LESIA - Observatoire de Paris

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

Séminaires passés

  • Mercredi 11 avril 2018 à 14h00 (Salle de conférence du bâtiment 17)

    Application of new inversion technique to the solar modelling problem and the determination of masses of red giants

    Gael Buldgen, University of Birmingham, HiROS group

    In the last decades, inversion techniques have been the most fruitful tools for the seismic analysis of the solar structure. They have allowed for the determination of the radial profile of thermodynamic quantities such as the density and the sound speed, thereby validating the so-called "standard" solar models, until the downward revision of the photospheric abundances of carbon, nitrogen and oxygen casted doubts on these models. This talk will show how inversion techniques of new quantities such as the entropy or the Ledoux discriminant can help provide new insights on the solar modelling problem and guide theoretical developments to its resolution.in addition to the solar problem, this seminar will discuss the application of mean density inversions to the radial oscillations of red giants. This part of the talk will show howmean density inversions can be used to determine more accurate values of this quantity than using the usual scaling laws, taking into account all the information of the radial oscillation spectrum.

    Les inversions sismiques ont été durant ces dernières décennies les approches les plus fructueuses pour l’analyse de la structure interne du soleil. Elles ont permis de déterminer les profils radiaux de grandeurs thermodynamiques telles que la vitesse du son et la densité, validant ainsi les modèles dits "standards" du soleil, jusqu’à ce que la révision de l’abondance photosphérique de carbone, azote et oxygène ne sème le doute sur les modèles théoriques. Ce talk présentera comment la combinaison de multiples inversions de nouveaux profils de grandeurs thermodynamiques telles que l’entropie et le discriminant de Ledoux permettent de jeter un regard neuf sur le problème de la métallicité solaire et peut guider les développements théoriques vers sa résolution. En plus de traiter du problème solaire, ce talk abordera également l’application des méthodes d’inversion sismiques aux modes radiaux des géantes rouges. Ce volet portera sur l’extension des inversions de densité moyenne à la branche des géantes, permettant une détermination plus exacte de cette grandeur qu’au moyen des lois d’échelles car tenant compte de toute l’information du spectre des oscillations radiales.


  • Jeudi 5 avril 2018 à 15h00 (Salle de réunion du bâtiment 16)

    A new look at the Sun with LOFAR : observations and implications

    Eduard Kontar (University of Glasgow, UK)


  • Jeudi 29 mars 2018 à 11h00 (Salle de conférence du bâtiment 17)

    PLATO (PLAnetary Tranists and Oscillations of stars) : statuts, concept instrumental et objectifs

    Martin Pertenaïs (Optical System Engineer of PLATO, DLR, Institute of Optical Sensor Systems)

    La mission spatiale de l’ESA PLATO – sélectionnée en 2014 par l’ESA dans le programme Cosmic Vision - progresse dans son développement qui l’amènera à un lancement vers L2 en 2026. Les deux objectifs scientifiques principaux de la mission sont la détection d’exoplanètes de type terrestres dans la zone habitable d’étoiles de type solaire, ainsi que le caractérisation de centaines de planètes de tous types.

    Durant ce séminaire, en plus du statut général de la mission, je rappellerai ces objectifs scientifiques et détaillerai les solutions instrumentales explorées pour résoudre ce challenge.


  • Mercredi 28 mars 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Evolution of magnetic structures in the pre-phase of X class flares

    Guiping Ruan (Shandong University, Weihai, Chine & LESIA)

    We analysed the long term pre-flare conditions of X2.1 and X1.8 flares. The events respectively includes a sigmoidal filament eruption, a coronal mass ejection, and a GOES X flare from NOAA active region 11283. Based on the HMI observation, for an area along the polarity inversion line underneath the filament, we found gradual pre-eruption decreases of both the mean strength of the photospheric horizontal field (Bh) and the mean inclination angle. We propose that the pre-event evolution of Bh may be used to discern the driving mechanism of eruptions. We also showed the expansion of the arcades overlying the filament until the reconnection. The reconnection occurred between the arcades and the pre-existing magnetic field. A NLFFF modelling confirmed the present scenario. I will present the two projects that I started at the LESIA :
    1. Study of eruptions and jets observed during a IRIS campaign with the MSDP of the solar tower.
    2. Study of the prominence plasma characteristics based on IRIS Mg II line profiles and MSDP Halpha line and radiative codes developed in Czech Republic.


  • Vendredi 23 mars 2018 à 14h00 (Salle de conférence du bâtiment 17)

    An ADCS 3-axis testbed system for nano/micro satellites

    Hoang The Huynh (USTH & LESIA)

    The development of nano/micro satellites is a new trend in space. The amount of those satellites is increasing rapidly by the time. Testing attitude determination and control system (ADCS) is playing an important role to the success of a satellite project. A testbed system is an essential tool to support the satellite developers. The talk will present the project of a hardware-in-the-loop system to test control laws and algorithms for ADCS of nano/micro satellites managed by the Vietnam National Space Center (VNSC). The system consists of three space environment condition simulators : zero gravity, Earth’s magnetic field and Sun light. This testbed system is expected to test various ADCS configurations of nano/micro satellites. Firstly, it will be a test platform for ADCS of NanoDragon satellite at VNSC. In addition, the talk will also present the PhD topic “Advanced Attitude Determination for Satellite ADCS Test-bed Using Multi-Sensor Fusion” that is one of the improvement research topics for above testbed system.

