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 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.

  • Lundi 5 mars 2018 à 14h00 (Salle de réunion du bâtiment 12)

    AO calibration strategy for the ELT : toward a Pseudo-Synthetic Interaction Matrix ?

    Cedric Taïssir Heritier (LAM/ONERA/Obs. Arcetri/ESO)

    To benefit from the full scientific potential of the future Extremely Large Telescope, its instruments will rely on Adaptive Optics (AO) systems. However, the design of the ELT will provide a challenging environment for the AO calibration, as there will be moving elements in the system and no calibration source. To overcome these constraints, a complete rethinking of the AO calibration procedures becomes necessary.

    Some strategies have already been identified and tested on 8-meters facilities such as the VLT-AOF or the LBT-FLAO and the first results seem to lead to a Pseudo-Synthetic approach, merging on-sky measurement and synthetic models. After introducing the context, I will present the current developments of a synthetic model to reproduce the LBT-FLAO Pyramid WFS behavior in the AO simulator OOMAO.

  • Jeudi 1er mars 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Impact of atomic diffusion on the structure and surface abundances of G and F type stars : stellar parameter determinations and effects of rotation

    Morgan Deal (LESIA)

    Atomic diffusion, including the effect of radiative accelerations on individual elements, leads to variations of the chemical composition inside the stars as well as the surface abundances evolution. Indeed the accumulation in specific layers of the elements, which are the main contributors of the local opacity, modifies the internal stellar structure and surface abundances. Here we show that the variations of the chemical composition induced by atomic diffusion in G and F type stars can lead to an increase of the iron surface abundance and to an increase of the Rosseland mean opacity at the bottom of the surface convective zone. This induces a modification of the size of the surface convective zone, of the radius of the star, of some seismic parameters, and more importantly of [Fe/H]. We also show the effect of the coupling between rotation and atomic diffusion is this kind of stars. These processes need to be taken into account in stellar evolution models as the observations are more and more precise, especially in the context of the future space missions TESS and PLATO.

  • Mercredi 28 février 2018 à 11h00 (Salle de réunion du bâtiment 16)

    Small-scale structures in the upper atmosphere of the Sun

    Krzysztof Barczynski (LESIA)

    Numerous small-scale structures (sizes of the order of megameters) constitute the background for the large-scale structures in the solar atmosphere. Their large number suggests that they play an important role in the energy transport and the magnetic structuring in the solar atmosphere.

    Properties of the small-scale structures in the solar atmosphere will be discussed. Particular attention is given to miniature loops (with a length of approximately 1 Mm) observed for the first time at coronal temperature (> 1 MK), and their relation between the emission of the small-scale structures and the underlying magnetic field. We also make a focus on the structures which are unresolved by modern instruments. We investigate the relation between emission from the different part of the solar atmosphere and underlying magnetic field. This study provides a statistical proxy of the properties of unresolved small-scale structures. We present study based on UV and EUV observation (images, spectra) with a combination of photospheric magnetic field maps.

    We show that miniature loops are a small-scale version of the hot coronal loop. We also find how the correlation and intensity-magnetic field relations (presented in our study as a power-law) change moving up from the upper photosphere to the transition region and discuss possible interpretations of obtained dependencies.

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