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

  • Mardi 30 juin 2009 à 11h00 (Salle de confĂ©rence du bât. 17)

    Interstellar dust in the solar system : space research meets astronomy

    Ingrid Mann (Kindai University, Higashi-Osaka, Osaka, Japan)

    Interstellar dust plays a major role in the physics and chemistry of the interstellar medium and for the formation of stars and their planetary systems. The motion of the Sun relative to the surrounding interstellar medium causes a direct flow of interstellar dust into the solar system. During the past years the interstellar dust flux has been measured with dust detectors on spacecraft in the inner solar system approximately between the orbits of Mercury and Jupiter. Current understanding of the interstellar dust, however, is based on astronomical observations. The astronomical observations reveal the light scattering and thermal emission properties of interstellar dust and these often show similarities to the observed properties of cometary dust in the solar system. The interstellar dust measurements from spacecraft are so far limited to the detection of dust fluxes and measurements of the mass and velocity of the particles, yet they reveal a different picture of interstellar dust and of the local interstellar medium surrounding the solar system.

  • Friday 19 June 2009 à 14h00 (Salle de confĂ©rence du bât. 17)

    Imaging and characterizing the habitable zones of nearby planetary systems with the Pupil mapping Exoplanet Coronagraphic Observer (PECO)

    Olivier Guyon (Subaru Telescope, National Observatory of Japan, Hawaii)

    The Pupil-mapping Exoplanet Coronagraphic Observer (PECO) is a NASA-funded mission concept which uses a coronagraphic 1.4-m space-based telescope to both image and characterize extra-solar planetary systems at optical wavelengths. PECO delivers 1e10 contrast at 2 lambda/D separation (0.15") using a high-performance Phase-Induced Amplitude Apodization (PIAA) coronagraph which remaps the telescope pupil and uses nearly all of the light coming into the aperture. For exoplanet characterization, PECO acquires narrow field images simultaneously in 16 spectral bands over wavelengths from 0.4 to 0.9 micron, utilizing all available photons for maximum wavefront sensing and sensitivity for imaging and spectroscopy. The optical design is optimized for simultaneous low- resolution spectral characterization of both planets and dust disks using a moderate-sized telescope. PECO will image the habitable zones of about 20 known F, G, K stars at a spectral resolution of R 15 with sensitivity sufficient to detect and characterize Earth-like planets and to map dust disks to within a fraction of our own zodiacal dust cloud brightness. PECO technologies are actively being developed and tested in laboratory testbeds. Some of these techniques also have interesting scientific potential for ground-based coronagraphic imaging systems.

  • Jeudi 28 mai 2009 à 14h15 (Salle de confĂ©rence du bât. 17)

    The Formation and Evolution of Millisecond Pulsars

    Jason Hessels (ASTRON / UniversitĂ© d’Amsterdam)

    Millisecond pulsars are neutron stars that spin hundreds of times a second. Their fantastic rotation rates are commonly believed to be the result of an accretion phase in which an old neutron star accretes both matter and angular momentum from a binary companion, and is "recycled" to become a radio pulsar once again - now spinning much faster than before and with a significantly reduced magnetic field strength. I will discuss our current understanding of radio millisecond pulsars, focusing primarily on those that are found in Galactic globular clusters, but also discussing those objects found in the field.

  • Mardi 19 mai 2009 à 11h00 (Salle de confĂ©rence du bât. 16)

    L’instabilitĂ© par ondes de Rossby dans les disques d’accrĂ©tion : simulations numĂ©riques 3D

    Héloïse Méheut (APC, Université Paris 7- Denis Diderot)

    Les ondes de Rossby sont connues de longue date en mĂ©tĂ©orologie et ocĂ©anographie mais ce n’est que plus rĂ©cemment qu’elles ont Ă©tĂ© utilisĂ©es en astrophysique. Leur physique devient particulièrement intĂ©ressante dans le cas des disques en rotation diffĂ©rentielle puisqu’elles peuvent dĂ©stabiliser l’ensemble du disque et donc modifier leur courbe de lumière. Je dĂ©crirai plusieurs applications de cette instabilitĂ©, en particulier je montrerai comment celle-ci permet d’expliquer les oscillations quasi-pĂ©riodiques observĂ©es lors des Ă©ruptions (flares) du trou noir central de la Galaxie. Jusqu’Ă  prĂ©sent l’instabilitĂ© par onde de Rossby n’avait Ă©tĂ© mise en Ă©vidence et Ă©tudiĂ©e qu’Ă  deux dimensions, son existence dans les disques d’accrĂ©tion rĂ©els restant incertaine. Je montrerai donc finalement une Ă©tude tridimensionnelle de l’instabilitĂ© et les mĂ©thodes numĂ©riques utilisĂ©es.

