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

Séminaires passés

  • Jeudi 24 janvier 2019 à 16h00 (Salle de conférence du bâtiment 17)

    FIRST interferometer - Exploring new photonic technologies for next generation high-contrast imaging instrumentation

    Nick Cvetojevic (LESIA)

    FIRST (Fibered Imager foR a Single Telescope) is a post-extreme AO instrument module undergoing commissioning at the Subaru Telescope that enables high-contrast imaging at sub-diffraction limit spatial scales. With the aim of increasing the instrument’s stability, sensitivity, and dynamic range, a comprehensive upgrade of FIRST’s interferometric components was undertaken in Meudon, developing a new series of photonic on-chip beam combiners and automated optoelectronic delay lines for rapid phasing of each sub-pupil. Integrated into the new photonic chips are waveguides in crystalline electro-optic material (Lithium niobate) that enable on-chip active phase control of the light at high speeds (up to kHz). This new technology enables unprecedented active control of the starlight (nulling/fringe tracking) to be miniaturised and integrated into the photonics themselves, as well as opening up new avenues of how fringes are obtained. This type of photonic architecture has not been implemented previously for astronomical interferometry and provides FIRST with key advantages over similar instruments, as well as testing the technology for future ELT instrumentation.


  • Mercredi 23 janvier 2019 à 11h00 (Salle de réunion du bâtiment 14)

    Polarimetry : a detective magnifying glass to probe astronomical objects

    Frédéric Marin (Observatoire de Strasbourg)

    One of the main challenges in astrophysics is that we are trying to understand cosmic objects without fully spatially resolving them. Many small scale details remain elusive. Since we do not have yet a complete knowledge of both the large scale and small scale structures, composition or magnetic topology of our favorite cosmic objects, we are ultimately struggling to understand the physical mechanisms that govern them. High-angular, high-resolution imaging, spectroscopy, timing or interferometric techniques provide us with exquisite details but they all remain poorly sensitive of the morphology, magnetic fields or gravitational effects governing the small scales. To reveal the truth, polarimetry is probably our best tool. As we will see this during the presentation, polarimetric techniques are highly sensitive to the smallest geometric details, from the morphology of accretion disks to the thinnest shells of supernova remnants. Polarization can also reveal the magnetic topology in solar flares, the magnetic reconnections in disk’s atmospheres or the synchrotron emission in jets. Starting the presentation with a detective enigma, I will then review what polarimetry can do and what are the expected discoveries to be made in virtually all astrophysical fields thanks to the new generation of polarimeters that will arrive in the close future.


  • Mardi 15 janvier 2019 à 11h00 (ATTENTION : changement de salle : bâtiment 14)

    In-flight photometry extraction of PLATO targets : optimal apertures based on noise-to-signal-ratio and frequency of threshold crossing events

    Victor Marchiori (LESIA)

    ESA’s PLATO space mission is devoted to unveiling and characterizing new extrasolar planets and their host stars. It will encompass a very large (>2200 deg2) field-of-view, granting it the potential to survey up to one million stars depending on the final observation strategy. Spacecraft telemetry budget cannot handle transmitting individual images for such a huge stellar sample, so the development of an appropriate strategy to perform on-board data reduction is mandatory. We employ mask-based (aperture) photometry to produce stellar lightcurves in-flight. Our aim is thus to find the mask model that optimizes the scientific performance of the reduced data. Three distinct aperture structures are considered. First, we set up a weighted mask delivering global minimum noise-to-signal ratio, computed through a novel fast convergence algorithm. A second weighted mask is built following a Gaussian function. Lastly, we define a mask containing exclusively binary weights. Each strategy is tested on synthetic imagettes generated for 50,000 potential PLATO targets. Stellar population is extracted from Gaia DR2 catalogue. A pioneer criteria is adopted for choosing the optimal solution (structure) : the one providing the best compromise between sensibility to detect true and capability to reject false planet transits, determined based on noise-to-signal ratio and frequency of threshold crossing events. Our results show that, although binary mask statistically presents a few percent higher noise-to-signal ratio compared to weighted masks, all three strategies have nearly the same efficiency in detecting legit planet transits. When it comes to avoid spurious signals from contaminant stars though, binary mask statistically collects significantly less contaminant flux than weighted masks, making the former to be 30% less likely to deliver false transit signatures at 7.1sigma detection threshold. In planet transit finding context thus, choosing apertures based exclusively on how well a transit-like signal can be detected may not be the optimal approach.


  • Mardi 15 janvier 2019 à 11h00 (Salle de réunion du bâtiment Jean-Louis Steinberg (bât. 16))

    Density turbulence in the solar wind using low-frequency angular broadening observations

    K. Sasikumar Raja (LESIA)

    Various types of remote sensing observations have been used so far to probe the weakly compressible density turbulence in the extended solar corona and solar wind. Using the angular broadening observations of radio celestial point like sources, we have studied various turbulent parameters in the solar wind : anisotropic broadening, amplitude of density turbulence, density fluctuations, proton heating rate and the dissipation scales. For this study, we used the observations of Gauribidanur radioheliograph, Very Large Array and other historical observations carried out during 1952-2017. In this talk, I will discuss, the way these parameters vary with heliocentric distance and the solar cycle. The newly launched Parker Solar Probe and upcoming Solar Orbiter missions are going to play a crucial role in addressing these issues and other long standing mysteries of the solar wind.


