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Institut d'Astrophysique et
de Géophysique (Bât. B5c)

Quartier Agora
Allée du 6 août, 19C
B-4000 Liège 1 (Sart-Tilman)

Tel.: 04.366.9779
Fax: 04.366.9729
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Mois précédent Mois suivant Janvier 2013
Séminaire suivant  1er séminaire : lundi 28 janvier, 15h45
Galaxy clusters in large X-ray surveys
Nicolas Clerc (Max Planck Institute for Extraterrestrial Physics, Munich)

Large sample of galaxy clusters collected in X-ray surveys are able to tightly constrain cosmological scenarios and provide a deep understanding of the physics of galaxy clusters. I will describe the main results, expectations and challenges faced in the exploitation of current and future large X-ray surveys. I will first introduce XMM-XXL, a 50deg2 XMM survey complemented by multi-wavelength data. I will describe its expected goals based on recent results obtained from (i) the pilot XMM-LSS survey (10deg2 in a contiguous area, catalogue of ~150 clusters) and (ii) X-CLASS, an archival survey (100deg2 scattered across the extragalactic sky, catalogue of ~800 clusters). In particular, I will particularly insist on practical aspects of these XMM-based cluster searches. Finally, I will present the German-Russian mission eRosita to be launched in 2014, and depict scientific applications from the expected discovery of 2-3 millions Active Galactic Nuclei and 100,000 galaxy clusters.
Mois précédent Mois suivant Février 2013
Séminaire précédant Séminaire suivant 2ème séminaire : mardi 05 février, 14h00
Sea ice biogeochemistry and interactions with atmosphere
Daiki NOMURA (Norwegian Polar Institute and Hokkaido University)

Sea ice cover has not been considered in estimations of biogeochemical cycles in ice-covered seas because of the assumption that sea ice acts as a barrier for the atmosphere-ocean systems. However, recent results have indicated that processes of sea ice formation and melting affect biogeochemical cycles in ice-covered seas. In this presentation, new results obtained in the Svalbard, Arctic Ocean (2011 and 2012) and the east Antarctica (2012) will be presented, in addition to my previous studies regarding sea ice biogeochemistry in the Sea of Okhotsk, Barrow, Alaska and the Lützow-Holm Bay, Antarctica.
Séminaire précédant Séminaire suivant 3ème séminaire : mardi 26 février, 15h45
Spectropolarimetry of Massive Binary Stars
Jamie Lomax (University of Denver)

Spectropolarimetry is an especially powerful tool when used in combination with other methods, such as photometry, spectroscopy and interferometry, and is used across the full range of wavelengths. It can provide important information about magnetic fields and the physical structure of complex systems that may not be revealed by other means. For example, it can constrain the properties of circumstellar disks, hot star winds, and complex binary star systems. In this talk, I will give examples of some applications of spectropolarimetry by reviewing some recent results from various massive star studies. Additionally, I will discuss results from new and recalibrated data taken with the University of Wisconsin’s Half-Wave Spectropolarimeter (HPOL) which have implications for our current understanding of three different massive binary systems: beta Lyrae, V356 Sgr, and V444 Cyg. Using broad band and spectropolarimetric analysis, I place constraints on the geometrical distribution of material within these systems that can help quantify the amount of mass lost from massive interacting binary systems.
Séminaire précédant Séminaire suivant 4ème séminaire : mercredi 27 février, 14h00
The flavor problem & flavor symmetry
Stefano Morisi (University of Wurzburg, Germany)

I give an introduction to the flavor problem. One of the most popular and attractive possibility to approach such a "problem" is that SM must be extended assuming an horizontal symmetry acting between the three generations of families, namely a flavor symmetry. I give a review on non-abelian discrete flavor symmetries first introduced for the so called tri-bimaximal neutrino mixing that is now ruled-out. I give the status of the situation about the flavor symmetries and I will mention their phenomenology.
Séminaire précédant Séminaire suivant 5ème séminaire : jeudi 28 février, 14h30
Flavor symmetries and DM stability
Eduardo Peinado (INFN-LNF, Italy)

