Vendredi 11 juin 2021 à 10h00 (En visioconfĂ©rence)
Baptiste Cecconi (LESIA)
L’instrument PWS de Galileo a observĂ© des occultations des Ă©missions radio basse frĂ©quence de Jupiter par les satellites galilĂ©ens. Nous montrons que le code ExPRES (Exoplanetary and Planetary Radio Emission Simulator) modĂ©lise avec prĂ©cision l’occurrence temporelle des occultations dans toute la gamme spectrale observĂ©e par Galileo/PWS. Ceci valide le code ExPRES. La mĂ©thode ĂȘtre appliquĂ©e pour prĂ©parer la planification des opĂ©rations scientifiques des survols des lunes Europe, Ganymede et Callisto par la mission JUICE.
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olga.alexandrova obspm.fr
Vendredi 28 mai 2021 à 11h00 (En visioconfĂ©rence)
James Waters (University of Southampton)
Auroral Kilometric Radiation (AKR) is terrestrial radio emission that is generated via the electron-cyclotron maser instability and has long been observed to be associated with energy deposition and interactions between the magnetosphere and ionosphere. Emitted from both hemispheres, an increase in the amplitude of a particular AKR source is indicative of the presence of strong, parallel electric fields in the auroral zone, while the emission frequency of AKR gives direct insight into the altitudinal extent of the source region. The viewing geometry is complex, however, due to the primary confinement of the source regions to local times on the nightside and the anisotropy of the beaming pattern seen at each pole. A new empirical technique has been developed that allows AKR emission to be isolated from remote sensing observations made by the Wind spacecraft. The method is applied to a month-long period in 1999 over which Wind completed close to two, precessing petal orbits while the Cassini spacecraft performed a close flyby of Earth and also recorded AKR. My talk will discuss the details of the selection and present the results of its application to these observations, which reproduces the AKR viewing geometry as well as previously observed diurnal modulations. Current and future uses of the resulting dataset will also be explored.
Jeudi 29 avril 2021 à 16h00 (En visioconfĂ©rence)
Paul NICAISE (GEPI - Observatoire de Paris)
Kinetic Inductance Detectors (KIDs) are spectrophotometers capable of recording single-photon events faster than atmospheric turbulence whilst providing energy resolution (R 10) without any added optics. KIDs are superconducting resonators featuring no dark current noise, very low thermal noise and can be easily multiplexed into large arrays of thousands of pixels. Only 20 years after they were first introduced, KIDs are now established as one of the strongest choices for many ground-based astronomical applications ranging from direct-imaging of exoplanet to millimetric observations of the CMB radiation. At Observatoire de Paris, the SpectroPhotometric Imaging in Astronomy with KID (SPIAKID) instrument is a demonstrator that aims to study the stars ages and metallicities in Ultra-Faint Dwarf (UFD) galaxies by 2024. I will present the KIDs technology as well as my contribution to the SPIAKID project that involves the investigation of enhanced optical coupling between incoming photons and the detector.
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Jeudi 15 avril 2021 à 16h00 (En visioconfĂ©rence)
David CORNU (LERMA - Observatoire de Paris)
This presentation will cover the basics of Convolutional Neural Networks (CNN) construction and usage. Through simple and illustrative examples, we will discuss why CNN are better suited for images than classical fully connected neural networks. We will explain how CNN work on simple image classification and describe some widely used CNN architectures for object detection inside images.
We will present some of the results of the Minerva team working on SKA Science Data Challenges that consist in detecting galaxies in very large simulated 2D images or 3D spectral cubes. In this context, we will discuss why astronomical images are challenging for CNN object detection methods and present our approach to overcome some of the encountered difficulties.
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Lundi 22 mars 2021 à 16h00 (En visioconfĂ©rence)
Joana Marques Oliveira (LESIA)
Triton is the largest of Neptuneâs satellites with a radius of 1353 km. It possesses a significant atmosphere, mainly composed of molecular nitrogen N2, that is in vapour pressure equilibrium with the N2 frost at the surface. Between 1990 and 2010, an “extreme solstice” occurred, where latitudes of up to 50Âș S were directly and constantly illuminated by the Sun. This occurs only every 650 years, due to a combination of Neptuneâs heliocentric motion and Tritonâs orbital precession. In this talk, we will be discussing the few stellar occultations by Triton that have been observed. We will particularly focus on the event of the 5 October 2017, observed from Europe, north Africa, and USA. This event yielded 90 positive light curves, making it the most observed event of Triton. Our work focused on constraining the evolution of Tritonâs atmospheric pressure since the Voyager 2 epoch, as well as deriving the shape of the lower atmosphere from the analysis of the 25 central flashes obtained. We will show the results obtained for the pressure in 2017, and discuss if there has been a surge during the extreme solstice. We also present a new analysis of the original Voyager 2 data, where we were able to extract new information.
Lundi 15 mars 2021 à 16h00 (En visioconfĂ©rence)
Thamiris Desantana (LESIA)
The Pluto-Charon system is very interesting due its complex dynamics. Its four small satellites Styx, Nix, Kerberos and Hydra were discovered close to, but not in, the 1:3, 1:4, 1:5, and 1:6 mean motion resonances with Charon, respectively (Cheng et al. 2014). Later, Styx, Nix, and Hydra were found to be in a three body resonance, a special configuration not so common among satellites and that provides clues to the origin of the system (Showalter and Hamilton 2015).
