Observatoire de Paris Institut national de recherche scientifique français Univerité Pierre et Marie Curie Université Paris Diderot - Paris 7

Sauf exception, les séminaires ont lieu sur le site de Meudon, dans la salle de conférence du bâtiment 17.

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Séminaires passés

  • Vendredi 16 décembre 2022 à 11h00 (Salle de conférence du bâtiment 17)

    Searching for singularities in fluid turbulence

    Thierry Lehner (Luth)

    We study hydrodynamical turbulence (for Euler and Navier Stokes=NS equations, mainly for the moment in incompressible cases with 3D homogeneous and isotropic turbulence) by two original ways.
    1) by postulating singularities (of the self similar Leray kind) and by detecting them indeed in real experiments data , which may lead to explain intermittency and dissipation of the turbulent kinetic energy. These predictions go beyond the Kolmogorov scaling laws. An in course proposal model on the possible nature of these singularities (finite time explosion) makes use of non linear Schrödinger equations (NLSE).
    2) by applying the scale relativity theory (born and developed in Meudon by L Nottale) we transform the NS equation into a macroscopic Schrodinger one , which allows to derive new results in turbulence , in particular on Lagrangian intermittency.
    3) we shall compare these current attempts with recent mathematical results on the existence of singularities for the related kind of Euler and NS equations.

    Nous étudions la turbulence hydrodynamique (équations de Euler et de Navier-Stokes=NS , surtout déjà dans des cas incompressible et 3D homogène et isotrope) par deux approches originales .
    1) en postulant des singularités (de type self-similaires à la Leray) et en les recherchant avec succès dans des data réels d’expérience , qui peuvent conduire à une explication de l’intermittence et être la source de la dissipation de l’énergie cinétique turbulente ; ces prédictions diffèrent des lois d’échelle à la Kolmogorov. Une modélisation proposée (en cours) de la nature possible de ces singularités (explosion en temps fini) est faite à l’aide d’équations de Schrodinger non linéaires .
    2) en appliquant la théorie de la relativité d’échelle (née et développée à Meudon par L Nottale) on transforme l’équation de NS en une équation de type Schrodinger au niveau ‘macroscopique’ , ce qui nous permet d’obtenir des résultats nouveaux en turbulence dont sur l’intermittence lagrangienne.
    3) on confrontera ces 2 approches avec des résultats mathématiques récents obtenus sur l’existence de singularités pour les équations de type Euler et de NS.

  • Vendredi 9 décembre 2022 à 11h00 (Salle de conférence du bâtiment 17)

    Bayesian modelling of beta-meteoroid observations with Solar Orbiter

    Samuel Kociscak (LESIA)

  • Jeudi 1er décembre 2022 à 16h00 (Salle de réunion du bâtiment 16 et visioconférence)

    Photonics, astrophysics, and mid-infrared glass

    Harry Kenchington Goldsmith (LESIA)

    Direct imaging of exoplanets promises the detection of new worlds in the habitable zone around distant stars. Interferometry uses multiple, or one-segmented, telescopes to make direct observations of exoplanets. In the mid-infrared the light of the star outshines the exoplanet by 8 orders of magnitude and interferometry can overcome this by nulling the star. New technologies in photonics may provide a pathway to do this. Photonics is the study of light guided and manipulated within transmissive materials. It has roots in the telecommunication industry and is now being used by astronomers in visible light and near-infrared light, with mid-infrared light to follow in the near future. The basic applications in astronomy are to move away from telescopes with large lab benches and environmentally sensitive fibre optics, to small and stable photonic chips. In this talk I will detail my work in mid-infrared photonics, highlighting the production of evaporatively deposited photonic chips and including specific issues with adapting photonic chips for astronomy applications, with the goal to create a nulling interferometer to detect exoplanets.

    Lien pour assister au séminaire :

  • Lundi 28 novembre 2022 à 16h00 (Salle de conférence du bâtiment 17)

    Caractérisation des phases d’altération dans les roches et les sols de Mars par LIBS avec l’instrument ChemCam (Mars Science Laboratory/Curiosity)

    Gael DAVID (LESIA)

    Les conditions environnementales passées de la surface de Mars sont accessibles à travers les enregistrements géologiques de la planète, et plus particulièrement via les minéraux d’altération qu’ils contiennent. Ces phases secondaires peuvent constituer de véritables marqueurs géochimiques des conditions durant l’altération aqueuse.