I will discuss the connection of non-abelian discrete flavor symmetries and its connection with the stability of dark matter (DM). First I will review the discrete dark matter mechanism, where the same non-abelian discrete flavor symmetry which accounts for the observed pattern of fermion masses and mixings, spontaneously breaks to a subgroup which renders DM stable. I will also sketch another possibility, where in the supersymmetric scenario the flavor symmetry forbids the R-parity violating terms giving a stable lightest supersymmetric particle (LSP).
Séminaire précédant Séminaire suivant 6ème séminaire : jeudi 28 février, 15h45
Observations of selected gravitational lenses at Maidanak observatory (brief review)
Talat Akhunov (AGO)

During several years we were carrying out photometric observations of selected gravitationally lensed quasars - SBS1520+530, UM673, SBS0909+52, PG1115+07, H1413+117, B1422+23, Q2237+030 and others. Photometric handling has been carried out. All these quasars show active variability. With these measurements we got observed values of time delays and evidence of microlensing in these objects.
Mois précédent Mois suivant Mars 2013
Séminaire précédant Séminaire suivant 7ème séminaire : jeudi 07 mars, 15h45
Double beta decay and the LHC
Martin Hirsch (IFIC, Valencia)

Neutrinoless double beta decay (0nbb) is mostly known as a sensitive probe for Majorana neutrino masses. However, since double beta decay is a process which violates lepton number, in principle all kind of extensions beyond the standard model of particle physics which have lepton number violation are constrained by limits from double beta decay, examples are left-right models or R-parity violating supersymmetry. Up to very recently double beta decay provided the most stringent constraints for many parameters of such models, but the data coming from the LHC experiments is already competitive with Sqrt(s)= 8 TeV and 5/fb of statistics. After some fairly general introduction to double beta decay, this talks discusses the general decomposition of the 0nbb decay operator and the possible tests of the short-range part of the 0nbb decay amplitude at the LHC.
Séminaire précédant Séminaire suivant 8ème séminaire : jeudi 14 mars, 15h45
The environment of Active Galactic Nuclei
Elias Koulouridis (National Observatory of Athens)

Nowadays, it is widely accepted that the accretion of material into a massive black hole, located at the galactic centre, is responsible for the detected excess emission (radiation not emitted by stellar photospheres) in the AGN's spectra, and these black holes do exist in all elliptical galaxies and spiral galaxy bulges, including our own. However, we still lack complete understanding of the various aspects of the nuclear activity, for example, the triggering mechanism and the feeding of the black hole, the physical properties of the accretion disk and the obscuring torus predicted by the unified scheme, the origin of jets in radio loud objects, the connection with star formation and the role of the AGN feedback. This talk is about different environments of AGN host galaxies and their significant role in the triggering, evolution, and quenching of the nuclear activity.
Séminaire précédant Séminaire suivant 9ème séminaire : jeudi 21 mars, 16h00
Joint colloquium :

Giant telescopes and the distant Universe
Paul Hickson (University of British Columbia)

The next generation of optical and infrared telescopes will transform astronomy. Equipped with adaptive optics and suites of sophisticated instruments, these telescopes will provide a view of the Universe with unparalleled resolution and sensitivity. Their science programs range from studies of the first luminous objects in the Universe to the characterization of extrasolar planets and the search for extraterrestrial life. Three such facilities are expected to begin construction within the next two or three years. I will give an overview of these projects, highlight some of the scientific opportunities that they will provide, and describe the novel technologies that make these telescopes possible.
Mois précédent Mois suivant Avril 2013
Séminaire précédant Séminaire suivant 10ème séminaire : jeudi 18 avril, 15h45
QCD collinear factorization, its extentions and the partonic distributions
Lech Szymanowski (National Center for Nuclear Research (NCBJ), Warsaw, Poland)

I summarize basic facts about the QCD factorization theorems. I start with a reminder of the standard form of the QCD factorization applied to the description of inclusive DIS in terms of the integrated partonic distribution functions (PDFs) and for the description of some hard exclusive processes which involve non-forward generalizations of PDFs: GPDs, GDAs, TDAs. Then I discuss a form of the QCD factorization relevant for the description of semi-inclusive DIS or Drell-Yan processes in terms of the unintegrated, transverse momentum dependent partonic distributions (TMDs). Finally, I discuss the open problem of TMD factorization breaking in hadroproduction of hadrons with high pT.
Séminaire précédant Séminaire suivant 11ème séminaire : lundi 22 avril, 16h00
Joint colloquium :