  • Mercredi 9 janvier 2019 à 11h00 (Salle de conférence du bâtiment 17)

    X-ray diagnostics of high energy solar phenomena : from the RHESSI spacecraft to the FOXSI sounding rocket

    Sophie MUSSET (University of Minnesota, USA)

    Solar X-rays provide the most direct diagnostics of energetic electron populations during solar flares. I will present recent results concerning particle acceleration and propagation in the solar corona from X-ray observations with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and extend the discussion to the need for new X-ray instrumentation using X-ray focusing optics. This instrumental concept was demonstrated with the FOXSI sounding rocket program and I will present results on X-ray diagnostics of microflares obtained with FOXSI observations.


  • Jeudi 20 décembre 2018 à 16h00 (Salle de conférence du bâtiment 17)

    Accurate Mass Measurements for Planetary Microlensing Events Using High Angular Resolution Observations with KECK and HST

    Jean-Philippe Beaulieu (IAP)

    The microlensing technique is a unique method to hunt for cold planets over a range of mass and separation, orbiting all varieties of host stars in the disk of our galaxy. It provides precise mass-ratio and projected separations in units of the Einstein ring radius. In order to obtain the physical parameters (mass, distance, orbital separation) of the system, it is necessary to combine the result of light curve modeling with lens mass-distance relations and/or perform a Bayesian analysis with a galactic model. A first mass-distance relation could be obtained from a constraint on the Einstein ring radius if the crossing time of the source over the caustic is measured. It could then be supplemented by secondary constraints such as parallax measurements, ideally by using coinciding ground and space-born observations. These are still subject to degeneracies, like the orbital motion of the lens. A third mass-distance relation can be obtained thanks to constraints on the lens luminosity using high angular resolution observations with 8 m class telescopes or the Hubble Space Telescope. The latter route, although quite inexpensive in telescope time is very effective. If we have to rely heavily on Bayesian analysis and limited constraints on mass-distance relations, the physical parameters are determined to 30–40% typically. In a handful of cases, ground-space parallax is a powerful route to get stronger constraint on masses. High angular resolution observations will be able to constrain the luminosity of the lenses in the majority of the cases, and in favorable circumstances it is possible to derive physical parameters to 10% or better. Moreover, these constraints will be obtained in most of the planets to be discovered by the Euclid and WFIRST satellites. We describe here the state-of-the-art approaches to measure lens masses and distances with an emphasis on high angular resolution observations. We will discuss the challenges, recent results and perspectives.


  • Mardi 11 décembre 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Importance of radiative accelerations for the transport of chemical elements in main-sequence stars

    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 have significant impact on their structure, stellar parameters and seismic properties. We will also discuss the effect of the coupling between rotation and atomic diffusion for such 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 space missions TESS and PLATO.


  • Vendredi 30 novembre 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Co-évolution de la vie et de l’atmosphère de la Terre primitive

    Benjamin Charnay (LESIA) et Boris Sauterey (IBENS, Institut de Biologie de l’Ecole Normale Supérieure)

    La vie et l’atmosphère de la Terre primitive semblent avoir évolués de façon couplée. L’exemple le plus frappant est la Grande Oxygénation de l’atmosphère, provoquée par l’apparition de la photosynthèse oxygénique, qui a bouleversée la chimie atmosphérique et océanique, la minéralogie, l’évolution de la vie et potentiellement le climat avec le déclenchement des glaciations Huroniennes. Il a également été proposé qu’avant la Grande Oxygénation, l’atmosphère archéenne était riche en méthane issu d’organismes méthanogènes. Ce méthane aurait provoqué un effet de serre permettant de compenser la plus faible insolation à cette époque, et donc de maintenir un climat tempéré. Cependant, aucune modélisation globale et réaliste du système Terre incluant la biosphère primitive n’a été réalisée pour tester ces hypothèses.

    Notre objectif est d’étudier les interactions entre les écosystèmes et l’environnement (atmosphère, océan et activité géologique) de la Terre primitive, ainsi que leur évolution. Pour cela, nous avons couplé un modèle climatique incluant la photochimie, à un modèle du cycle du carbone et un nouveau modèle réaliste et dynamique d’écosystème primitifs, qui inclut les différents métabolismes non photosynthétiques.

    Durant cette présentation, nous décrirons le modèle climat-photochimique (le GCM Générique du LMD), le modèle du cycle du carbone et le modèle d’écosystème. Nous présenterons ensuite les résultats obtenus en couplant ces différents modèles, qui nous permettent de quantifier les teneurs en méthane durant l’Hadéen/Archéen et d’étudier les rétroactions climatiques causées par les écosystèmes de méthanogènes. Nous terminerons en discutant brièvement des différentes perspectives que nous offre ce modèle unique.


  • Lundi 26 novembre 2018 à 14h00 (Salle de conférence du bâtiment 17)

    Cours d’évolution stellaire sur la branche des géantes rouges

    Arlette Noels (Université de Liège, STAR institute)

    La Professeure Arlette Noels fera un cours d’évolution stellaire avancée pour les étoiles de faible masse sur la phase des géantes rouges. Cours en langue française.


  • Mardi 13 novembre 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Forecasting geoeffeciency of coronal mass ejections and their sheath regions

    Emilia Kilpua (University of Helsinki, Finlande)

    Coronal mass ejections (CMEs) are key drivers of magnetospheric storms at Earth. Two key structures in CMEs are the turbulent sheath and the magnetic flux rope with smoother variations of solar wind plasma and magnetic field parameters. In this presentation I will discuss the status and challenges in predicting in advance (at least half-a-day) geoefficiency of flux ropes and sheath regions. The key focus will also be on observational and data-driven modelling efforts at the University of Helsinki space physics team to predict the magnetic field characteristics of flux ropes.


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