LESIA - Observatoire de Paris

Plato2.0 : Diversité, Formation et Evolution des systèmes planétaires

Filippo Pantellini — 2014-05-23T13:54:51Z

La mission PLATO (PLAnetary Transits and Oscillation of stars), sélectionnée par l'ESA pour un lancement en 2024, a pour objectif la détection et caractérisation de milliers de planètes (dont des jumelles de la Terre). De plus, la mission permettra d'étudier la diversité, la formation, et l'évolution des systèmes "planètes-étoiles" améliorant ainsi fondamentalement notre compréhension des systèmes planétaires. Ces objectifs seront atteints grâce à la combinaison de la méthode des transits planétaires et de la sismologie des étoiles hôtes. La préparation de la mission s'appuiera sur l'expertise et l'expérience acquise grâce aux missions du CoRoT (CNES) et Kepler (NASA), qui ont démontré l'efficacité et la richesse de cette approche. De plus, PLATO bénéficiera d'une forte synergie avec la mission GAIA ainsi que des suivis spectroscopique depuis le sol.

Dans une première partie, les objectifs de la mission ainsi que l'organisation du consortium seront présentés. La seconde partie sera consacrée à la présentation de l'architecture de la charge utile PLATO constituée de 34 télescopes, 34 électroniques de lecture des détecteurs CCD et 16 Unités de Traitement Numérique. La philosophie de développement sera présentée dans ses grandes lignes. Une présentation de la participation des laboratoires d'Ile de France au Segment Sol clôturera la présentation.

Black hole variability : from Galactic center flares to microquasars oscillations

Filippo Pantellini — 2014-03-08T15:11:01Z

Black hole systems can be affected by variable phenomena. This temporal variability scales with the mass of the black hole and it is thus likely that it is related with strong-field gravity. This seminar is dedicated to examining some possible scenarios of black hole variability and link them when possible to future observations. During this talk, I will discuss two kinds of variable phenomena :

– the Galactic center flares of radiation : I will present simulations of astrometric observations of flares by the very-near future (May 2015) infrared interferometer GRAVITY. I will discuss the possibility that GRAVITY could help distinguish different theoretical flare scenarios ;
– microquasars oscillations : I will present a model of microquasar high-frequency quasi-periodic oscillations, namely the oscillating torus model. I will present simulated temporal power spectra for this model and discuss them in the perspective of the 3:2 resonance that some microquasars exhibit.

Planetary formation in transition and debris disks

Filippo Pantellini — 2014-02-03T11:20:12Z

I will present recent results regarding two peculiar objects in which planetary formation is most likely on-going. First, I will discuss in detail the 7 Myr old transition disk around TCha. With a large interferometric dataset, we are able to finely characterize the geometry of this transition disk, and revisit the previous detection of a candidate companion found to be in the gap. Nonetheless, the disk displays several features indicating that giant planet formation is on-going. Second, I will discuss the case of the 10-16 Myr old debris disk around HD113766A. With Herschel/PACS and VLTI/MIDI observations we demonstrate that the debris disk is twofold, with an inner dusty belt close to the star. Modeling results, such as the size of this belt and specific mineralogical markers indicate that we are witnessing the outcome of a collision between differentiated planetesimals. Terrestrial planet formation is known to be a highly unstable period of time and may explain our findings.

Fluctuations Alfvéniques dans le vent solaire : influence de l'orientation du champ magnétique local sur la vitesse radiale du plasma et implications pour les futures mesures près du Soleil

Filippo Pantellini — 2014-01-30T15:04:59Z

Il est avéré que les fluctuations Alfvéniques jouent un rôle important dans la caractérisation du profil du vent solaire et sont responsables des variations de vitesse observées sur l'échelle de l'heure à 1AU. Nous avons récemment montré qu'en conséquence du haut niveau d'Alfvénicité présent dans le vent solaire rapide, en particulier le vent polaire mesuré par la sonde Ulysses, il existe une corrélation bien établie entre la vitesse des protons et la direction du champ magnétique locale (Matteini et al, GRL 2014). Je discuterai les conséquences de cette corrélation respectivement sur l'évolution des fluctuations Alfvéniques de basse fréquence en fonction de la distance au Soleil, sur l'origine des renversements de champ magnétique (magnetic switchbacks) et sur la statistique de l'anisotropie des spectres turbulents. Je discuterai les possibles implications de cette étude dans le cadre des mesures près du Soleil par les sondes Solar Orbiter et Solar Probe+.