A case of non-quantum quantization
Yves Couder (Laboratoire Matière et Systèmes Complexes, Université Paris 7 Diderot)

How can a single structure have simultaneously the properties of a localized particle and those of a wave? This is the central question at the scale of elementary particles. Perfectly characterized in the formalism of quantum mechanics, this duality is usually thought to have no possible equivalent in classical physics. We were driven into revisiting this question when we found that a droplet bouncing on a vibrated bath could become self-propelled by its coupling to the surface waves it excites. I will focus on experiments where the droplet is submitted to a central force and show that several quantum-like behaviors emerge, all related to the "path memory" contained in the wave field.
Séminaire précédant Séminaire suivant 12ème séminaire : jeudi 25 avril, 15h45
Sterile neutrinos and violation of lepton flavor universality
Avelino Vicente (Université Paris-Sud)

We study the violation of lepton flavour universality in light meson decays due to the presence of non-zero mixings between the active neutrinos with new sterile states. The modified W l nu vertices, arising from a non-unitarity leptonic mixing matrix intervening in charged currents, might lead to a tree-level enhancement of R = Gamma (P -> e nu) / Gamma (P -> mu nu), with P=K, pi. These enhancements are illustrated in the case of the inverse seesaw, showing that one can saturate the current experimental bounds on R for kaons and pions, while in agreement with the different experimental and observational constraints.
Mois précédent Mois suivant Mai 2013
Séminaire précédant Séminaire suivant 13ème séminaire : jeudi 02 mai, 15h45
Axion cold dark matter
Javier Redondo (Max-Planck-Institut für Physik (Werner-Heisenberg-Institut) München)

We will briefly review the strong CP problem and the solution proposed by Peccei and Quinn, the existence of a new degree of freedom that restores C and P conservation in the colour sector: the axion. Then we will review the cosmology of axions focusing on their role as dark matter, and the perspectives for its discovery.
Séminaire précédant Séminaire suivant 14ème séminaire : mercredi 08 mai, 14h00
A virtual tour to the Atacama Large Millimeter-submillimeter Array (ALMA)
Thodori Nakos (ALMA, Chile)

ALMA is the most advanced and complex observatory ever built. By the end of construction, foreseen for September 2013, the ALMA interferometer will consist of 66 array elements, situated at 5,000 m altitude in the Atacama desert. The first Early Science results, taken with about just 20 antennas, have clearly demonstrated the superiority of ALMA with respect to any other sub-mm facility, by a factor of 10 - 30, both in terms of sensitivity and angular resolution. This virtual tour will give you an insider's view to the challenges we are facing every day to complete the construction, how to achieve the high-precision (e.g. surface, pointing accuracy) of the array elements, and present some of its spectacular scientific results, although ALMA is still in its infancy.
Séminaire précédant Séminaire suivant 15ème séminaire : mercredi 15 mai, 15h45

Santiago Triana (Instituut voor Sterrenkunde, KU Leuven, Belgium)

A 3-meter diameter liquid-metal spherical-Couette flow, composed of a rotating spherical container and a differentially rotating internal core, exhibits a sequence of sharp and strong inertial mode oscillations when the (inner core to outer sphere) rotation rate ratio is less than one or negative. At larger rotation ratios the flow exhibits bi-stable fluctuations. Non-linear self-interactions of inertial modes lead to zonal flows, whose strength diverges as the Ekman number vanishes, therefore enhancing dissipation substantially. This is particularly relevant when considering dissipation mechanisms of gravito-inertial modes in stars and planets (excited either by tidal forces or differential rotation) as their Ekman number is vanishingly small. A good understanding of the modes\\\' driving mechanism (and dissipation) in the laboratory experiments might be of great help to elucidate the nature of the gravito-inertial oscillations observed very recently in stars.
Séminaire précédant Séminaire suivant 16ème séminaire : jeudi 30 mai, 16h00
Joint colloquium :

