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

Séminaires passés

  • Mercredi 20 mars 2019 à 14h00 (Salle de conférence du bâtiment 17)

    Fourier-based WaveFront Sensing at LAM : a quick overview

    Olivier Fauvarque (LAM)

    At the condition to use adequate focal masks, optical Fourier filtering turns out to convert phase fluctuations into intensity variations with a great efficiency. If two wave front sensors in particular - the Pyramid and the Zernike – have, during the last decades, shown very promising performance making them serious candidates for the next AO systems of the Extremely Large Telescopes, a general and theoretical scheme to finely define their behavior was missing.

    This seminar intends to present the latest theoretical results that have been done by the LAM AO team and its partners in this emerging instrumental field of research. In particular, the problem of Optical Gain tracking will be adressed through the powerful Kernel mathematical framework. We will then see how to choose proper phase reconstructor depending on the sensors’ optical characteristics. We will also introduce a new kind of optical device allowing to improve the linearity of Fourier-based WFSs without any loss of sensitivity. Finally, a presentation of the LOOPS test bench (designed to generate and test a large diversity of Fourier-based WFSensors) will serve as conclusion.


  • Mercredi 20 mars 2019 à 11h00 (Salle de conférence du bâtiment 17)

    Recovering thermodynamics from spectral profiles observed by IRIS using machine and deep learning techniques

    Alberto Sainz Dalda (Lockheed-Martin Solar and Astrophysics Laboratory / BAERI, USA)

    We present three novel methods to recover the physical information from spectral profiles suitable to be inverted from an iterative solution of the radiative transfer equation. We combine the meaningful results provided by these traditional methods with machine and deep learning techniques to obtain similar-quality results in a easy-to-use, faster way. We have applied these new methods to Mg II h&k lines observed by IRIS. As a result, we are able to obtain the thermodynamics in the chromosphere and high photosphere in a few CPU-minutes, speeding up the process in a factor of 105-106. The open-source code developed to this aim will allow the community to use IRIS observations to open a new window to a host of solar phenomena.


  • Mardi 19 mars 2019 à 11h00 (Salle de conférence du bâtiment 17)

    What can observations of solar plasma composition tell us about physical processes occurring on the Sun ?

    Deborah Baker (University College London, Mullard Space Science Laboratory, UK)

    Elemental abundance variations are tracers of physical processes throughout the Universe, with the cosmic reference standard being the solar elemental composition. Knowledge of the Sun’s elemental composition underpins our understanding of the flow of mass and energy from deep in the interior, through the outer atmosphere, into the heliosphere. As the Sun’s outer atmosphere originates from the photosphere, it is not trivial that the elemental abundances of the photosphere and corona are different. Recent Hinode/EIS and SDO/EVE results suggest that the observed distribution and evolution of elemental composition are closely linked with the magnetic activity and heating processes in the Sun’s outer atmosphere. I will review the key results and show how variation in elemental composition may be used as a tracer of physical processes on the Sun.


  • Lundi 4 mars 2019 à 14h00 (Amphithéâtre Evry Schatzman, bâtiment 18)

    Deep learning exoplanets and the solar system

    Ingo P. Waldmann (Deputy Director UCL Centre of Space Exoplanet Data, Dept. of Physics & Astronomy, University College London)

    The field of exoplanetary spectroscopy is as fast moving as it is new. Analysing currently available observations of exoplanetary atmospheres often invoke large and correlated parameter spaces that can be difficult to map or constrain. This is true for both : the data analysis of observations as well as the theoretical modelling of their atmospheres. Issues of low signal-to-noise data and large, non-linear parameter spaces are nothing new and commonly found in many fields of engineering and the physical sciences. Recent years have seen vast improvements in statistical data analysis and machine learning that have revolutionised fields as diverse as telecommunication, pattern recognition, medical physics and cosmology. In many aspects, data mining and non-linearity challenges encountered in other data intensive fields are directly transferable to the field of extrasolar planets as well as planetary sciences. In this seminar, I will discuss our new deep learning framework, ExoGAN (Tzingales & Waldmann, 2018, AJ), designed to address some of these atmospheric modelling challenges using generative adversarial networks. I will then proceed to discuss our new hyper-spectral image classification code, PlanetNET (Waldmann & Griffith, in press, Nat. Astr.), able to automatically and accurately map Saturn’s clouds using Cassini/VIMS data. As we firmly move into the era of ‘big data’ for both planetary (e.g. Juno) and exoplanetary sciences (e.g. JWST, Ariel), intelligent algorithms will play an important part in facilitating the analysis of these rich data sets in the future.