Angular Momentum Evolution in Low-Mass Stars : A Fresh Look

Filippo Pantellini — 2014-01-10T09:12:10Z

How angular momentum evolves in low-mass stars is one of the outstanding questions in stellar astrophysics, with implications for topics ranging from solar and stellar activity to the habitability of orbiting planets. The evolution is determined by a complex interplay between initial conditions during star formation, the evolution of stellar structure, and the behaviour of stellar magnetic fields, but a detailed theory has remained elusive. We present here an analytic model to address this issue. We show that the empirical picture of angular momentum evolution arises naturally if rotation is related to magnetic field strength instead of (as assumed in prior studies) to magnetic flux. The evolution then becomes a strong function of stellar radius, explaining the main trends in rotation observed in low mass stars from open clusters to the field at a few Gyrs. Our model is also able to reproduce the empirical Skumanich law for solar-type stars (for very different reasons than usually assumed), as well as the observed lifetime of magnetic activity in very low-mass stars. Finally, we present recent modifications to the theory — including more realistic wind velocities, field geometries and core-envelope decoupling — which bring our theory even closer in agreement to the observations. We also briefly discuss further anticipated tests of the theory, and some implications for planet habitability around M dwarfs.

Planets and Stellar Activity : Hide and Seek in the CoRoT-7 system

Filippo Pantellini — 2013-11-28T11:37:16Z

Since the discovery of the transiting Super-Earth CoRoT-7b, several investigations have been made of the number and masses of planets present in the system, but they all yield different results, owing to the star's high level of activity. We re-observed CoRoT-7 in January 2012 with both HARPS and CoRoT, so that we now have the benefit of simultaneous radial-velocity and photometric data. This allowed us to use the off-transit variations in the star's lightcurve to model the radial-velocity variations induced by stellar activity, according to the model described by Aigrain et al. (2012). This model was incorporated into a Monte Carlo Markov Chain in order to make a precise determination of the orbits of CoRoT-7b and CoRoT-7c. We use Bayesian methods to assess the evidence for the presence of CoRoT-7d.

La mission MarcoPolo-R

Filippo Pantellini — 2013-11-18T11:13:06Z

MarcoPolo-R est une mission de retour d'échantillons d'un astéroïde géocroiseur, en compétition à l'ESA dans le cadre du programme Cosmic Vision - M3. MarcoPolo-R est une mission multidisciplinaire, qui s'inscrit dans une nouvelle ère d'exploration du système solaire, et qui permettra à l'Europe de contribuer pleinement à l'effort international de retour d'échantillons.

L'objectif scientifique principal de la mission MarcoPolo-R est le retour d'échantillons de matière inaltérée provenant de l'astéroïde géocroiseur primitif classé potentiellement dangereux (PHA) 2008 EV5. Cet astéroïde possède un albédo qui suggère qu'il s'agit d'un corps hydraté très primitif, peutêtre présentant un lien génétique avec les comètes. L'échantillonnage d'une telle matière et sa préservation d'intéractions avec la géosphère permettra d'obtenir de ce fait des mesures à l'échelle atomique qui ne peuvent pas être effectuées in situ par une sonde spatiale robotique. Ces mesures viseront à déterminer l'environnement stellaire du système solaire à sa naissance, l'héritage de matière pré-solaire, la chronologie fine de la condensation et de la formation des premiers solides, l'origine des éléments volatils et biotiques sur les planètes, les processus évolutifs en oeuvre sur un astéroïde.

MarcoPolo-R a le potentiel de révolutionner notre compréhension des propriétés du matériel primitif présent dans le Système Solaire et de nous éclairer sur la nature des petits corps, ce qui constitue une étape essentielle pour comprendre les conditions primordiales responsables de la formation des planètes et de l'émergence de la Vie sur Terre. Enfin, elle fournira des informations cruciales sur les géocroiseurs pour développer des stratégies appropriées pour protéger la Terre d'un impact de l'un de ces objets potentiellement dangereux.

L'état de la mission et les résultats de l'étude à l'ESA seront présentés.

Explosive volcanism : Mercury, the Moon and Bepi-Colombo

Filippo Pantellini — 2013-11-18T11:09:25Z

Volcanism is a common processes in the Solar System whether it is ice or silicate based. In the inner Solar System, explosive and effusive volcanism are two common expressions of silicate based volcanism, including the Moon and Mercury. The goal of this presentation is to highlight how the latest observations from remote-sensing instruments in orbit around these two planets are changing our view of the origin, the occurrence and the mineralogical composition of explosive volcanism. For instance, explosive volcanism was not expected on Mercury's surface and lunar explosive volcanism is showing a range of mineralogical composition much larger than previously thought. Since explosive volcanism is highly dependant on the volatile content, a better understanding is highly critical to constrain thermal, mineralogical and atmospheric evolution of a planet. Current and foreseen observations, especially with the suite of instruments on Bepi-Colombo, will certainly allow us to make very important observations to keep improving our understanding of volcanism in the Solar system.