  • Mercredi 8 avril 2009 à 11h00 (Salle de confĂ©rence du bât. 17)

    Retour vers Jupiter : la mission EJSM/Laplace (Europa Jupiter System Mission)

    Pierre Drossart (LESIA, Observatoire de Paris)

    La mission EJSM Ă  l’Ă©tude conjointement entre la NASA et l’ESA se compose de deux orbiteurs, le Jupiter Europa Orbiter (NASA) qui se satellisera autour du satellite Europe et le Jupiter Ganymede Orbiter (ESA) qui prendra lui la direction de Ganymede. Tous deux auront pendant une phase d’approche d’environ 18 mois la possibilitĂ© d’Ă©tudier l’atmosphère et la magnĂ©tosphère de Jupiter ainsi que ses autres satellites. Une prĂ©sentation gĂ©nĂ©rale du scĂ©nario de la mission sera exposĂ©e, avec ses objectifs scientifiques et ses dĂ©fis techniques.

  • Lundi 6 avril 2009 à 11h00 (Salle de confĂ©rence du bât. 17)

    Microwave and HXR Observations of an Impulsive Flare : the Nature of Quasiperiodic Pulsations

    Tim Bastian (National Radio Astronomy Observatory, Charlottesville, VA, USA)

    Oscillations and quasi-periodic pulsations (QPPs) have been observed in the radio and X-ray emission from solar flares for many years. Generally speaking, radio QPPS have been attributed to nonlinear, self-organizing wave-wave or wave-particle interactions in the case of coherent emission, and to modulation of the source parameters in the case of incoherent emission. In particular, incoherent gyrosynchrotron emission at centimeter wavelengths is typically attributed to variations in the coronal magnetic field due to the excitation of MHD modes and/or the modulation of the acceleration and injection of energetic electrons into the source. This talk concerns some recent work on an impulsive flare that displayed QPPs in both its microwave and HXR emission. Observations by the Nobeyama Radioheliograph, the Nobeyama Polarimeters, the Owens Valley Solar Array, and the Ramaty High Energy Solar Spectroscopic Imager were obtained and a Fourier analysis of the radio and X-ray data was performed. Together with observations of the time variability of the radio polarization and spectral index, these data show that MHD oscillations are unlikely to play a significant role in modulating the source ; quasi-periodic acceleration and injection of the energetic electrons is the more likely explanation.

  • Vendredi 27 mars 2009 à 16h00 (Salle de confĂ©rence du bât. 17)

    DĂ©veloppement de la coronographie de phase aux US

    Dimitri Mawet (JPL)

    Longtemps considĂ©rĂ©es comme Ă©trangetĂ© aux US, la coronographie de phase est en train de fleurir au pays de l’oncle Sam. En 2005, Gene Serabyn et Pierre Haguenauer ont installe un coronographe a 4 quadrants sur le tĂ©lescope Hale du Mont Palomar. En dĂ©finissant une sous-ouverture non-obstruĂ©e hors axe de 1.5 m et en y concentrant l’action de l’optique adaptative existante, le 4Q y bĂ©nĂ©ficie de conditions extrĂŞme AO optimales. Je prĂ©senterai une partie des rĂ©sultats observationnels obtenus grace a ce tandem unique, sur le sujet des naines brunes et de disques circumstellaires. Fort de cette expĂ©rience fructueuse avec les coronos de phase qui a mis en exergue leur avantage observationnel sur les coronographes de Lyot, le JPL a dĂ©cidĂ© d’investir dans le dĂ©veloppement du concept prometteur de vortex optique vectoriel (OVVC). Je prĂ©senterai les derniers rĂ©sultats expĂ©rimentaux en infrarouge et visible de cette nouvelle famille de coronographe, proche de l’idĂ©al coronographique, ainsi que les perspectives d’intĂ©gration des OVVCs dans des instruments au sol (instruments de seconde gĂ©nĂ©ration pour la dĂ©tection et caractĂ©risation d’exoplanètes) et dans l’espace.

