LESIA - Observatoire de Paris

  • Mardi 15 janvier 2019 à 11h00 (Salle de conférence du bâtiment 17)

    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 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.


  • Vendredi 30 novembre 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Co-évolution de la vie et de l’atmosphère de la Terre primitive

    Benjamin Charnay (LESIA) et Boris Sauterey (IBENS, Institut de Biologie de l’Ecole Normale Supérieure)

    La vie et l’atmosphère de la Terre primitive semblent avoir évolués de façon couplée. L’exemple le plus frappant est la Grande Oxygénation de l’atmosphère, provoquée par l’apparition de la photosynthèse oxygénique, qui a bouleversée la chimie atmosphérique et océanique, la minéralogie, l’évolution de la vie et potentiellement le climat avec le déclenchement des glaciations Huroniennes. Il a également été proposé qu’avant la Grande Oxygénation, l’atmosphère archéenne était riche en méthane issu d’organismes méthanogènes. Ce méthane aurait provoqué un effet de serre permettant de compenser la plus faible insolation à cette époque, et donc de maintenir un climat tempéré. Cependant, aucune modélisation globale et réaliste du système Terre incluant la biosphère primitive n’a été réalisée pour tester ces hypothèses.

    Notre objectif est d’étudier les interactions entre les écosystèmes et l’environnement (atmosphère, océan et activité géologique) de la Terre primitive, ainsi que leur évolution. Pour cela, nous avons couplé un modèle climatique incluant la photochimie, à un modèle du cycle du carbone et un nouveau modèle réaliste et dynamique d’écosystème primitifs, qui inclut les différents métabolismes non photosynthétiques.

    Durant cette présentation, nous décrirons le modèle climat-photochimique (le GCM Générique du LMD), le modèle du cycle du carbone et le modèle d’écosystème. Nous présenterons ensuite les résultats obtenus en couplant ces différents modèles, qui nous permettent de quantifier les teneurs en méthane durant l’Hadéen/Archéen et d’étudier les rétroactions climatiques causées par les écosystèmes de méthanogènes. Nous terminerons en discutant brièvement des différentes perspectives que nous offre ce modèle unique.


  • Lundi 26 novembre 2018 à 14h00 (Salle de conférence du bâtiment 17)

    Cours d’évolution stellaire sur la branche des géantes rouges

    Arlette Noels (Université de Liège, STAR institute)

    La Professeure Arlette Noels fera un cours d’évolution stellaire avancée pour les étoiles de faible masse sur la phase des géantes rouges. Cours en langue française.


  • Mardi 13 novembre 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Forecasting geoeffeciency of coronal mass ejections and their sheath regions

    Emilia Kilpua (University of Helsinki, Finlande)

    Coronal mass ejections (CMEs) are key drivers of magnetospheric storms at Earth. Two key structures in CMEs are the turbulent sheath and the magnetic flux rope with smoother variations of solar wind plasma and magnetic field parameters. In this presentation I will discuss the status and challenges in predicting in advance (at least half-a-day) geoefficiency of flux ropes and sheath regions. The key focus will also be on observational and data-driven modelling efforts at the University of Helsinki space physics team to predict the magnetic field characteristics of flux ropes.


  • Mardi 6 novembre 2018 à 14h00 (Salle de conférence du bâtiment 17)

    Dynamique du milieu interstellaire au Centre galactique : des nouveaux objects type G2 autour du trou noir supermassif

    Anna Ciurlo (UCLA)

    Au centre de la Voie lactée, l’environnement du trou noir supermassif (SMBH) présente un mélange complexe de jeunes étoiles, de gaz et de poussière. Le parsec central du Centre galactique a été observé à haut résolution avec le Keck pendant plus de 20 ans, le but principal étant de surveiller les étoiles en orbite autour du SMBH. Cependant, les caractéristiques du gaz situé au centre peuvent être examinées de près en utilisant cette base de données unique.

    En particulier, les observations effectuées avec le spectromètre à intégrale de champ OSIRIS au Keck nous permettent d’examiner les propriétés dynamiques du gaz et d’identifier un certain nombre d’objets aux propriétés inhabituelles. Certains d’entre eux ont de très hautes vitesses radiales (200-800 km/s) et changent de vitesse, positon ou de forme au cours du temps. Parmi ces objets on observe plusieurs nouveaux objets de « type G » (Gillessen et al. 2012) ou objets stellaires poussiéreux (dusty stellar objects DSO, Eckart et al. 2013).

    Nous présentons une étude de la morphologie et de la dynamique orbitale de ces objets compacts. Nous discutons des hypothèses pour leur formation et leur évolution. Ces objets pourraient jouer un rôle dans l’accrétion sur le SMBH. Initialement supposés être des nuages ​​de gaz compacts et poussiéreux, les DSO sont probablement des objets stellaires entourés d’enveloppes gonflées de poussière et de gaz. Diverses hypothèses ont été proposées mais aucun consensus général n’a encore été atteint sur leur nature. Jusqu’à présent, deux de ces objets ont été identifiés et étudiés en détail : G2 et G1. Nous décrivons ici les observations de trois autres DSO, y compris deux signalés pour la première fois, et utilisons cette population DSO pour contraindre leur nature intrinsèque.