    L’instrument ChemCam, à bord du rover Mars Science Laboratory (Curiosity) est le premier instrument de spectroscopie sur plasma induit par laser (LIBS) déployé à la surface de Mars. Il offre un moyen inédit de caractériser in situ et à une échelle d’analyse submillimétrique, la composition chimique de la surface de la planète. Durant ce séminaire, je vais présenter quelques travaux réalisés avec cet instrument sur la détection de phases d’altération (i.e. sulfates et oxydes de fer) dans les sols et les roches sédimentaires du cratère de Gale. Leurs processus de formation seront également discutés ainsi que l’histoire géologique qui en découle.

  • Lundi 21 novembre 2022 à 16h00 (Salle de conférence du bâtiment 17)

    Directly imaging cold and temperate exoplanets in reflected starlight : atmospheric characterization and target selection

    Oscar Carrion (LESIA)

    With more than five thousand exoplanets discovered to date, the next milestone is the atmospheric characterization of a significant number of these worlds. The observing techniques that are currently available have biased the discovery and atmospheric characterization of exoplanets towards hot, giant planets on short-period orbits. Direct-imaging observations will be needed to overcome these biases and enable the analysis of cold and temperate long-period exoplanets and their atmospheres.

    Studying this population of planets, which has remained out of reach thus far, is a key to understand the diversity of exoplanets and, in particular, the existence of habitable conditions beyond the Earth. Direct-imaging observations of temperate exoplanets have indeed been identified as a scientific priority for the coming decades both by the US Astro 2020 Decadal Survey and by the ESA Voyage 2050 report. In this talk we address the physical fundamentals of direct-imaging observations in reflected starlight, and derive practical conclusions for the planning and interpretation of the upcoming measurements.

    We also analyse the prospects for atmospheric characterization of exoplanets from reflected-starlight spectra with different observing strategies. Finally, we compute which of the known exoplanets are potentially observable with several upcoming direct-imaging space telescopes, and discuss some interesting science cases.

  • Lundi 10 octobre 2022 à 16h00 (Salle de conférence du bâtiment 17)

    A year of near-IR spectroscopy on Mars with Perseverance/SuperCam

    Clément Royer et Lucia Mandon (LESIA)

    The Perseverance rover (Mars 2020 mission, NASA) landed in Jezero crater, Mars, the site of an ancient lake, on February 2021. The main science objectives of the mission are the characterization of past environments, the search for preserved biosignatures and the collection of samples to be returned to Earth by the MSR mission (Mars Sample Return). The payload of Perseverance includes the SuperCam instrument, which combines various remote-sensing techniques to investigate the elemental and mineralogical composition of rocks and soils. In particular, the near-infrared reflectance spectrometer (IRS), which covers the 1.3–2.6 µm range, allows for the identification of a wide variety of mineral phases, and especially hydrated ones.

    From a technical point of view, IRS is a miniaturized spectrometer based on AOTF (Acousto-Optic Tunable Filter) technology. This optical element is the cornerstone of the IRS’ performance and its flight calibration will be presented in this seminar. Then, we will give an overview of the minerals detected by the IRS during the first year of the mission and the implications on the geological history of the crater floor in terms of igneous and alteration processes. As the IRS is the first near-infrared spectrometer operating directly on the surface of Mars, we will also discuss how these results bring insights into the orbital studies using near-infrared data.

  • Lundi 3 octobre 2022 à 16h00 (Salle de conférence du bâtiment 17)

    The Dragonfly landing site seen by the Cassini RADAR

    Léa Bonnefoy (Cornell University)

    The Selk crater region is the future landing site of NASA’s Dragonfly mission to Titan. The region was imaged by the Cassini RADAR at incidence angles from 5° to 72° and at various polarization angles. Using this data set, we assembled backscatter curves for six terrains. By fitting different models to these data we derived new composition and roughness constraints for each terrain. Topography from radarclinometry also revealed a non-uniform crater rim, less eroded than previously thought.

  • Jeudi 29 septembre 2022 à 16h00 (Salle de réunion du bâtiment 16 et visioconférence)

    Data reduction and inverse problem approaches - An efficient (and cheap) way to push the instrumental limits

    Anthony BERDEU (LESIA)

    Study of astrophysical objects requires always more complex models (black hole surrounding, exoplanet atmosphere, accretion disk, galaxy formation and evolution, …) whose inputs imply always more precise measurements. On the other side, the astronomical instruments start to reach their fundamental limits (negligible sensor readout noise, precise mirror polishing, technologies harder to scale up to extremely large telescopes, …). During my talk, I will discuss how data science applied for optimal data reduction and processing via inverse problem approaches can bridge this gap, by pushing the experimental limits without the need of further instrumental developments. I will introduce different applications of my research : integral field spectroscopy (SPHERE/IFS), blind deconvolution and PSF reconstruction (SPHERE/ZIMPOL) and extreme adaptative optics for coronagraph (Evanescent Wave Coronagraph, EvWaCo).

    Lien pour assister au séminaire :