Les premiers résultats cosmologiques de PLANCK
Jean-Loup Puget (IAS - Institut d'Astrophysique Spatiale, Orsay)

Les principaux résultats cosmologiques du satellite Planck seront résumés, après leur diffusion et la mise à disposition des données le 21 mars dernier. Planck HFI, réalisé sous la maîtrise d'oeuvre de l'IAS, est le fruit d'une collaboration internationale impliquant le CSL.
Mois précédent Mois suivant Juin 2013
Séminaire précédant Séminaire suivant 17ème séminaire : jeudi 06 juin, 16h00
Joint colloquium :

Cosmic strings in multiferroics
Nicola A. Spaldin (ETH Zürich)

A key open question in cosmology is whether the vacuum contains topological defects such as cosmic strings, believed to have formed as a result of symmetry-lowering phase transitions in the early universe. An inexpensive, laboratory-based route to shedding light on the answer is to test the predicted scaling laws for topological defect formation (the so-called Kibble-Zurek mechanism) in condensed matter systems. Here we show that the multiferroic hexagonal manganite oxides -- with their coexisting magnetic, ferroelectric and antiphase orderings -- have an appropriate symmetry-lowering phase transition for testing the Kibble-Zurek scenario. We present an analysis of the Kibble-Zurek theory of topological defect formation applied to the hexagonal magnanites, show that the recently observed domain vortex cores are formally topologically protected, and that recent literature data are quantitatively consistent with our predictions from first-principles electronic structure theory. Finally, we explore experimentally for the first time to our knowledge the cross-over out of the Kibble-Zurek regime and find a surprising "anti-Kibble-Zurek" behavior.
Séminaire précédant Séminaire suivant 18ème séminaire : jeudi 20 juin, 15h45
Quark masses and mixing from spontaneous symmetry breaking
Chee Sheng Fong (LNF (Laboratori Nazionali di Frascati, INFN))

In this talk, I will discuss the idea of spontaneous breaking of quark symmetry SU(3)^3 to explain the structure of Yukawa couplings in the Standard Model. In particular, I will show that quark mass hierarchy and mixing can be realized in the symmetry breaking ground state. In addition, I will also show that there is no strong CP violation in that ground state.
Séminaire précédant Séminaire suivant 19ème séminaire : vendredi 21 juin, 15h00
Joint colloquium :

Quantum Tunneling of Atomic Waves
Ennio Arimondo (Università di Pisa)

Tunneling as a quantum mechanical effect takes place in a classically forbidden region between two regions of classically allowed motion. A standard example of tunneling across static barriers is the motion in a double-well potential. The two potential wells are separated by a potential barrier which is impenetrable for a low-energy classical particle. The quantum mechanical solution shows that the wave packet initially localized in one of the wells performs oscillations between the two classically allowed region. Tunneling takes place between two levels nearly degenerate in energy, and in most cases the investigated tunneling takes place between the lowest energy states, for instance of the double well. In the last decade, the experimental techniques used in atom and quantum optics have made it possible to control the external and internal degrees of freedoms of ultracold atoms with a very high degree of precision. Thus, ultracold bosons or fermions loaded into the periodic optical potential created by interfering laser beams (double-well, lattices and superlattices) are optimal realizations of quantum mechanical processes and phenomena proposed and studied in other contexts of solid-state physics. We will present the observation and investigation of different quantum tunneling phenomena for a Bose-Einstein condensate of rubidium atoms loaded within the periodic potential created by interfering laser beams
Mois précédent Mois suivant Août 2013
Séminaire précédant Séminaire suivant 20ème séminaire : jeudi 29 août, 15h45
The controversy about quark and gluon angular momentum: what's it all about and does it matter?
Elliot Leader (Imperial College, London)

An important aspect of understanding the internal structure of the nucleon is to know how much spin and orbital angular momentum is carried by the quarks and gluons, and various experiments are designed to measure these quantities. Surprisingly, it turns out that it is far from trivial to decide how to define these quantities. So, given that they are supposed to be measurable, some clarification is essential! My talk hopes to provide that.
Mois précédent Mois suivant Septembre 2013
Séminaire précédant Séminaire suivant 21ème séminaire : jeudi 05 septembre, 15h45
Magnetism in Massive Stars
Véronique Petit (University of Delaware)