  • Mardi 6 novembre 2018 à 14h00 (Salle de réunion du bâtiment 14)

    Impact of an axisymmetric magnetic field on gravity waves in rapidly rotating stars

    Vincent Prat (AIM, CEA)

    Internal gravity waves provide us with a unique way to probe the stellar interior of intermediate-mass and massive stars. Those waves are also able to transport angular momentum. However, the propagation, the frequencies of waves and the transport they induce may be strongly affected by rotation and the presence of a magnetic field. For slow rotators, rotation can be taken into account using perturbative methods. For rapid rotators, which is the case of many early-type pulsators such as gamma Doradus, delta Scuti, SPB and Be stars, such methods fail, and the eigenvalue problem is fully 2D and computationally expensive. The traditional approximation of rotation, which neglects the horizontal component of the rotation vector, allows for efficient computation of modes. Another approach is to consider only small-wavelength waves and model them as propagating rays. Using these two complementary methods, I will discuss the impact of an axisymmetric magnetic field on gravito-inertial waves and the implications for the detection of deep magnetic fields.


  • Mercredi 24 octobre 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Precursors of Magnetic Flux Emergence in the Moat Flows of Active Region AR12673

    I will describe observations of magnetic disturbances in active region AR12673 between 1 and 3 September 2017 seen as a disruption of the moat flow several hours before the onset of strong flux emergence near the main sunspot. The moat flow is commonly known as a radially oriented strong outflow of photospheric plasma surrounding sunspots, which ends abruptly and shapes an annular pattern around the penumbra. Using highly accurate methods of tracking this photospheric flow applied to SDO/HMI data, we are able to describe the evolution of the moat surrounding the main sunspot of AR 12673. We find that several hours before the emergence of strong magnetic flux near the main sunspot, the moat boundaries are broken at these very same locations. This behavior is observed both on 1 and 3 September. There is no such behavior observed in the absence of flux emergence. These observational results pose the question of how often they occur in other active regions and whether the disruption of the moat flow might be, like in this case, an indication of impending enhanced magnetic activity or simply a coincidental event. I will also describe ongoing work using my new photospheric tracking framework adapted specifically for HMI and that enables systematic analyses of the moat flows as well as the relationship between supergranular flows and the magnetic evolution in emerging active regions.


  • Mardi 23 octobre 2018 à 11h00 (Salle de conférence du bâtiment 17)

    Magnetic Imprints and Momentum Processes in Major Solar Eruptions

    Xudong SUN (University of Hawaii, USA)

    The solar active region photospheric magnetic field evolves rapidly during major eruptive events, suggesting appreciable feedback from the corona. Using high-cadence vector magnetogram sequence, multi-wavelength coronal imaging, and numerical simulation, I will show how the observed photospheric "magnetic imprints” are highly structured in space and time, and how it can in principle be used to estimate the impulse of the Lorentz force that accelerates the coronal mass ejection (CME) plasma. In an archetypical event, the Lorentz force correlates well with the CME acceleration, but the total force impulse surprisingly exceeds the CME momentum by almost two orders of magnitude. Such a trend holds for the majority of the major eruptions from our survey sample. I propose a "gentle photospheric upwelling" scenario, where most of the Lorentz force is trapped in the lower atmosphere layer, counter-balanced by gravity of the upwelled mass. This unexpected effect dominates the momentum processes, but is negligible for the energy budget. I will discuss our follow-up project, how the upcoming high-sensitivity observations and new-generation numerical models may help elucidate the problem.


  • Jeudi 18 octobre 2018 à 16h00 (Salle de conférence du bâtiment 17)

    A Monte-Carlo radiative transfer code to explore the polarisation signatures of dusty disks and torii and its application to torii in active galactic nuclei

    Lucas Grosset (LESIA)

    One of the main observational challenges for investigating the central regions of active galactic nuclei (AGN) at short wavelengths, using high angular resolution, and high contrast observations, is to directly detect the circumnuclear optically thick material hiding the central core emission when viewed edge-on. I will present my work on the dust structures of these inner regions and the main results I obtained during my four years in the LESIA. But I will focus particularly on one of the tool I used for this purpose, the simulation code MontAGN, computing the high angular resolution maps of dusty structures, including polarisation, through Monte-Carlo radiative transfer methods in the near infrared. Even at early development stages, simulated maps through MontAGN allowed me to constrain the geometry and density of the structures surrounding the AGN, in particular the dusty torus and the ionisation cone. Furthermore, after 2 years of development, the code is now almost ready for its first release and will be soon available after few upgrades.


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