Abstract : In this work, we propose and seek evidence for a path to explain the satellitesâ current positions. Our idea is to start with an early Charon migrating under tidal forces from Pluto, and the initial resonant capture of one or more of the small satellites into two-body resonance with Charon. Perhaps the most likely situation is that Styx was first captured into the 3:1 mean motion resonance. Subsequently, Styx would move outward while simultaneously having its eccentricity raised by the resonance. We hypothesize that additional captures of Hydra and Nix into mean motion resonances can, when activated together, turn on the three-body resonance. If the three body resonance takes control, Styx could, in principle, be adiabatially removed from its resonance with Charon and its eccentricity driven back toward zero.
Jeudi 11 mars 2021 à 16h00 (En visioconfĂ©rence)
Olivier Flasseur (LESIA)
Astronomy is a field of study in which optical advances have driven the design of new generations of instruments always more efficient and dedicated to specific tasks. In particular, the detection of exoplanets and their characterization by direct imaging from the Earth is a hot topic. Beyond the detection of exoplanets, the reconstruction of circumstellar disks made of gas and dust is of primary astrophysical interest since exoplanets could form inside such structures by accretion.
Microscopy is another field of study in which recent advances in terms of resolution and sensitivity have opened the door to new medical diagnoses. Among the large variety of imaging modalities, digital holography appears to be a cost-effective method of choice for characterizing microscopic objects.
For both application fields, the detection, characterization and reconstruction of the objects of interest are very challenging due to the underlying low signal-to-noise ratio regime, thus requiring a fine processing of the data by dedicated and versatile algorithms. In this seminar, we will present some of the processing algorithms we have proposed in the context of high-contrast direct imaging, in astronomy, and of digital holography, in microscopy. The underlying imaging challenges are formalized within an inverse problems framework. The main focus is put on the use of statistical and/or physics-based approaches to derive reliable and quantitative estimates characterizing the detected objects. Information redundancies (e.g., temporal, multi-spectral) are also exploited. Robust processing strategies are also considered to improve their systematic deployment on data often corrupted by outliers. All the developed algorithms are totally unsupervised : weighting and/or regularization parameters are estimated in a data-driven fashion making the methods efficient for the processing of real data of uneven quality.
Pour assister au séminaire, contacter johan.mazoyer obspm.fr
Jeudi 25 fĂ©vrier 2021 à 16h00 (En visioconfĂ©rence)
Lucas Grosset (SOFIA - USRA - NASA Ames Research Center)
From visible to far-infrared, polarisation has proven helpful in bringing the extra constraints needed to improve our understanding of some complex astrophysical objects. This was especially true in the active galactic nuclei (AGN) field where polarisation was the corner stone in the development of the unified model for AGNs. Indeed, polarisation bring new observable to classical imaging or spectroscopy, setting stronger constraints on the physical mechanisms occurring at the observed location, but it also allows to get rid of some contrast issues. Furthermore, just like classical imaging, polarisation benefits a lot from high angular resolution that can separates two regions with very different polarisation, previously combined into a low resolution with a different polarisation (usually, polarisation degree increase with better resolution).
I will present two AGNs where the combination of polarisation and high angular resolution, in the near-infrared and in the far-infrared, helped to investigate the dust distribution surrounding the central engine of the AGN and the organization of magnetic field at larger scale. I will present both the observations and the numerical methods used to investigates these polarimetric observations, including the radiative transfer code MontAGN.
Pour assister au séminaire, contacter johan.mazoyer obspm.fr
Jeudi 11 fĂ©vrier 2021 à 16h00 (En visioconfĂ©rence)
Anna CIURLO (UCLA)
The proximity of the Galactic Center makes it unique for studying the kinds of interactions that might be taking place between the interstellar medium, the stars and the supermassive black holes in galactic nuclei. We used 13 years of data collected by OSIRIS, the NIR IFU at Keck Observatory to study the dynamics and temporal variability of gas. We highlighted several compact emission sources in BrÎł emission line which are orbiting closely around the central supermassive black hole. These objects appear to have many of the same characteristics as the tidally-interacting object, G2. The discovery and characterization of these additional objects demonstrate the existence of a population of these "G-objects". The G objects are possibly the residuals of binaries merged under the influence of the black hole. These objects potentially offer a mechanism to episodically transport material onto the black hole and enhance its activity. The accretion rate of cooler gas very near the black hole is not strongly constrained. With our large OSIRIS data-set we produced the highest signal-to-noise near-infrared spectrum of the immediate vicinity of the central black hole and test models of the accretion flow.
Pour assister au séminaire, contacter johan.mazoyer obspm.fr
Lundi 8 fĂ©vrier 2021 à 16h00 (En visioconfĂ©rence)
Yun ZHANG (Laboratoire Lagrange/CNRS, UniversitĂ© CĂŽte dâAzur, Observatoire de la CĂŽte dâAzur)
Most small bodies with sizes larger than tens of meters are made of numerous pieces of rocks and sometimes ices that have coalesced under the influence of gravity (so called “rubble piles”). As their own gravity is not enough to squeeze them into spheres, these small bodies exhibit a variety of shapes and surface morphologies, recording their life stories. Here I will show by numerical simulations how small bodies are reshaped in general subject to various effects, e.g., collisions, solar radiation, close encounters, based on our recent studies. Our interpretations allow us to characterize their physical properties and constrain their formation and evolution. Some examples including the formation of the first discovered interstellar object âOumuamua will be given (Zhang & Lin, Nature Astronomy 2020).
