LESIA - Observatoire de Paris

  • Wednesday 15 December 2010 à 11h00 (** Salle 204 du bat. 18 **)

    Simulating observations to test general relativity in its strongest regime at the Galactic Center with the instrument GRAVITY.

    Frederic Vincent (LESIA)

    Probing gravity in the immediate vicinity of a supermassive black hole would be a powerful test of general relativity (GR). The Galactic Center is an ideal laboratory to do so. Indeed, flares are regularly observed there, which could be due to a hot spot orbiting on the last stable orbit of the black hole: a perfect probe of strong gravity.
    The precise observation of such events will be possible in the near future thanks to the second generation VLTI beam combiner GRAVITY. To determine the level at which this instrument will put constraint on general relativity, a full GR ray-tracing code has been developed: GYOTO. The aim of this talk is to present recent results from this code. I will insist on the simulation of a hot spot orbiting either around a Kerr black hole, or an alternative compact object.
    These results are in turn used as inputs to a code simulating the instrument GRAVITY. The final objective is to determine to what extent GRAVITY will allow to constrain the nature of the central compact object and to probe space-time in its surroundings.


  • Tuesday 14 December 2010 à 11h00 (Salle de conférence du bât. 17)

    Insigths into stars and their environment with Kepler data

    Savita Mathur (High Altitude Observatory, Boulder, USA )

    Seismology is the best tool that we have to probe the solar and stellar interiors. Many results have been obtained with the Sun, such as its structure, its rotation, and its magnetic activity. Asteroseismology has progressed the last ten years thanks to ground-based observations and space missions (WIRE, MOST).
    In 2006, a big change occurred thanks to CoRoT allowing the observation of a few tens of solar-like stars and few hundreds of red giants during 150 days. In many of these stars, acoustic modes could be detected and studied leading to estimation of mass, radius, age, and magnetic activity for some of these stars. A bigger revolution started with the NASA Kepler mission. Indeed it led to a drastic change in term of quality and quantity of data by observing thousands of stars for at least 3.5 years. Primarily intended to search for transiting exoplanets, more specifically Earth-like planets around Solar-like stars, Kepler data are also perfect for asteroseismic analyses. Indeed, the exquisite data provided by Kepler are now allowing us to perform analyses as an "ensemble asteroseismology", to investigate scaling laws, stellar activity, and to test stellar evolution theory even inside clusters. On the other hand, detailed studies of host stars provide very valuable information to the exoplanet search. We can determine very accurately the size of the star, therefore of the planet, fix the habitable zone around a star, and estimate the age of the planetary system.
    In this talk, we will show and discuss the most interesting and exciting results obtained in asteroseismology with CoRoT and Kepler as well as their impact on their environment and the exoplanet search as exoplanet search and asteroseismology have to be studied hand to hand.


  • Friday 10 December 2010 à 15h00 (Salle de conférence du bât. 17)

    Séminaire par 4 chercheurs de l’« Institute of Radio Astronomy, Kharkov, Ukraine »

    A. Konovalenko, V. N. Melnik, M. A. Sidorchuk, S. Stepkin (Institute of Radio Astronomy, Kharkov, Ukraine)

    A. Konovalenko: 50-years of low-frequency radio astronomy in Ukraine
    Low-frequency (decameter wavelengths) radio astronomy in Ukraine has a long history. It was initiated at the beginning of 1960’s by Prof. S. Ya. Braude (1911-2003). Now the Institute of Radio Astronomy of NASU (Kharkov, Ukraine) operates the largest existing decameter array UTR-2 and the VLBI system URAN. Many new astrophysical results were obtained with these instruments. Many studies are carried on in international collaboration, especially with the Paris-Meudon Observatory, involving exploitation of the existing radio telescopes as well as development of new generation low-frequency instruments.

    V. N. Melnik: Solar radio emission at low frequencies
    Recent results obtained on decameter radio emission of the Sun by the radiotelescope UTR-2 are presented. Through the use of modern receiving facilities, new phenomena were observed in the range 10-30 MHz : fast Type III bursts, fine time structure of usual Type III bursts, solar S-bursts, Type IV bursts and their fine « zebra » structure, the third harmonics of Type II bursts, and bursts in absorption at different time scales. New properties of the well-known Type II, III and Iib bursts and of drift pairs were also measured.