Massive star magnetism is often considered an astronomical "wildcard", as it is hard to predict in which stars it may occur. This reflects our fundamental ignorance of the origin of massive star magnetism, and compels us to better understand the scope of its influence on massive stars individually, and also as a population. In the last decade, our understanding of this phenomenon has made a giant leap forward thanks to a new generation of powerful spectropolarimeters capable of measuring the Zeeman effect in the spectra of these stars. Over the past 5 years, ambitious projects such as the Magnetism in Massive Stars (MiMeS) Collaboration have been seeking out magnetic massive stars in the Galaxy, to better understand their origins, physical properties, and how they influence observable stellar characteristics.
Séminaire précédant Séminaire suivant 22ème séminaire : lundi 09 septembre, 15h45
Clouds in Brown Dwarfs and Exoplanets: The Burgeoning Field of Astrometeorology
Adam Burgasser (UC San Diego, USA)

The atmospheres of low-temperature brown dwarfs (L, T and Y dwarfs) and warm exoplanets are conducive to complex chemistry, including the formation of condensate grains of metals, minerals, salts and ices. The distribution of condensate clouds across the surface of these objects, and the variation of cloud properties with temperature, pressure and composition all play critical roles in shaping emergent spectral energy distributions, energy balance and temporal variability. In this talk I summarize our current evidence and theoretical understanding of clouds on brown dwarfs and exoplanets, with an emphasis on the most recently discovered (by TRAPPIST) variable brown dwarf system Luhman 16AB.
Séminaire précédant Séminaire suivant 23ème séminaire : jeudi 19 septembre, 15h45
Fermi Bubbles under Dark Matter Scrutiny
Wei-Chih Huang (Scuola Internazionale Superiore di Studi Avanzati di Trieste)

The quest for Dark Matter signals in the gamma-ray sky is one of the most intriguing and exciting challenges in astrophysics. In this talk, I will present the analysis of the energy spectrum of the Fermi bubbles at different latitudes, making use of the gamma-ray data collected by the Fermi Large Area Telescope. By exploring various setups for the full-sky analysis we achieve stable results in all the analyzed latitudes.

At high latitudes, |b|=20°-50°, the Fermi bubbles energy spectrum can be reproduced by gamma-ray photons generated by inverse Compton scattering processes, assuming the existence of a population of high-energy electrons. At low latitudes, |b|=10°-20°, the presence of a bump at a gamma energy around 1 to 4 GeV, reveals the existence of an extra component compatible with Dark Matter annihilation.

Our best-fit candidate corresponds to annihilation into bottom-antibottom with a dark matter mass M= 61.8+6.9-4.9 GeV and cross section <sigma v> = 3.30+0.69-0.49 10-26 cm3 s-1. In addition, using the energy spectrum of the Fermi bubbles, we derive new conservative but stringent upper limits on the Dark Matter annihilation cross section.
Séminaire précédant Séminaire suivant 24ème séminaire : jeudi 26 septembre, 15h45
Investigating pre-main sequence stellar evolution with asteroseismology
Konstanze Zwintz (Institute of Astronomy of KU Leuven.)

Young stars that have recently been formed gain their energy mostly from gravitational contraction and can become pulsationally unstable during their evolution to the main sequence. Within the last decade, several dozens of pulsating pre-main sequence (PMS) stars of different types were discovered using data obtained by the space telescopes MOST and CoRoT. The asteroseismic investigations yielded new insights into the connection between pulsation and early stellar evolution. It was shown, for example, that the evolutionary stage of a star can be constrained by asteroseismology, if a dense enough pulsation frequency spectrum is observed. Also, the study of young pulsating stars in the cluster NGC 2264 (observed simultaneously in the optical by the MOST and CoRoT satellites, the infrared by the Spitzer space telescope and the X-ray by the Chandra satellite mission) yields exciting results. An overview of the current status and latest results in the field of asteroseismology of pre-main sequence stars will be given.
Mois précédent Mois suivant Octobre 2013
Séminaire précédant Séminaire suivant 25ème séminaire : lundi 07 octobre, 15h45
Leptogenesis and neutrino masses
Enrico Nardi (INFN-Laboratori Nazionali di Frascati)

