LESIA - Observatoire de Paris

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

Prochains séminaires

  • 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.

  • 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.

  • 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.

Séminaires passés

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

    Active Galacti Nuclei at very high energies - observations and modelisation

    Andreas Zech (LUTh)

    Until now, more than 70 active galactic nuclei, nearly all blazars, have been identified as emitters of gamma rays at very high energies (TeV), by means of Cherenkov telescope networks like the HESS experiment in Namibia. The spectral and temporal information collected from these sources allows to better understand the extreme conditions in the region of emission. The main tool for linking observations to the physics of the sources is the modelisation of spectral distributions incorporating models of radiative transfer. I will present recent observations in this domain and I will discuss their interpretation with leptonic and hadronic models. Projections for CTA will also be discussed.

  • Jeudi 15 février 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Calibration of mixing length parameters with 3D simulation models

    Sonoi Takafumi (LESIA)

    Observation by space missions such as CoRoT and Kepler have provided with a wealth of high-quality data of stellar oscillations. Particularly, rich spectra of solar-like oscillations should allow us to perform precise determination of stellar global parameters such as age, mass and radius, and interior structure. To make the best of such data, we need theoretical stellar models with precise near-surface structure, which has significant influence on solar-like oscillation frequencies. Mixing length parameters of the convection models are a key factor to determine the near-surface structure. However, we have not yet a definitive recipe for giving its value.

    We aim at calibrating values of these parameters across the Heltzsprung-Russell (HR) diagram based on 3D hydrodynamical models, provided by the CO5BOLD code. Although previous calibration with 3D models have limited to the classical mixing length theory (MLT), we analyze also the full spectrum turbulence (FST) models proposed by Canuto & Mazzitelli (1991) and Canuto, Goldman & Mazzitelli (1996). We perform calibration by matching entropy profiles of 1D envelope models with those of the 3D models. For atmosphere of the 1D models, we compare the Eddington grey T-tau relation and the one with the solar calibrated Hopf function based on Vernazza et al. (1981).

    For both the MLT and FST models with a mixing length l=alpha*H_p, calibrated alpha values increase with increasing surface gravity or decreasing effective temperature. For the solar model, the calibrated alpha values for the MLT and FST models with the Eddington T-tau relation are found to be in good agreement with previous works which performed alpha calibration with the Eddington T-tau by matching with the observables of the Sun. It is found that the solar Hopf T-tau relation generally gives photospheric minimum entropy closer to a 3D model than the Eddington T-tau in a wide range of the HR diagram. Since the alpha values substantially vary with effective temperature and surface gravity, 1D computation of stellar evolution should not be performed with the alpha value fixed, but the calibrated alpha values should be implemented into such computation.

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

    Numerical Simulation of a Global Superflare from Kappa-1 Cet

    Benjamin Lynch (Space Science Laboratory, U. Berkeley, USA)

  • Mercredi 31 janvier 2018 à 15h00 (Salle de conférence du bâtiment 17)

    Modelling of entire prominences with their multiple fine structures : the 3D Whole-prominence fine structure model

    Stan Gunár (Astronomical Institute of the Czech Academy of Sciences, République tchèque)

    We present the 3D whole-prominence fine structure (WPFS) model (Gunár & Mackay 2015) that combines a 3D magnetic field configuration of an entire prominence obtained from non-linear force-free field simulations, with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along hundreds of fine structures within the 3D magnetic model. The prominence plasma has realistic density and temperature distributions including the prominence-corona transition region. Thanks to this the 3D WPFS model provides us with a representation of a prominence with complexity that approaches the real prominences.

    This fact was demonstrated by H-alpha visualization of the simulated prominence done by Gunár & Mackay (2015, 2016). To produce the high-resolution synthetic H-alpha images of the WPFS model we use the fast approximate radiative transfer visualization technique developed by Heinzel et al. (2015). This technique allows us for the first time to produce images of simulated prominences in emission on the solar limb and filaments in absorption against the solar disk using a single model. By employing such a visualization we can study connections between the local configuration of the prominence magnetic field and the observable structure of the prominence/filament plasma. In addition, we are able to consistently study the influence of the varying photospheric flux distribution on the prominence magnetic field configuration and its effect on the observable prominence plasma during prominence evolution.

    In addition to the H-alpha line, we have also developed and used a novel technique for synthesis of the emergent radiation at the millimeter/sub-millimeter wavelengths which are employed by the Atacama Large Millimeter/sub-millimeter Array (ALMA) - see Gunár et al. 2016, 2018.