    M. A. Sidorchuk, et al.: Decametric continuum investigations at UTR-2
    We present studies of continuum radiation in the decameter 10-30 MHz range carried out with the largest existing radiotelescope, UTR-2 (effective area 150000 m2), at an angular resolution of 70′–25′ respectively. The survey of the Northern Sky has been the main observing program at UTR-2 for 40 years, combining imaging and discrete sources survey. Maps of the sky have been built for declinations 29°-55° at frequencies 14.7, 16.7, 20 and 25 MHz, with sensitivity ∼3.5-1.1 mK at 1σ level. These maps have the best available resolution and sensitivity at these low frequencies. Their analysis is ongoing. Published catalogs of discrete sources contain information about ∼2300 discrete sources. Systematic observations for declinations ≥ 60° are near completion. Maps of continuum radiation over the whole Northern sky (-15° < δ < +85°) have also been obtained with a coarser resolution, using one section of the UTR-2 radiotelescope (resolution ∼12°×4°) and with the complete URAN-2 radiotelescope at 25 MHz (resolution of 3.5°×7°). These maps adequately represent the large-scale structure of the decametric background radiation of the Galaxy in the Northern hemisphere. We also present investigations of ≥20 extended HII regions, supernova remnants, and clusters of galaxies. Those represent a mere start of the studies of the low frequency radio continuum to come.

    S. Stepkin: Low frequency radio recombination lines
    Radio spectroscopy at extremely low frequencies provides unique opportunities for studies of the cold low-density partially ionized interstellar medium, which plays a significant role in many astrophysical phenomena (among them, for example, are processes of star formation). Using the radiotelescope UTR-2 we have detected series of radio recombination lines in absorption, arising from bound carbon atoms in the cool tenuous medium located in the Perseus arm in front of the supernova remnant Cassiopeia A, undergoing transitions corresponding to quantum levels up to ∼ 1009 and even larger values of n). Such atoms have a classical diameter of about ∼0.1 mm.


  • Tuesday 30 November 2010 à 11h00 (Salle de conférence du bât. 17)

    2D multi-thread modeling of prominence fine structures - models and recent results.

    Stanislav Gunar (Astronomical Institute, Academy of Sciences, Czech Republic)

    The modeling of quiescent prominences provides us with a useful tool for understanding of these complex and beautiful solar features. We employ a state-of-the-art 2D multi-thread models to describe the prominence fine structures in the form of vertical 2D threads suspended in a dipped horizontal magnetic field in a magneto-hydrostatic equilibrium. The temperature structure of these threads includes the prominence-corona transition region. We consistently solve the 2D non-LTE radiative transfer within threads to obtain the synthetic hydrogen spectra which we compare with observations. In this seminar I will briefly outline the properties of the 2D multi-thread models and present our recent results. Those include the analyses of the effect of the magnetic field orientation with respect to the LOS on the shape of the hydrogen Lyman lines, study of the Lyman line asymmetries, and statistical analyses of two observed quiescent prominences.

    Co-authors: P. Heinzel, U. Anzer, B. Schmieder, P. Schwartz


  • Mardi 23 novembre 2010 à 11h00 (Salle de conférence du bât. 17)

    Première détection claire des oscillations globales de Jupiter

    Patrick Gaulme (IAS)

    La connaissance de la structure interne de Jupiter apporterait de fortes contraintes sur la formation du système solaire. La mesure de la masse de son noyau et de sa composition globale indiqueraient si la planète s’est formée par accrétion ou instabilité gravitationnelle. Aujourd’hui, la structure interne de Jupiter est mal contrainte et la sismologie, qui consiste en l’identification de ses modes acoustiques propres, est la seule manière de mesurer profondément le profil de densité. La sismologie de Jupiter fut considérée à partir des années 1970, et jusqu’à maintenant, les différentes tentatives ont, au mieux, conduit à des résultats ambigus. Dans ce séminaire, je présenterai la détection des modes d’oscillation propres de Jupiter à partir de mesures de vitesse radiales, qui ont été effectuées avec le tachymètre de Fourier SYMPA (Observatoire de la Côte d’Azur). Les paramètres sismiques globaux ont été mesurés, à savoir la fréquence d’amplitude maximale à 1213±32 µHz, la grande séparation moyenne à 155.2±2.1 µHz et l’amplitude maximale des modes à 49 (-10/+8) cm s-1. L’estimation de la grande séparation moyenne est cohérente avec les modèles actuels de Jupiter, qui présentent un noyau de plusieurs masses terrestres. Ces résultats mettent un terme au débat sur la détectabilité des oscillations de Jupiter et ouvre la voie à la contrainte de la structure interne des planètes géantes du système solaire.