Leptogenesis is a class of scenarios in which the cosmic baryon asymmetry originates from an initial lepton asymmetry generated in the decays of heavy sterile neutrinos in the early Universe. I explain why leptogenesis is an appealing mechanism for baryogenesis, and I review its motivations and the basic ingredients. I will then address in some detail the connections between leptogenesis and the scale of the light neutrino masses generated via the seesaw mechanism. I conclude by discussing which future experimental results could provide circumstantial evidences for leptogenesis.
Séminaire précédant Séminaire suivant 26ème séminaire : jeudi 10 octobre, 15h45
Higgs monopoles in modified gravity
André Füzfa (Université de Namur)

When the Higgs field is nonminimally coupled to gravity, there exists a family of spherically symmetric particlelike solutions to the field equations. These monopoles are the only globally regular and asymptotically flat distributions with finite energy of the Higgs field around compact objects. Moreover, spontaneous scalarization is strongly amplified for specific values of their mass and compactness. The existence of such solutions can shed some light on the interplay between particle masses and gravitation.

In Higgs inflation, these monopoles could form and if they are not washed out by the exponential expansion, they could constitute a candidate for dark matter, with a mass range similar to the one of primordial black holes below the evaporation limit. However, as they also interact through their Higgs external field, the phenomenology is expected to be distinct from the one of black holes or other dark matter candidates.
Séminaire précédant Séminaire suivant 27ème séminaire : jeudi 17 octobre, 15h45
Taking extrasolar planet imaging to a new level with vector vortex coronagraphy: the ERC/ARC VORTEX project
Olivier Absil (AGO)

Vector vortex coronagraphs (VVC) are among the most promising solutions to directly image faint extrasolar planets by dimming the glare of their nearby host star. Manufacturing and efficiently operating such devices is however a challenging enterprise, especially in the thermal infrared regime where warm planets radiate most of their energy. For several years, we have been developing at the University of Liège a new class of VVC, called the Annular Groove Phase Mask (AGPM) coronagraph. Etched on a diamond substrate (in collaboration with the Uppsala University), this coronagraph can be operated at any wavelength, including the thermal infrared, thanks to the excellent transparency properties of diamond. We are now at a stage where the first components have been manufactured and tested. The aim of the ERC/ARC VORTEX project is three-fold. First, we will install and exploit the first generation of AGPM coronagraphs on large telescopes in world-leading observatories, putting a special emphasis on the development of optimal data analysis techniques that will unleash the full potential of the AGPM. Second, we aim at designing and manufacturing new AGPM coronagraphs for the next generation of imaging instruments, featuring improved performance. Finally, we will study, develop and test a ground-breaking concept that could improve very significantly the on-sky performance of VVCs in general. This concept is based on the quantum properties of light and in particular on the fact that an optical vortex induces an orbital angular momentum on the input starlight. In this seminar, I will describe the main goals of the VORTEX project and review the first results recently obtained at the VLT with our first AGPM coronagraph.
Séminaire précédant Séminaire suivant 28ème séminaire : mardi 22 octobre, 15h45
Le projet SPECULOOS : la recherche de planètes telluriques en transit autour des étoiles ultra-froides les plus proches
Michael Gillon (AGO)

Les 1000 étoiles ultra-froides (2700K et moins) les plus proches représentent une opportunité unique pour la recherche de vie au-delà de notre système solaire. En raison de leur faible luminosité, leur zone habitable est 30-100 fois plus proche que celle du Soleil, les périodes orbitales correspondantes allant de un à quelques jours seulement. Cette proximité rend les possibles transits de planètes habitables beaucoup plus probables et fréquents que pour des systèmes analogues au nôtre. La taille minuscule de ces étoiles (environ 1 rayon de Jupiter) conduit les transits de planètes de la taille de la Terre à être suffisamment profonds pour une détection depuis le sol. De plus, une planète habitable qui transiterait l'une des étoiles ultra-froides les plus proches serait propice à une caractérisation poussée de son atmosphère avec les télescopes géants en développement comme le JWST ou l'ELT, y compris la recherche de biosignatures. Toutes ces raisons nous ont motivés à mettre sur pied un survey photométrique optimisé pour la détection de planètes telluriques transitant les étoiles ultra-froides australes les plus proches. Le nom de ce projet est SPECULOOS, acronyme typiquement local de Search for habitable Planets EClipsing ULtra-cOOl Stars. Tout juste financé par la commission européenne au-travers d'un ERC Starting Grant, il sera basé sur deux télescopes robotiques de classe 1m qui opéreront à partir de 2015 depuis le désert de l'Atacama au Chili. Ce séminaire présente le concept de SPECULOOS, son statut, et quelques résultats de son prototype en opération depuis 2011.
Séminaire précédant Séminaire suivant 29ème séminaire : mardi 29 octobre, 15h45
La forme de la terre
Maurice Gabriel (AGO)