  • Mardi 30 janvier 2018 à 14h00 (Salle de conférence du bâtiment 17)

    La spectroscopie de Fourier en astronomie : de ses origines à nos jours

    Jean-Pierre Maillard (IAP)

    Les principes de la spectroscopie dite « par transformation de Fourier » ont été établis en France au début des années 60 (thèse de Jeanine Connes). Ils s’inscrivent en prolongement direct des travaux d’Albert Michelson à la fin du 19e siècle et l’interféromètre qui porte son nom.

    Mais cette méthode de spectroscopie non conventionnelle car non dispersive, n’a été redécouverte et pris son essor que trois-quarts de siècle plus tard, grâce aux développements concomitants des détecteurs photo-électriques, des moyens d’enregistrements des données, des ordinateurs et au rôle de quelques pionniers. Ce nouveau type de spectromètre, développé avant tout dans le domaine infrarouge, a pu être considéré dans les années 70 comme le spectromètre idéal. Les applications en astronomie, avec plusieurs résultats marquants, de la composition de l’atmosphère des planètes au rayonnement du fond cosmologique, ont largement contribué à sa généralisation. Mais aujourd’hui, avec le développement des mosaïques de détecteurs de grand format dans tous les domaines spectraux, particulièrement en astronomie, les spectromètres à réseau ont pris l’avantage. La méthode de Fourier conserve toutefois quelques niches spécifiques, comme dans le domaine de l’infrarouge lointain spatial ou pour la spectroscopie intégrale de grands champs. Le séminaire rendra compte de cette aventure scientifique débutée il y a plus de 130 ans et qui se poursuit de nos jours.

  • Vendredi 26 janvier 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Alpha du Centaure, trois étoiles pour une mission interstellaire

    Pierre Kervella (LESIA)

    Alpha Centauri AB et Proxima sont nos plus proches voisines, à environ 270 000 unités astronomiques. Les deux composantes principales sont similaires au Soleil (A et B), alors que Proxima est une naine rouge de très faible masse (1/8ème de la masse du Soleil, et 1/6ème de son rayon) autour de laquelle orbite une planète tellurique dans sa zone habitable (Proxima b). Je présenterai un résumé ce que nous savons de ces trois étoiles. Outre leur importance en physique stellaire, Alpha Cen pourrait être la première cible d’une sonde balistique interstellaire miniaturisée. La faisabilité d’une telle mission est actuellement étudiée par le projet Breakthrough Starshot, dont je décrirai brièvement les objectifs.

  • Vendredi 19 janvier 2018 à 14h00 (Salle de conférence du bâtiment 17)

    Adaptive Optics Facility : un peu d’histoire, pas mal de technologie, beaucoup de résultats !

    Pierre-Yves Madec (ESO)

    L’Adaptive Optics Facility (AOF) est un projet mené et réalisé par l’ESO qui a permis de transformer Yepun (UT4) en un télescope adaptatif assisté par lasers, et d’associer à HAWK-I (imageur infrarouge grand champ) et MUSE (spectrographe 3D dans le visible) deux modules d’analyse de surface d’onde. Cette association permet d’améliorer la qualité d’image pour chacun de ces instruments en éliminant la contribution des basses couches turbulentes (Ground Layer Adaptive Optics) et même d’offrir un mode Laser Tomography (LTAO) dans le visible pour le Narrow Field Mode de MUSE. Initié en 2015, ce projet aura permis en particulier d’améliorer la technologie des miroirs secondaires déformables et de développer une nouvelle filière de lasers Sodium industriels répondant complètement aux besoins de l’astronomie. Les premiers éléments de l’AOF ont été installés sur Yepun en Mai 2015. L’intégration sur le télescope des 4 unités lasers a été achevée en Avril 2016. Le nouveau miroir secondaire déformable est en opération depuis Janvier 2017. La Science Verification du Wide Field Mode de MUSE associé à l’AOF a eu lieu en Septembre 2017. Celle de HAWK-I couplé à l’AOF s’est tenue début Janvier 2018.

    Au cours de ma présentation, je rappellerai les principaux éléments techniques de la conception de l’AOF en me focalisant sur le 4 Laser Guide Star Facility (4LGSF), le Deformable Secondary Mirror (DSM) ainsi que sur GRAAL et GALACSI, les deux modules d’analyse de surface d’onde associés respectivement à HAWK-I (imageur infrarouge grand champ) et MUSE (spectrographe 3D dans le visible). Je décrirai ensuite la stratégie de contrôle du DSM et son interaction avec l’optique active du télescope. Je présenterai finalement le processus d’intégration de l’AOF sur Yepun et les premiers résultats obtenus au cours des missions techniques de vérification des performances.