  • Jeudi 18 novembre 2010 à 11h00 (Salle de conférence du bât. 17)

    Le survol de (21) Lutetia par Rosetta

    M.A. Barucci, J. Crovisier, S. Erard, S. Fornasier, M. Fulchignoni, C. Leyrat (LESIA)

    Le 10 Juillet 2010 la sonde Rosetta de l’ESA a effectué le survol de l’astéroïde (21) Lutetia. Les résultats recueillis par les instruments embarqués OSiRIS (système d’imagerie), VIRTIS spectromètre V et IR), RSS (Radio science), MIRO (microwaves instrument) seront présentés et discutés par les chercheurs du LESIA qui sont engagés dans les différentes équipes en tant que co-investgateurs ou interdisciplinary scientist.


  • Mardi 9 novembre 2010 à 11h00 (Salle de conférence du bât. 17)

    Étude de la formation des disques circumstellaires autour des étoiles Be avec l’instrument VEGA/CHARA

    Omar Delaa (Observatoire de la côte d’Azur)

    Les étoiles Be sont des rotateurs rapide de la séquence principale de type spectral B et présentant ou ayant présenté par le passé des raies de la série de Balmer en émission (appelé communément "Phénomène Be"). Il est généralement admis que ces raies d’émission proviennent d’un environnement circumstellaire constitué essentiellement d’hydrogène éjecté par l’étoile. Cependant, l’origine de la formation de ces environnements autour des étoiles Be reste un problème fondamental non résolu. Durant ce séminaire, je présenterai les mécanismes connus à ce jour pouvant être responsables de la formation de ces environnements et je tenterai d’apporter des contraintes sur ces possibles mécanismes grâce à l’étude de la géométrie et de la cinématique de leurs environnements circumstellaires.


  • Friday 22 October 2010 à 15h00 (Salle de conférence du bât. 17)

    Utilisation of Electromagnetic Angular Momentum in Radio Studies of Space (parts 3/4 and 4/4)

    Bo Thidé (Swedish Institute of Space Physics, Uppsala)

    Two 45 minute seminars separated by a coffee break will be presented on this day.

    Most of the information we obtain about the Universe is extracted from the electromagnetic radiation from space that reaches our telescopes and other sensor systems on Earth and on board spacecraft orbiting Earth.

    Up till now, virtually all radiation from Nature has been analyzed only with respect to its intensity, spectral content, direction of arrival, and polarization. While polarization is a manifestation of the physical fact that electromagnetic radiation does not only carry linear momentum but also angular momentum, polarization measurements alone do not provide an exhaustive characterization of the total angular momentum state of electromagnetic radiation. Already in the early 1900s it was shown theoretically that collimated beams can carry angular momentum. In the 1930s and 1940s, elegant Einstein-de Haas type experiments for photons carried out by Beth in optics and by Carrara in radio showed that angular momentum can be transferred from electromagnetic beams to mechanical bodies. In the years that followed, very high angular momentum states of nuclei were discovered via their photon decays. Still, it was not until the 1990s that laser beams and microwave radio beams carrying both spin angular momentum (polarization) and orbital angular momentum (OAM) could be readily generated, controlled, and detected. Soon thereafter it was experimentally proved that individual photons can be endowed with OAM and be entangled in these states, showing that electromagnetic radiation can be characterized, analyzed, and utilized more fully than what was commonly known at the time. Recently the possibility of using the OAM degrees of freedom of light and radio in astrophysics and space physics has come to the fore.

    It is namely reasonable to assume that electromagnetic OAM is radiated by some astrophysical sources or is imparted upon radiation through interaction with plasma in space, at least under certain conditions. The characterization of the OAM of light or radio beams intercepted by telescopes on Earth or in space can provide new and crucial information about the physical processes involved. Following this assumption, many authors have proposed studies with new instruments to detect and manipulate the electromagnetic OAM, henceforth referred to as POAM (photon orbital angular momentum) and also performed new experiments at the telescope. This represents a single face of a more complex evolution occurring now in astronomy. As described by Harwit, POAM permits new types of measurements and paves the way for the utilization of topological and more generic degrees of freedom in astronomy and space sciences.