    Proposé par Boris Segret et Benoit Mosser.


  • Jeudi 22 mars 2018 à 11h00 (Salle de conférence du bâtiment 17)

    About Type II Radio Bursts (to be defined)

    Iver Cairns (University of Sydney, Australie)


  • Mardi 20 mars 2018 à 14h00 (Salle 204 du bâtiment Evry Schatzman (18))

    Ringdowns and echoes from the optical geometry perspective

    Marek Abramowicz (Varsovie et Göteborg)

    An observational proof for the existence of the event horizon is fundamentally impossible. However, there are many strong "near proofs". One of them, recently pointed out by Thibault Damour, and further discussed by Vitor Cardoso and others, is an absence of echoes in the gravitational wave ringdowns. I will give a pedagogical explanation of the issue in terms of the optical geometry. The optical geometry was introduced (in a different context) by Garry Gibbons and later explained dynamically by Brandon Carter, Jean-Pierre Lasota and me, and explored by many other authors.


  • Jeudi 15 mars 2018 à 16h00 (Salle de conférence du bâtiment 17)

    Observations of fast-moving structures in the debris disk of AU Microscopii : 3 years of follow-up with SPHERE

    Anthony Boccaletti (LESIA)

    The instrument SPHERE was installed at the VLT in 2014 and provides a significant gain in terms of contrast with respect to the previous generation of instruments. As a result, we now have access to very high contrast in the close environment of bright stars in particular the young systems in order to search for giant planets and circumstellar disks. During the commissioning in Aug 2014, SPHERE has revealed several structures (several AU in size) in the form of arches or undulations in the midplane of the debris disk around the star AU Microscopii. This disk is seen edge on and the system is conveniently close ( 10 pc) and young as well ( 20 Myr). The comparison of these SPHERE observations with the ones from STIS/HST 4 years before, not only allowed us to re-identify the structures in older data but most importantly led us to conclude that these structures were moving outwards in the disk, some with very large projected speed (4-10 km/s) hence possibly escaping the system. Several assumptions were considered to explain this behaviour, one of the them involves a body in Keplerian motion releasing some dust under the influence of the star’s activity. Since then, the object is regularly observed with SPHERE as part of the GTO and during a monitoring program. After a short introduction on the instrument and its modes as well as the achieved performance, I’ll remind the initial results from 2014 which led to the discovery of these fast-moving structures. Then, I’ll present the recent observations obtained from the last 3 years which unambiguously confirm the motion of the structures. The hypothesis of a parent body emitting an outflow of dust will be discussed in the light of these observations.


  • Vendredi 9 mars 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Finding new classes of exoplanetary systems with modern direct imaging techniques

    Elodie Choquet (JPL-Caltech)

    Within 2 decades, our classical view of planetary systems and of their formation mechanisms have been revolutionized by the observation of thousands of exoplanets. We now know that most stars host planets, that these planets are surprisingly diverse and often different from our Solar system’s, and that they probably form through a range of complex mechanisms. All these findings were obtained by looking within the first inner AUs only of exoplanetary systems, with indirect observing methods. How do planetary systems look like beyond 5 AU ? How common are planets there, what are their physical properties, how do they interact with the outer disks of dust and planetesimals ? Direct imaging can answer these questions by offering complete views of the outer regions of extrasolar systems. In the visible and near-infrared, the compelling regimes for studying planet atmospheres and dust properties, this observing method faces technical challenges that limit detections to the brightest objects. Here I will present recent works that improve the detection limits of direct imaging instruments, and how they lead to discoveries of a new class of faint objects. I will present my contribution to these developments and analyses, and I will discuss prospects toward detections and characterization of extrasolar systems with JWST.


  • Mardi 6 mars 2018 à 14h00 (Salle de conférence du bâtiment 17)

    Société NanoXplore, FPGA BRAVE, des FPGA durcis européens

    Proposé par Moustapha Dekkali

    NanoXplore est une société pionnière dans la conception de réseaux logiques programmables à grande échelle. Elle est actuellement engagée, au côté du CNES et de l’ESA, dans un programme ambitieux visant à produire une gamme de FPGA durcis européens : les FPGA BRAVE qui offrent à la fois la flexibilité et la performance.

    Le consortium mené par NanoXplore, en partenariat avec ST Microelectronics, a produit un premier FPGA : le NG-MEDIUM, un FPGA reprogrammable durci aux radiations, dont le kit de développement et les outils NanoXmap sont disponibles. Deux autres versions sont à l’étude : le NG-Large et le NG-XLarge.

    La présentation portera sur la gamme de FPGA BRAVE, leurs statuts et les outils associés.


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