  • Mardi 26 février 2019 à 11h00 (Salle de réunion du bâtiment 14)

    The strongest magnetic fields in sunspots and their statistical properties

    Joten Okamoto (NAOJ, Japan)

    Sunspots are concentrations of magnetic fields on the solar surface. Then, where is the strongest field in each sunspot ? It is generally located in an umbra, but sometimes stronger fields are found outside umbrae, such as a penumbra and a light bridge. The formation mechanism of such strong fields outside umbrae is still puzzling. Now we have numerous high-quality datasets taken with the Hinode/Spectro-Polarimeter over 10 years, which motivate us to address this question via a statistical analysis of strongest fields in sunspots. Hence, we complied a ranking list of active regions by their largest field strengths and investigated conditions for appearance or formation of strong magnetic fields. In this seminar, we will introduce a sunspot with a field strength of 6250 G as a case study, and then discuss the key features to produce strong fields in a statistical sample.


  • Vendredi 22 février 2019 à 14h00 (Salle de réunion du bâtiment 14)

    WhoSGlAd : A new stellar seismic probing method

    Martin Farnir (University of Liège - STAR Institute, Belgium)

    Most of the information we gather about the Universe emanates from the stellar light. A good understanding of stars is therefore needed to, for instance, trace back the evolution of our Galaxy or of exoplanetary systems. In the last decades, the launch of the space-borne missions CoRoT (2006-2014) and Kepler (2009-2018) enabled a revolution in stellar physics. They provided us with photometric information on thousands of stars with an unprecedented quality. The amount and precision of the data allowed a direct probing of the internal properties of stars through the study of their oscillations, or asteroseismology. This additional piece of information leads to stringent constraints on stellar evolution that were not accessible with ‘classical’ techniques.

    In this talk, I will present my work about the study of acoustic glitches in main-sequence stars. Acoustic glitches represent the small signature in the oscillation frequency pattern resulting from sharp gradients in the stellar structure. Those glitches are essential to our understanding of stellar structure and evolution as, for example, they provide information about the surface helium content in low-mass stars or on the extent of the mixed regions, inaccessible by other means. In order to analyse such signatures and provide significant inferences, methods as precise and accurate as possible are necessary. However, previous works interested in the information carried by acoustic glitches focused only on those signatures and neglected information contained in the oscillation spectrum as a whole. The resulting parameters characterising the stellar structure were correlated with rather important error-bars. Thus, in this presentation, I will introduce a new stellar seismic probing method that we developed, called WhoSGlAd (Whole Spectrum and Glitches Adjustement) which analyses in a comprehensive way stellar oscillation spectra. This leads to smaller error-bars and thus stricter constraints. Our method is precise and fast, and may be used with any minimisation scheme to find best fit models representing seismic data. This provides precise inferences about their structure and access to the surface helium content. As the TESS mission has been launched and PLATO will follow, providing methods able to fully benefit from the wealth of exquisite quality data while reducing at most the uncertainties of the inferences is in order. Therefore, WhoSGlAd should reveal to be of great help in characterising the targets of both missions.


  • Jeudi 21 février 2019 à 16h00 (Salle de conférence du bâtiment 17)

    Accretion discs from young stars to compact objects

    Zakaria Meliani (LUTH - Observatoire de Paris)

    Accretion discs are known to form around young stars and compact objects. They play an important role in the central object evolution. They allow the accretion by extraction of angular momentum outward. In this seminar, I will present what we know about the physics of accretion disc at different scales and the relation with the associated wind and jet.