On connait généralement la mesure d’Eratosthène et éventuellement celle de Posidonius dans l’antiquité. On connait aussi les premiers travaux faits en France par Fernel et les grandes expéditions française en Laponie et au Pérou qui ont démontré l’aplatissement de la Terre. Mais on connait moins les travaux des Arabes et comment ils ont conduit Christophe Colomb à la conclusion que le voyage en Chine en navigant vers l’ouest était plus court. Aussi on sait moins que les mesures conduisaient à des valeurs incompatibles entre elles et avec les prédictions théoriques et l’histoire des longs efforts qui ont conduit à leur réconciliation presque parfaite qui ont demandé l’introduction du géoïde et de la notion d’équilibre isostatique. Nous nous proposons de parcourir rapidement les développements historiques de ces problèmes.
Séminaire précédant Séminaire suivant 30ème séminaire : mardi 29 octobre, 15h45
Dark matter realization of the Weinberg neutrino mass operator
Diego Restrepo (Instituto de Física - Universidad de Antioquia)

The large set of possible realizations of the Weinberg operators at one-loop level is filtered with the requirement to have at least one consistent dark matter candidate. The resulting models are presented as extensions of the several inert-scalar dark matter models.
Séminaire précédant Séminaire suivant 31ème séminaire : mercredi 30 octobre, 14h00
Icecube: The south pole neutrino observatory - a review
Athina Meli (U. Gent)

This seminar will be informal, pedagogical and informative about the status and results of the icecube and icetop experiments.
Mois précédent Mois suivant Novembre 2013
Séminaire précédant Séminaire suivant 32ème séminaire : jeudi 07 novembre, 15h45
Jupiter’s Dynamic Magnetosphere
Marissa Vogt (U. Leicester)

The gas giant Jupiter possesses a strong internal magnetic field that produces the largest magnetosphere in our solar system and leads to spectacular auroral emissions. Eight spacecraft have now visited the Jovian system, yielding a wealth of information about Jupiter’s rotation-dominated magnetosphere and its internal plasma source, the volcanically active moon Io. The Galileo spacecraft, which was in orbit around Jupiter from 1995 to 2003, has provided the most extensive spatial coverage of the magnetosphere and the opportunity to study temporal changes on long time scales. In this talk I discuss the dynamic nature of Jupiter’s magnetosphere and aurora, using both in situ spacecraft measurements and Earth-based telescope observations to illustrate changes on time scales from days to several years. I will focus on recent work in which we characterize long-term changes in Jupiter’s magnetic field configuration due to current sheet variability and will discuss the resulting implications for the aurora. Finally, I will consider the relative roles of the solar wind and internal factors, such as rotational stresses or changes in Io’s plasma production, in driving the many temporal variations in Jupiter’s magnetosphere.
Séminaire précédant Séminaire suivant 33ème séminaire : jeudi 07 novembre, 16h00
Analysis of small-b_T expansion for TMD PDF
Alexey Vladimirov (Department of Astronomy and Theoretical Physics, Lunds University, Sweden)

The factorization procedure for the transverse momentum dependent (TMD) processes involves two subsequent factorizations: factorization of hard-scale and b_T scale. I present the method to separate the scales of these factorization. I show that the complete two-scale structure, which is novel in the subject, allos one to significantly decrease the theoretical uncertainty, to obtain intuitive interpretation of Collins-Soper equations, and also to consider the higher terms of the small-b_T expansion in a self-consistent way. The main part of discussion will be on \phi^3_ model (scalar QCD), however, the QCD case will also be presented.
Séminaire précédant Séminaire suivant 34ème séminaire : jeudi 14 novembre, 15h45
Numerical modeling of Gravitational Microlensing processes
Elena Fedorova (AGO)