    These seminars describe and discuss some of these new opportunities, from theoretical foundations to applications of POAM in space physics.


  • Thursday 21 October 2010 à 15h00 (Salle de conférence du bât. 17)

    Utilisation of Electromagnetic Angular Momentum in Radio Studies of Space (parts 1/4 and 2/4)

    Bo Thidé (Swedish Institute of Space Physics, Uppsala)

    Two 45 minute seminars separated by a coffee break will be presented on this day. Two more seminars will be presented on Friday.

    Most of the information we obtain about the Universe is extracted from the electromagnetic radiation from space that reaches our telescopes and other sensor systems on Earth and on board spacecraft orbiting Earth.

    Up till now, virtually all radiation from Nature has been analyzed only with respect to its intensity, spectral content, direction of arrival, and polarization. While polarization is a manifestation of the physical fact that electromagnetic radiation does not only carry linear momentum but also angular momentum, polarization measurements alone do not provide an exhaustive characterization of the total angular momentum state of electromagnetic radiation. Already in the early 1900s it was shown theoretically that collimated beams can carry angular momentum. In the 1930s and 1940s, elegant Einstein-de Haas type experiments for photons carried out by Beth in optics and by Carrara in radio showed that angular momentum can be transferred from electromagnetic beams to mechanical bodies. In the years that followed, very high angular momentum states of nuclei were discovered via their photon decays. Still, it was not until the 1990s that laser beams and microwave radio beams carrying both spin angular momentum (polarization) and orbital angular momentum (OAM) could be readily generated, controlled, and detected. Soon thereafter it was experimentally proved that individual photons can be endowed with OAM and be entangled in these states, showing that electromagnetic radiation can be characterized, analyzed, and utilized more fully than what was commonly known at the time. Recently the possibility of using the OAM degrees of freedom of light and radio in astrophysics and space physics has come to the fore.

    It is namely reasonable to assume that electromagnetic OAM is radiated by some astrophysical sources or is imparted upon radiation through interaction with plasma in space, at least under certain conditions. The characterization of the OAM of light or radio beams intercepted by telescopes on Earth or in space can provide new and crucial information about the physical processes involved. Following this assumption, many authors have proposed studies with new instruments to detect and manipulate the electromagnetic OAM, henceforth referred to as POAM (photon orbital angular momentum) and also performed new experiments at the telescope. This represents a single face of a more complex evolution occurring now in astronomy. As described by Harwit, POAM permits new types of measurements and paves the way for the utilization of topological and more generic degrees of freedom in astronomy and space sciences.

    These seminars describe and discuss some of these new opportunities, from theoretical foundations to applications of POAM in space physics.


  • Mercredi 20 octobre 2010 à 11h00 (Salle de conférence du bât. 17)

    Seminaire PHASE / LESIA : Imagerie directe d’exo-planètes : coronographie et contrôle de front d’onde

    Laurent Pueyo (Johns Hopkins University, Baltimore, Md.)

    La détection directe d’exo planètes depuis la terre ou l’espace nécessite une considérable suppression cohérente de la lumière de l’étoile. Dans une première partie je présenterai ensuite des résultats obtenus au Princeton High Contrast Laboratory, axés sur l’optique adaptative spatiale mais généralisable au sol, prouvant la faisabilité du contrôle simultané d’amplitude et de phase en utilisant deux miroirs déformables en série. Je montrerai ensuite comment les modèles développés dans le cadre, cette expérience peuvent être utilisés afin d’affiner notre compréhension du coronographe PIAA, et en particulier ouvrent la possibilités d’implémenter un tel système a l’aide de miroirs déformables. La seconde partie se focalisera sur des résultats expérimentaux récents obtenus outre-atlantique dans le domaine de la mesure de front d’onde, et l’extinction dynamique de « speckles » via des miroirs déformables. Je présenterai tout d’abord les tout derniers résultats du banc d’essai de l’interféromètre de calibration du Gemini Planet Imager, ou des mesures de phase avec 5 nm de précision ont été réalisées. Une telle mesure, découplée du détecteur scientifique, permettra une fois sur le télescope de corriger les erreurs de front d’onde non communes dont les constantes de temps sont plus courtes que les poses scientifiques, et donc d’affiner le contraste des observations depuis le sol.


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