The Oldest Computer, the Antikythera Mechanism: Epitome of Greek Philosophy

Filippo Pantellini — 2013-10-15T09:56:41Z

"The origin of all technical achievements is the divine curiosity [of Socrates/Plato] and the play instinct of the working and thinking researcher as well as the constructive fantasy of the inventor..."

Albert Einstein, speech on the radio at the opening of the 7 Deutsche Funkausstellung in Berlin, 1930.

The so called Antikythera Mechanism, or Pinax or Sphere, as its original name was, is the oldest known advanced scientific instrument, the first computer and mechanical universe.

Subverts everything believed about the lack of interest of the Greeks in technology. Seems to be offseason, in fact it is the epitome of Greek Philosophy.

It is the epitome of philosophy because to build a mechanical Cosmos, such as the mechanism, you need to understand, embrace and practice the Greek philosophy, the philosophy of the Ionian philosophers, and you got to put it to work. We can say, in fact, that the signature of Pythagoras is in the mechanism, as on one of the gears the Pythagorean pentagon is engraved right in the middle of a gear, around its shaft. The Mechanism is the culmination of Pythagorean philosophy, their teaching and understanding that led to our knowledge of the Cosmos with the introduction of mathematics to understand and predict natural phenomena. This process was based on observations, experiments and the perception that Nature is harmonious, and that the Cosmos vibrates with the so called Music of the Spheres. The Pythagoreans discovered all these with properly designed and realized experiments with musical instruments, hammers, strings etc and appropriate measurements followed with appropriate theoretical analysis with mathematics that eventually led them to the inductive thinking formulation of the laws of physics and modern civilization with today's technology.

Built by Greeks, probably between 150 and 100 BC and, as demonstrated by appropriate calculations based on our measurements on the mechanism, the instrument is based on measurements taken by Archimedes and his students at a philosophical school that he had in Syracuse. It turns out now that Archimedes was a physicist and astronomer and had school and his students continue astronomical work for at least few decades, obtain measurements of eclipses using a clock (like the one we know from detailed description Archimedes constructed). The pupils of Archimedes eventually send tables with astronomical data, including eclipses observations, to another Greek who constructs the instrument. As Hipparchus is perhaps the only and the greatest astronomer Greek at that time who works in Rhodes, that has a lot of money and excellent tradition in metallurgy and technological constructions.

The mechanism is a complex exact analog and digital computer that works with carefully designed and manufactured gears (Bytes) with small teeth (bits). The gears perform certain mathematical operations as they move around and drive shafts and indicators and pointers showing the position of various heavenly bodies, the Sun, the Moon and possibly the planets in circular and spiral scales (analog part). That was the first mechanical universe, the first planetarium.

Findings of the wreck (statuettes and conical weights) combined with ancient texts, lead us to a working hypothesis that perhaps the mechanism was at a weight and float and might have read in automatically as texts describing clock of Archimedes or as medieval clocks.

Of particular importance is the discovery that the motion of the Moon follows to a good approximation Kepler's second law, and perhaps even all three laws of Kepler, discovery completes initial study five years ago. The motion of the Moon is very realistic using a train of 4 gears, two of them linked with an elliptical bond (pin in an elliptical slot) and the trajectory and velocity of the Moon probably follows the three laws of Kepler.

Accélérer les calculs a l'aide de GPUs

Filippo Pantellini — 2013-09-18T08:14:43Z

L'arrivée des GPU (Graphics Processing Unit) a profondément changé la manière de concevoir la notion de coprocesseur. A moins de 500€, il est désormais possible d'avoir à sa disposition une puissance de calcul qui n'était réservée jusqu'à un passé récent qu'aux grands centres de calcul. La société Nvidia, en mettant au point l'environnement CUDA, a fourni à la communauté des développeurs des moyens simples et efficaces pour rendre cette puissance de calcul accessible au plus grand nombre.

Ce séminaire sera l'occasion de découvrir toutes les possibilités qu'offrent ce nouveau type d'accélérateur matériel, à travers une introduction à cette nouvelle architecture et tout l'écosystème qui s'y rapporte donnée par François Courteille, Senior Professional Solution Architect chez Nvidia ainsi que des exemples d'applications. Damien Gratadour nous présentera ensuite les résultats obtenus par l'équipe optique adaptative du pôle HRAA du LESIA pour la simulation et le contrôle temps-réel de systèmes d'optique adaptative à l'aide de GPU. Les présentations seront suivies d'une table ronde de questions-réponses

Mardi 15 octobre 2013 de 14:00 à 16:00 à l'amphithéâtre du bât 18.

Inscription et programme détaillé sur : https://indico.obspm.fr/conferenceDisplay.py?confId=24