  • Mardi 10 mars 2009 à 11h00 (Salle de confĂ©rence du bât. 17)

    Reconstruction of magnetic fields in the solar corona

    Gherardo Valori (Astrophysical Institute Potsdam, Germany)

    The quiescent solar corona is regularly modified by very fast ejections of coronal material and magnetic field (CME) that occur preferably above active regions. Most CME models require the formation of twisted magnetic field structures (flux ropes) before or during such events. Unfortunately, the coronal magnetic field is not directly measurable at present, and therefore it is difficult to verify the validity of different CME models. In order to remove this obstacle, the extrapolation of photospheric magnetic field measurements can be used to reconstruct the missing coronal information. In such applications it is necessary to estimate how well an extrapolation code can reproduce all aspects of highly nonlinear structures such as flux ropes. This is of course possible only using test fields.
    In this talk I will present a series of applications of the magneto-frictional extrapolation code to both test fields and measured data. One of the considered test field is the Titov and DĂ©moulin force-free equilibrium (Titov and DĂ©moulin, Astr. and Astrophys. 351, 707, (1999), hereafter TD). The TD equilibrium models a semi-circular, 3D current-carrying flux rope by means of a current ring embedded in a potential field. The parameters of the TD model can be adjusted to create both stable and kink- and torus-unstable configurations. Employing the TD equilibrium as a test field, I will show that our magnetofrictional extrapolation code can reproduce the energy and the twist of the magnetic field within a percent accuracy. This information is essential for the reconstruction of coronal fields involved in eruptions because the twist is, together with the height profile of the overlying potential field, the most important stability parameter - at least as long as the TD equilibrium is a good model of an active region. In the talk I will also show extrapolations of measured magnetograms. In this case, the assumption made in the derivation of the extrapolation methods are not entirely fulfilled, leading to a lower self-consistency of the reconstructed field. The conflict between measurements’ properties and model’s assumption can be partially reconciled employing a data-preprocessing method. However, a significant dependence of the extrapolation results on the details of the employed methods and on their numerical implementation is found. Therefore, in applications to measured magnetograms, an extensive comparison of the extrapolated field with subsidiary observations is essential to assess the reliability of the reconstructed coronal field.

  • Mardi 3 fĂ©vrier 2009 à 11h00 (Salle de confĂ©rence du bât. 17)

    Migration planĂ©taire : application aux systèmes extrasolaires et solaire

    Aurélien Crida (DAMTP, University of Cambridge, UK)

    Au cours de leur formation dans les disques protoplanĂ©taires, les planètes Ă©changent du moment cinĂ©tique avec le gaz du disque. Ainsi, elles peuvent ouvrir un sillon dans le disque, et migrer, gĂ©nĂ©ralement vers l’Ă©toile centrale. Après une introduction Ă  ces phĂ©nomènes, et une revue des processus impliquant une planète seule, ce sĂ©minaire se focalisera sur la migration de plusieurs planètes Ă  fois.

    Deux planètes peuvent entrer en rĂ©sonance au cours de leur migration, ce qui permet d’expliquer les paramètres orbitaux de nombreux systèmes extra-solaires. En particulier, nous avons reproduit le système de GJ 876 au moyen de simulations numĂ©riques hydro-dynamiques. Sous certaines conditions, la migration d’une paire de planètes en rĂ©sonance se produit vers l’extĂ©rieur. Le cas des exo-planètes rĂ©cemment dĂ©couvertes par imagerie directe Ă  plusieurs dizaines d’AU de leur Ă©toile en est peut-ĂŞtre l’illustration.

    Enfin, dans le cas du système solaire, la migration des planètes gĂ©antes a Ă©tĂ© de très faible amplitude. Une rĂ©sonance entre Jupiter et Saturne l’expliquerait. Une telle configuration s’avère compatible avec le modèle de Nice, dans lequel les planètes gĂ©antes atteignent leurs orbites actuelles 600 millions d’annĂ©es après la dissipation du disque protoplanĂ©taire. Les consĂ©quences de ce modèle sur la structure de la NĂ©buleuse Solaire de Masse Minimale seront discutĂ©es.

  • Vendredi 16 janvier 2009 à 11h00 (Salle de confĂ©rence du bât. 17)

    Turbulence MHD Hall et vent solaire

    Sébastien Galtier (IAS, Université Paris-Sud, IUF)

    Le vent solaire est souvent considĂ©rĂ© comme un laboratoire astrophysique unique pour l’Ă©tude des processus non linĂ©aires dans les plasmas. Dans ce sĂ©minaire, je ferai une revue des rĂ©sultats observationnels et thĂ©oriques sur les fluctuations magnĂ©tiques Ă  basses (f < 1Hz) et hautes (f > 1 Hz) frĂ©quences. La question principalement soulevĂ©e concerne l’origine physique d’une seconde zone inertielle observĂ©e Ă  hautes frĂ©quences qui ne peut ĂŞtre associĂ©e Ă  des processus relevant de la MHD, comme c’est le cas Ă  basses frĂ©quences. Dans ce contexte, je prĂ©senterai de rĂ©cents rĂ©sultats thĂ©oriques et numĂ©riques sur la turbulence MHD Hall et discuterai de l’importance d’inclure l’anisotropie pour interprĂ©ter correctement les donnĂ©es spatiales.