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

    New insights on stellar rotation thanks to asteroseismology

    Rhita-Maria Ouazzani (LESIA)

    Helioseismology and asteroseismology of red giant stars have shown that distribution of angular momentum in stellar interiors, and its evolution with time remains an open issue in stellar physics. Owing to the unprecedented quality and long baseline of Kepler photometry, we are able to seismically infer internal rotation rates in Γ Doradus stars, which provide the main-sequence counterpart to the red-giants puzzle. Here, we confront these internal rotation rates to stellar evolution models which account for rotationally induced transport of angular momentum, in order to test angular momentum transport mechanisms.

    On the one hand, we used a stellar model-independent method developed by Christophe et al. in order to obtain accurate, seismically inferred, buoyancy radii and near-core rotation for 37 Γ Doradus stars observed by Kepler. We show that the stellar buoyancy radius can be used as a reliable evolution indicator for field stars on the main sequence. On the other hand, we computed rotating evolutionary models of intermediate-mass stars including internal transport of angular momentum in radiative zones, following the formalism developed by Zahn and Maeder, with the cestam code. This code calculates the rotational history of stars from the birth line to the tip of the RGB. The initial angular momentum content has to be set initially, which is done here by fitting rotation periods in young stellar clusters.

    We show a clear disagreement between the near-core rotation rates measured in the sample and the rotation rates obtained from the evolutionary models including rotationally induced transport of angular momentum following Zahn, 1992. These results show a disagreement similar to that of the Sun and red giant stars in the considered mass range. This suggests the existence of missing mechanisms responsible for the braking of the core before and along the main sequence. The efficiency of the missing mechanisms is investigated.

    The transport of angular momentum as formalized by Zahn and Maeder cannot explain the measurements of near-core rotation in main-sequence intermediate-mass stars we have at hand.

  • Lundi 8 janvier 2018 à 11h00 (Salle de conférence du bâtiment 17)

    A Wide-Field Coronal EUV Imager-Spectrometer

    Leon Golub (Harvard-Smithsonian Center for Astrophysics, USA)

    We have designed a novel wide field, dual-use EUV imager to observe the dynamics of solar coronal streamers and other large-scale structures from the solar surface out to at least 3 R_sol. The COSIE instrument is proposed for implementation on the ISS, with the objectives of : 1.) understanding the dynamics of the Transition Corona, the region of the upper corona in which the plasma beta changes from low to high and the atmosphere transitions from being dominated by magnetically confined closed structures to high beta with generally open radially-directed regions with outflowing solar wind streams, and 2.) providing new tools for space weather forecasting via early detection of coronal mass ejections (CMEs), tracking of CMEs during their main acceleration phase and early path changes, and modeling of the CME magnetic configuration at event initiation. The imaging channel has 500X greater sensitivity than existing EUV imagers and is capable of detecting streamers out to at least 2.5 R_sol and CMEs to substantially greater distances. A novel feature of COSIE is that the observing mode is switchable between ultra-high sensitivity direct EUV imaging and a global spectroscopic imaging mode. The overlapped spectra can be unfolded to provide spectral resolution of 20,000 over the 185-206Å passband covering a wide coronal temperature range, as well as providing full-Sun density images every 10 seconds. The sensitivity and field of view of the design are flexible and many observing locations are feasible, including L1, L4 and L5.

  • Lundi 11 décembre 2017 à 16h00 (Salle de conférence du bâtiment 17)

    Le carbone dans les particules de la comète 67P/Churyumov-Gerasimenko

    Hervé Cottin (université Paris-Est Créteil & LISA)

    La présence de matière organique solide dans les poussières cométaires a été établie par la détection d’éléments tels que le carbone, l’hydrogène, l’oxygène et l’azote dans les particules de la comète 1P/Halley en 1986. Ce matériau est généralement considéré comme ayant une origine dans le milieu interstellaire, mais il pourrait aussi s’être formé dans la nébuleuse solaire. Cette composante organique solide ne peut être observée à partir de la Terre, de sorte qu’elle a toujours échappé à une caractérisation sans ambiguïté. En revanche, de nombreuses molécules organiques en phase gazeuse ont été observées dans les comètes ; elles proviennent principalement de la sublimation des glaces à la surface ou dans le sous-sol du noyau cométaire. Au cours de ce séminaire, je présenterai la détection in situ, grâce au spectromètre de masse COSIMA de la sonde Rosetta, d’une phase organique solide dans les particules de poussière émises par la comète 67P / Churyumov-Gerasimenko. Le carbone dans cette composante organique est lié dans de très grandes structures de haut poids moléculaire, analogues à la matière organique insoluble trouvée dans les météorites de type chondrite carbonée. Une estimation de l’abondance de cette phase carbonée sera présentée et j’essaierai de présenter un bilan plus général de la nature du carbone présent dans le noyau de la comète 67P.

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