  • Lundi 18 février 2019 à 16h00 (Salle 204 du bâtiment 18 - ATTENTION, changement de salle !)

    On the chemical origin of volatile species in comets : the examples of molecular oxygen and methanol

    Vianney Taquey (Marie Sklodowska-Curie ASTROFIT2 Fellow Osservatorio Astrofisico di Arcetri, Florence, Italy)

    The Rosetta spacecraft analysed the Jupiter-family comet 67P/Churyumov-Gerasimenko (67P/C-G) in 2014 and 2015. The ROSINA mass spectrometer on board the Rosetta orbiter carried out a chemical census with unprecedented sensitivity of the coma of 67P/C-G and detected a “zoo” of molecules from simple di-atomic species to complex pre-biotic organics, such as glycine. The precise abundances and the signal correlations among species measured by Rosetta/ROSINA now give us invaluable constraints to infer the chemical origin of comet 67P/C-G, and possibly of our Solar System. In this talk, I will take the examples of molecular oxygen and deuterated organic molecules, such as methanol, whose signal correlations and abundance ratios with water ice can be used as powerful chemical tracers. We compared the ROSINA measurements with state-of-the-art astrochemical models applied to dynamical models by considering several scenarios, and with sub-mm interferometric observations of nearby low-mass protostars using the ALMA and NOEMA sub-mm interferometers. The comparison between comet measurements, model predictions and observations of protostars would favour a dark cloud (or “primordial”) grain surface chemistry origin for molecular oxygen and methanol in comets, albeit for slightly warmer and denser dark clouds than those usually considered as solar system progenitors.


  • Jeudi 7 février 2019 à 11h00 (Salle de conférence du bâtiment 17)

    The Future of Exoplanet Imaging : the Fast Atmospheric Self-Coherent Camera Technique

    Benjamin Gerard (University of Victoria, Canada)

    Direct detection and detailed characterization of exoplanets using extreme adaptive optics (ExAO) is a key science goal of future extremely large telescopes. However, quasi-static wavefront errors will limit the sensitivity of this endeavor. Additional limitations for ground-based telescopes arise from residual AO-corrected atmospheric wavefront errors, generating short-lived aberrations that will average into a halo over a long exposure, also limiting the sensitivity of exoplanet detection. We have developed the framework for a solution to both of these problems using the self-coherent camera (SCC), to be applied to ground-based telescopes, called the Fast Atmospheric SCC Technique (FAST). We will present updates of the ongoing coronagraph fabrication and testing for this method as well as future implementation, including a possible upgrade of the Gemini Planet Imager. Sensitivity improvement from this method could play an essential role in the future ground-based detection and characterization of lower mass and/or colder exoplanets.


  • Jeudi 31 janvier 2019 à 16h00 (Salle de conférence du bâtiment 17)

    High contrast imaging : from active correction to observation of circumstellar debris disks

    Johan Mazoyer (Jet Propulsion Laboratory)

    More than 3000 exoplanets have been discovered to date, but only a few have been imaged directly. However, by allowing the observation of circumstellar disks and planets (sometimes simultaneously around the same star, as in the case of β-pictoris), this method is a fundamental tool for the understanding the process of planetary formation. In addition, direct access to the light of the detected objects allows spectroscopy, paving the way to the full chemical analysis of exoplanets’ atmosphere and disks grains. Several coronagraphic instruments are currently observing to images of young Jupiters and/or Kuiper like disks. These instruments use coronagraphs optimized for circular, often un-obstructed apertures. Indeed, the remaining aberrations created by the atmosphere or optics defaults is limiting the contrast at levels far above the ones created by apertures discontinuities (inter-segment gap or secondary mirror mounts). However, the next generation of ground and space based telescopes will have to address the problem of apertures discontinuities in coronagraphy, if we want to obtain images and spectra of earth sized planets or dust grains below the snow line. My talk is about the next steps to make these detections a reality. First, I will present my current research to improve the contrast level of coronagraphs using deformable mirrors. Then, I will also show my work in the field of the post processing of high contrast images, specifically in the field of circumstellar disks, with the GPI instrument.


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