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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.: 04.366.9774
Fax: 04.366.9729
Recul rapide Année précédente Séminaires : Archives 2014 Année suivante  
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Mois précédent Mois suivant Janvier 2014
Séminaire suivant  1er séminaire : jeudi 09 janvier, 15h45
The core collapse supernovae and compact stellar remnants
Rupak Roy (AGO)

The core collapse supernovae are supposed to be the principal processes for the creation of compact objects like Neutron Stars, Magnetar or a stellar mass Black Hole. The nature of the explosions and the types of the compact objects are mainly dependent on the properties of the pre-SN stars. Multiwavelength observations of these events have revealed the diversity of these catastrophes. Here, I shall summarize the diversity in core collapse explosions along with their energetic and progenitor properties. The nature of the compact remnants produced by these explosive events will also be discussed.
Séminaire précédant Séminaire suivant 2ème séminaire : vendredi 10 janvier, 14h45
Search for νµ→ντ oscillation in the OPERA experiment
Valeria Troshina (Skobeltsyn Institute of Nuclear Physics (SINP MSU), Lomonosov State University)

OPERA (Oscillation Project with Emulsion-tRacking Apparatus) is a unique experiment on νµ→ντ oscillation search in appearance mode (by observing τ-leptons from ντ charge current interaction inside the detector). Nuclear emulsion is the main detector used in OPERA. It has a spatial resolution of about 1µm, so it becomes possible to observe the short-live τ-lepton. 5.79 events are expected during the 5 years of the experiment.

By now the data collection is completed, but OPERA experimentalists continue to analyze the data. They have already found 3 candidates to ντ interaction and have already published articles about a candidate event for νµ→νe oscillation and about non standard νµ→ντ oscillation searches involving more than 3 neutrino mass states.

The speaker will also discuss the reconstruction of the electromagnetic cascade primary particles parameters and the evaluation of the charm background contamination.
Séminaire précédant Séminaire suivant 3ème séminaire : jeudi 23 janvier, 14h45
E6SSM inspired 6-Higgs-doublet models
Venus Keus (University of Southampton)

N-Higgs-doublet models are amongst the simplest extensions of the Standard Model, motivated for instance by Supersymmetric scenarios. I will discuss various 6-Higgs-doublet models which are symmetric under different groups. These models, inspired by the E6 Supersymmetric Standard Model, contain viable dark matter candidates. I will describe some phenomenological aspects of these models, identifying signatures that can be observed at the LHC.
Séminaire précédant Séminaire suivant 4ème séminaire : mardi 28 janvier, 15h45
An observational path towards another Genesis
Amaury Triaud (MIT)

Recent exoplanet discoveries, and the detailed characterisation of some systems, have altered our perception of how we should search for life in the Universe. Instead of focusing on Earth analogs orbiting solar analogs, we now have a wide range of systems that we can consider with the hope to study habitability well beyond earth-like conditions. I will present some of my work related on the study and exploration of exoplanet diversity and how that relates to the ultimate goal of the search for life in solar neighbourhood.
Mois précédent Mois suivant Février 2014
Séminaire précédant Séminaire suivant 5ème séminaire : jeudi 06 février, 15h45
A No-Go Theorem in Multi-Higgs Flavour Models
Igor Ivanov (AGO)

Explaining the observed mass and mixing patterns of fermions as a natural consequence of some discrete flavour group will be very attractive resolution of the flavour puzzle. Many models based on various symmetry groups in multi-Higgs-doublet models try to pursue this idea. Here, I will present a no-go theorem, which states that the only way of obtaining a physical CKM mixing matrix and, simultaneously, non-degenerate and non-zero quark masses is requiring the vacuum expectation values of the Higgs fields to break completely the flavour group. Together with some recent results on the Higgs potential in multi-doublet models, this theorem strongly limits the list of symmetry groups compatible with the physical CKM matrix.
Séminaire précédant Séminaire suivant 6ème séminaire : jeudi 13 février, 11h15
Joint Colloquium :

The climate risks
Valérie Masson-Delmotte (LSCE, CEA-Saclay)

This presentation will focus on the key points of the fifth report of the Group I of the Intergovernmental Panel on Climate Change (IPCC-GIEC in French). I will present the main aspects of current climate change through the synthesis of observations, and the main results concerning the role of human activities in climate change in recent decades. Global warming unequivocally is marked by trends in air and ocean temperatures, by melting ice, and is accompanied by changes in the hydrological cycle, by an increase in heat waves, by heavy precipitation events and by an increase in mean sea level. Human influence is detected in many of these changes, and today it is extremely likely that it has been the main cause of the warming observed since the mid-20th century and of its various consequences on the climate system. Climate modeling indicates that new greenhouse gas emissions will imply a continuation of the warming affecting all components of the climate system. One of the major results of this fifth report is the close relation between total cumulative emissions of carbon dioxide and the temperature change at the surface of the Earth in the late 21st century and beyond. Finally, simulations of future carbon cycle and of climate indicate that most of the characteristics of climate change will persist for many centuries even if emissions of carbon dioxide are stopped.
Mois précédent Mois suivant Mars 2014
Séminaire précédant Séminaire suivant 7ème séminaire : mercredi 12 mars, 15h45
Milli-interacting dark matter interpretation of the direct-search experiments
Quentin Wallemacq (AGO)

