the outer belt. Analysis of the outer belt's architecture in conjunction with simple stirring models places a mass limit of 2.3 ± 0.4 M Jup on a companion interior to the belt ( a ≤ 78 au), a factor of two impro v ement o v er constraints from high contrast imaging.</p
Multi-Messenger Signatures of the Milky Way Galaxy in Cosmic Rays, Neutrinos, and Gamma Rays
No full textInternational audienceIn this talk, I reviewed recent measurements of the local Galactic cosmic ray flux up to the kneeregion, as well as the emission from the Milky Way galaxy in gamma-rays and neutrinos withinthe same energy range. I presented an overview of both observational data and example theoreticalmodels of multi-messenger diffuse emission from the Galaxy
Traitement des graines de tournesol par plasma froid à pression atmosphérique : augmentation de la tolérance au stress hydrique pendant la germination et développement précoce des plantules.
No full textInternational audienceThe aim of this study was to investigate the impact of ambient air plasma treatment on both sunflower seed germination and early steps of seedling development under water stress. Dry seeds were exposed to a cold atmospheric plasma (CAP) generated in a dielectric barrier device where excited molecular nitrogen and ozone were detected by optical emission spectroscopy and mass spectrometry respectively. Interestingly, we explain the crucial role of gap’s accuracy when treating seeds with CAP, especially to improve interaction between seeds and plasma and therefore ensure an efficient treatment. CAP significantly improved the germination rates of seeds of dormant and non-dormant sunflower genotypes under water stress, demonstrating its efficiency in alleviating seed dormancy and in improving germination under suboptimal conditions. Furthermore, greenhouse experiments demonstrated that plasma treatment also stimulated seedling development under water stress conditions. These findings highlight the potential of CAP treatment as an effective approach to promote the whole process of emergence of crop species.L’objectif de cette étude est d’étudier l’impact d’un traitement par plasma d’air ambiant sur la germination des graines de tournesol ainsi que sur les premières étapes du développement des plantules sous stress hydrique. Des graines sèches ont été exposées à un plasma froid à pression atmosphérique (CAP) généré dans un dispositif à barrière diélectrique, où l’azote moléculaire excité et l’ozone sont détectés respectivement par spectroscopie d’émission optique et par spectrométrie de masse. Nous mettons en évidence le rôle crucial de la distance interelectrode (gap) lors du traitement des graines, notamment pour améliorer l’interaction entre les graines et le plasma et ainsi garantir un traitement efficace. Le CAP a significativement amélioré les taux de germination de graines de génotypes de tournesol dormants et non dormants sous stress hydrique, démontrant son efficacité pour atténuer la dormance des graines et améliorer la germination dans des conditions sous-optimales. En outre, des expériences en serre ont montré que le traitement par plasma stimulait également le développement des plantules en conditions de stress hydrique. Ces résultats soulignent le potentiel du traitement par CAP comme approche efficace pour favoriser l’ensemble du processus d’émergence des espèces cultivées
Analyticity and positivity of Green's functions without Lorentz
No full textInternational audienceWe study the properties imposed by microcausality and positivity on the retarded two-point Green's function in a theory with spontaneous breaking of Lorentz invariance. We assume invariance under time and spatial translations, so that the Green's function depends on and . We discuss that in Fourier space microcausality is equivalent to the analyticity of when lies in the forward light-cone, supplemented by bounds on the growth of as one approaches the boundaries of this domain. Microcausality also implies that the imaginary part of (its spectral density) cannot have compact support for real . Using analyticity, we write multi-variable dispersion relations and show that the spectral density must satisfy a family of integral constraints. Analogous constraints can be applied to the fluctuations of the system, via the fluctuation-dissipation theorem. A stable physical system, which can only absorb energy from external sources, satisfies for real . We show that this positivity property can be extended to the complex domain: in the domain of analyticity guaranteed by microcausality. Functions with this property belong to the Herglotz-Nevanlinna class. This allows to prove the analyticity of the permittivities and that appear in Maxwell equations in a medium. We verify the above properties in several examples where Lorentz invariance is broken by a background field, e.g. non-zero chemical potential, or non-zero temperature. We study subtracted dispersion relations when the assumption at infinity must be relaxed
Classification photométrique des étoiles carbonées variables avec Gaia : application à un petit échantillon de test de 125 objets
No full textEtude photométrique automatisée d'un échantillon réduit de 125 étoiles carbonées provenant de Gaia DR3Nous présentons une étude photométrique automatisée d'un échantillon réduit de 125 étoiles carbonées provenant de Gaia DR3, dans le but de décrire leurs comportements de variabilité grâce à des diagnostics géométriques (surface de la boucle dans le diagramme couleur-magnitude CMD), temporels (décalage de phase entre la couleur et la magnitude) et photométriques (amplitudes, températures effectives). Nous proposons une classification basée sur quatre régimes principaux : la pulsation propre, le décalage linéaire, l'atmosphère dynamique et la prépondérance de la poussière. Les résultats révèlent une population dominée par des pulsations quasi-adiabatiques. Cependant une part importante d'objets est influencée par l'atmosphère dynamique et la poussière. Cette méthode offre une structure réplicable et adaptable pour l'analyse des étoiles variables carbonées en vue de son élargissement à un nombre plus important d'étoiles
Molecular gas in cool-core brightest cluster galaxies at z ≃ 0.4