1. Approximate formulae for the amplification of an image of the source near the caustic of gravitational microlens;

2. Modelling the high amplification event's (HAE) lightcurves using these formulas;

The first two sections concern approximate solutions of the general lens equation near fold caustics and cusp points. Analytic expressions of the first-order corrections to the well-known linear caustic approximation are obtained. The results are applied to derive new corrections to total amplifications of the images in Q2237+0205 (OGLE photometry). Possibilities to distinguish different source models (Gaussian, power-law, and limb-darkening source models) in observations are discussed on the basis of statistical simulations of microlensed light curves.

3. Astrometrical and photometrical appearances of GM: simulations.

In the third part astrometric and photometric microlensing effects in various cases of extended sources and extended lenses are analyzed, including a simple model of microlensing by extended dark matter clumps.
Séminaire précédant Séminaire suivant 35ème séminaire : vendredi 22 novembre, 15h45
Constrained formulations of Einstein equations
Isabel Cordero-Carrión (Laboratoire Univers et Théories (LUTh), Observatory of Paris-Meudon)

Einstein equations written in a covariant way, as elegant as they are, are not very useful for some applications, in particular for numerical simulations of astrophysical scenarios. There are different ways to rewrite these equations in a more practical way, known as formalisms of Einstein equations. The so-called 3+1 formalism is based on a slicing of spacetime by spacelike hypersurfaces. There are different posibilities for how this slicing is done and the mathematical and numerical properties of the resulting system of partial differential equations can be very different. In addition, the way the equations are solved numerically leads to different formulations. I will try to give a basic introduction to this topic and briefly mention some of the projects I have been involved with.
Séminaire précédant Séminaire suivant 36ème séminaire : jeudi 28 novembre, 15h45
Charged particle spectra as a probe for small x parton dynamics and saturation
Anastasia Grebenyuk (ULB)

In the high energy limit the parton dynamics are driven by the growth of gluon densities at low momentum fraction (x). At such high parton densities gluon recombination processes become possible which lead to saturation effects. Such effects have been measured experimentally with the cross section of leading tracks as a function of the track transverse momentum in pp collisions. High parton densities on the other hand lead also to special dynamical effects, when the longitudinal momenta of the gluons are of similar size as their transverse momenta. This is studied with a measurement of pt and eta spectra of charged particles in ep collisions at HERA.
Mois précédent Mois suivant Décembre 2013
Séminaire précédant Séminaire suivant 37ème séminaire : jeudi 12 décembre, 15h45
Deux nombres remarquables
Pierre Renson (AGO)

L'exposé sera consacré à deux nombres qui jouissent de propriétés vraiment remarquables. Le premier de ces deux nombres est connu de chacun, scientifique ou non, mais il nous paraît intéressant de dresser un tableau de ses propriétés, même si les principales sont bien connues. Le second nombre, bien qu'il soit étudié depuis longtemps déjà, est moins connu; nous ferons un panorama de ses propriétés essentielles dont nous verrons l'intérêt.
Séminaire précédant 38ème séminaire : mercredi 18 décembre, 11h00 
Local Observables in a Landscape of Infrared Gauge Modes
Federico Urban (ULB)

Cosmological local observables are at best statistically determined by the fundamental theory describing inflation. When the scalar inflaton is coupled uniformly to a collection of subdominant massless gauge vectors, rotational invariance is obeyed locally. However, the statistical isotropy of fluctuations is spontaneously broken by gauge modes whose wavelength exceed our causal horizon. This leads to a landscape picture where primordial correlators depend on the position of the observer. We compute the stochastic corrections to the curvature power spectrum, show the existence of a new local observable (the shape of the quadrupole), and constrain the theory using Planck limits.
Université de Liège > Faculté des Sciences > Département d'Astrophysique, Géophysique et Océanographie : CoWebAGO, Juin 2009.