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

Prochains séminaires

  • 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.


  • 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.


  • 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.


  • 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 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.


Séminaires passés

  • 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.


  • 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.


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