No full textInternational audienceThe brightest cluster galaxies (BCGs) of today are passive and very massive galaxies at the center of their clusters. They still accrete mass by swallowing companions and flows of cold gas, processes which are regulated by radio-mode active galactic nucleus (AGN) feedback. However, the formation history of BCGs is still a matter of debate. We report new findings based on millimeter observations performed with the Northern Extended Millimeter Array (NOEMA) interferometer mapping the cold molecular gas (CO) that feeds the star formation of distant BCGs. We selected three sources among the strongest cool-core BCGs at intermediate redshifts ( z ≃ 0.4), namely, RX 1532, MACS 1447, and CHIPS 1911. Previous unresolved millimeter observations and multi-wavelength analyses have shown that they are among the most star forming (SFR ≃ 100 M ⊙ /yr) and gas rich ( M H 2 ≃ 10 11 M ⊙ ) BCGs at intermediate redshifts. The selected sources are thus caught in a phase of rapid mass assembly, which makes them ideal targets for high-resolution observations of their molecular gas. We find that all three BCGs show point-like and steep-spectrum continuum emission at millimeter wavelengths, with a spectral slope of α = 0.6, which we interpret as being optically thin synchrotron emission from the AGN. By combining our NOEMA intensity and velocity maps with archival deep optical/infrared images from the Hubble Space Telescope , we unambiguously detected in situ star formation, filaments of accreting cold gas likely regulated by AGN feedback, a disturbed morphology associated with tidal tails of molecular gas, and gas compression and tails originating from the stripping of gas. While effective condensation of the intracluster medium is required to explain the large molecular gas reservoirs, the BCGs exhibit a broad variety of environment-driven mechanisms responsible for the processing of their cold gas: flows of cooling gas (RX 1532), ram pressure or sloshing of the intracluster medium (MACS 1447), and galactic tides (CHIPS 1911). This study thus provides new insights into the physical mechanisms responsible for the mass assembly of galaxies hosting AGN at the center of clusters
Quasi‐Thermal Noise Spectroscopy in Magnetized Space Plasma: Analysis of Wind/WAVES Measurements in Earth's Magnetosphere
No full textInternational audienceAbstract Quasi‐thermal noise is measured by an electric antenna embedded in a plasma, and is routinely used to obtain a diagnostic of, for example, the plasma density and electron temperature. Its applications have been so far to a large extent aimed at the analysis of unmagnetized plasmas, for which the plasma‐electron cyclotron frequencies ratio is large. The objective of this paper is to exploit measurements of magnetized quasi‐thermal noise, obtained by the Wind/WAVES TNR receiver in Earth's magnetosphere, deriving from them a full plasma diagnostic. To achieve such objective, we employ an analysis of the Wind/WAVES power spectrum and find the best fit between the measurements and two models for the quasi‐thermal noise, one of which is magnetized. From such analysis, we obtain the density and temperature of two electron populations, supposed Maxwellian, as well as of the magnetic field strength. We compare the diagnostic obtained with this technique with an approximated, but faster, approach also based on the quasi‐thermal noise analysis, and find that the different approaches provide coherent results within measurements uncertainties. Two crucial conclusions are drawn from this work. First: the quasi‐thermal noise is an alternative diagnostic technique for the plasma electrons in a magnetosphere. Second: the analysis of magnetospheric electric measurements might need to include the effects of quasi‐thermal noise, notably in the analysis of natural occurring waves
Spatiotemporal dynamics of nanosecond pulsed discharge in the form of a fast ionization wave: self-consistent two-dimensional modeling and comparison with experiments under negative and positive polarity
No full textInternational audienceNanosecond discharges are characterized by a shift in energy branching toward the excitation of electronic levels and dissociation, making them particularly attractive for plasma chemistry. Understanding the spatio-temporal structure of these discharges is especially important. This paper presents a detailed 2D-axisymmetric numerical analysis of a nanosecond discharge propagating in a long tube and in pure nitrogen. The modeling is conducted using a self-consistent plasma fluid solver under the local mean energy approximation, including photoionization. The discharge develops at moderate pressures, 1–10 Torr, in the form of a fast ionization wave (FIW). Simulations are performed for both negative and positive polarities of the voltage pulse applied to the high-voltage electrode. The computational results are validated against available experimental data, including FIW velocity within the studied pressure range, electron density, longitudinal electric field, and the radial distribution of N2(C3Πu) emission on a nanosecond timescale
Constraining the nature of the most extreme Galactic particle accelerator. H.E.S.S. observations of the microquasar V4641 Sgr
No full textInternational audienceMicroquasars have emerged as promising candidates to explain the cosmic-ray flux at petaelectronvolt energies. LHAASO observations revealed V4641~Sgr as the most extreme example so far. Using 100~h of H.E.S.S. data, we performed a spectro-morphological study of the gamma-ray emission around V4641~Sgr. We employed HI and dedicated CO observations of the region to infer the target material for cosmic-ray interactions. We detected multi-TeV emission around V4641~Sgr with a high significance. The emission region is elongated. We found a power-law spectrum with an index 1.8, and together with results from other gamma-ray instruments, this reveals a spectral energy distribution that peaks at energies of 100~TeV for the first time. We found indications (3) of a two-component morphology, with indistinguishable spectral properties. The position of V4641~Sgr is inconsistent with the best-fit position of the single-component model and with the dip between the two components. We found no significant evidence of an energy-dependent morphology. No dense gas was found at any distance towards V4641~Sgr. The peak of the SED at 100~TeV identifies V4641~Sgr as a candidate cosmic-ray accelerator beyond the so-called knee. The absence of dense target gas places stringent energetic constraints on hadronic interpretations, however. The H.E.S.S. measurement requires an unusually hard () spectral index for the protons. A leptonic scenario faces fewer obstacles if the particle transport is fast enough to avoid losses and to reproduce the observed energy-independent morphology. The absence of bright \xray emission across the gamma-ray emission region requires a magnetic field strength ~G, however. Our findings favour a leptonic origin of the gamma-ray emission. This conclusion does not exclude hadron acceleration in the V4641~Sgr system