I reinterpret the results of the direct searches for dark matter in terms of milli-interacting dark particles. The model reproduces the positive results from DAMA/LIBRA and CoGeNT and is consistent with the absence of signal in the XENON100, CDMS-II/Ge and LUX detectors. Dark atoms, interacting with standard atoms through a kinetic mixing between photons and dark photons and a mass mixing of <\sigma> mesons with dark scalars, diffuse elastically in terrestrial matter where they deposit all their energy. Reaching underground detectors through gravity at thermal energies, they form bound states with nuclei of the active medium by radiative capture, which causes the emission of photons that produce the observed signals. The parameter space of the model is explored and regions reproducing the results at the 2<\sigma> level are obtained for each experiment.
Séminaire précédant Séminaire suivant 8ème séminaire : jeudi 20 mars, 15h45
Cirrus heterogeneity effects on brightness temperatures and on cloud optical properties retrieved from space infrared radiometry.
Thomas Fauchez (Laboratoire d'Optique Atmosphérique (LOA), Université Lille 1)

In a global change context, cloud radiative forcing is one of the main uncertainties, in particular ice clouds (cirrus), having a strong impact on the greenhouse effect. It is, thus, important to know their properties. The satellite is a crucial tool in order to study clouds, allowing an almost continuously global coverage. Current retrieval algorithms of cloud properties assume that the observation pixel is homogeneous (Plan Parallel Approximation, PPA) and independant of the other (Independant Pixel Approximation, IPA). However, clouds are, in reality, complex and heterogeneous structures. These approximations may, thus, lead to errors on the retrieved cloud optical properties.

The objective of this PhD thesis was to estimate the impact of cirrus cloud heterogeneities on top of atmosphere brightness temperatures and on cloud optical properties retrieved from space infrared radiometry. To solve this question, we developped and used different modelling tools. Firstly, we adapted the tridimensionnal cloud generator 3DCLOUD to generate cirrus fields. This model allows us to generate clouds in a tridimensionnal atmosphere using simplifications of thermodynamics and dynamics equations as well as a Fourier framework to control scale invariant properties of clouds. Numerous cirrus clouds were generated, one of them was built according to measurements of the cirrus observed during the airborne campaign CIRCLE-2. Secondly, 3DCLOUD was coupled to the radiative transfer code 3DMCPOL, extended, during this PhD thesis, to the 3D radiative transfer simulation in the thermal infrared. This code is based on a Monte-Carlo algorithm and allows us to simulate, in a realistic way, radiative quantities such as radiances and brightness temperatures. The impact of cirrus heterogeneities have been studied in the observation conditions of the Imaging Infrared Radiometer (IIR) on-board the CALIPSO satellite for three thermal infrared channels (8.65 m, 10.60 um and 12.05 um) and for nadir observations at the spatial résolution of 1 km x 1 km.

We have shown that, at the IIR spatial resolution, heterogeneity effects are mainly dominated by the PPA bias, the IPA bias having a neglectful impact because photon horizontal transport is almost zero, in the thermal infrared, at this scale. Heterogeneity effects mainly depend on the standard deviation of the optical thickness inside the observation pixel and of the brightness temperature contrast beween the cloud top and the clear sky atmosphere. The bias becomes larger than the IIR instrumental accuracy of 1 K from an optical thickness of approximately 0.3, at 532 nm, corresponding to optically thick cirrus clouds. The heterogeneity effects on the retrieved optical properties can reach more than 50% on the effective diameter of ice crystals and by up to 20% on the cloud optical thickness. These effects are much more important that others possible errors and they would have to be considered in order to improve the cloud property retrieval.
Séminaire précédant Séminaire suivant 9ème séminaire : mardi 25 mars, 15h45
Binary system characterization with FIRST (Fibered Imager foR a Single Telescope)
Elsa Huby (ULg (AEOS/HOLOLAB))

My work during my Ph.D. has been carried out in the context of imaging at high angular resolution and high dynamic range, and in particular on the development of the instrument called FIRST, Fibered Imager foR a Single Telescope. Its principle relies on the novel technique of pupil remapping, combining aperture masking and spatial filtering of the wavefront thanks to single-mode fibers. The purpose of my thesis was to mount FIRST on a telescope, improve its performances and develop a pipeline for data reduction and analysis. First light of the instrument was obtained in July 2010 on the 3m Shane telescope at Lick Observatory. Afterwards, I have been working on different aspects of the optical and mechanical design, in order to improve its performances during observations. We have then conducted an observation campaign consisting of binary systems, which are ideal objects to assess the resolving power and the dynamic range of the instrument. For that purpose, I have developed a data reduction pipeline, allowing the estimation of interferometric observables from the fringe images and performing a fit of a binary model. I have obtained first results on the binary system Capella, which validate the concept of pupil remapping on-sky and demonstrate that this technique allows the recovery of the capabilities of a telescope at the diffraction limit and at visible wavelengths. Although the sensitivity of FIRST is still limited, these results are highly promising regarding observations to come at the Subaru telescope and more generally regarding future developments based on this technique, in particular in the context of exoplanet detection and characterization.
Séminaire précédant Séminaire suivant 10ème séminaire : jeudi 27 mars, 15h45
Optically bright Post-AGB stars in the Magellanic Clouds:
Candidate Selection, Spectral Energy Distributions and Spectroscopic Examination

Kamath Devika (KU Leuven)

Post-AGB stars are low- to intermediate-mass stars in a rapid transition from the AGB phase to the Planetary Nebula (PN) phase. Post-AGB stars bear signatures of the structural and chemical composition changes that occur during the AGB phase of evolution and therefore can be used to constrain AGB models and provide insight to the formation of PNes. In the Galaxy, the luminosities (and hence initial masses) of the diverse group of post-AGB candidates are badly affected by their unknown distances, making it difficult to use the observational characteristics of these interesting objects to study the poorly understood late stages of stellar evolution. In this talk, I will present the new results of a systematic search for post-AGB candidates in the Magellanic Clouds which became possible after the release of deep infrared surveys such as mid-infrared LMC and SMC Spitzer surveys. The catalog of post-AGB candidates in the Magellanic Clouds has been created by selecting candidates based on the existence of a mid-infrared excess and furthermore by obtaining low-resolution optical spectra. The optical spectra and broadband photometry from the optical to mid-infrared were used to derive luminosities, effective temperatures and masses for the post-AGB candidates. Using a combination of colour criteria and SED analysis, we were able to classify between single and binary post-AGB objects. We find that variability is displayed by several of the post-AGB candidates with the most common variability types being the Population II Cepheids(including RV-Tauri stars) and semi-regular variables. From the numbers of post-AGB candidates in the SMC and LMC, we were able to estimate evolutionary rates for the transient post-AGB phase. These catalogs of spectroscopically verified post- AGB candidates are expected to be a valuable resource for the study of the late stages of stellar evolution as a function of initial mass and metallicity because of the known distances to the Magellanic Clouds.
Mois précédent Mois suivant Avril 2014
Séminaire précédant Séminaire suivant 11ème séminaire : mercredi 02 avril, 15h45
MAVEN and Mars Express: Past, Present & Future Study of Mars Atmospheric Escape
Pr. Nicholas Schneider (LASP, University of Colorado)

NASA’s MAVEN mission, arriving at Mars in September, carries a powerful Imaging UV spectrograph (IUVS) to study Mars' upper atmosphere and the history of atmospheric loss. IUVS builds on the successes of the Mars Express SPICAM instrument, and adds capabilities for D/H isotopic ratio measurements, scanning and imaging capability, and enhanced scattered light rejection. The instrument will operate nearly continuously in five observing modes, generating thousands of spectral images daily. All data will be pipeline-processed into physical quantities such as density and temperature and archived on NASA’s Planetary Data System for use by the international Mars community. In addition to the overview of MAVEN and IUVS, I’ll report recent results by our group showing that the escape of hydrogen from Mars is surprisingly variable.
Séminaire précédant Séminaire suivant 12ème séminaire : lundi 07 avril, 15h45
Quasi-renormalisable quantum field theories, pseudofactorials and all that...
Maxim Polyakov (Ruhr-Universität Bochum)

Quasi-renormalisable quantum field theories, pseudofactorials and all that...
Séminaire précédant Séminaire suivant 13ème séminaire : jeudi 10 avril, 15h45
Using effective Lagrangians with gauge bosons to model 0nu2beta and neutrino masses
Alberto Aparici (IFIC Valencia)

The relationship between neutrinoless double beta decay (0nu2beta) and neutrino masses is a deep one, as it has been well known since the Schechter-Valle theorem was stated. Effective field theory is a well-suited tool to investigate this relationship, for the new physics responsible for lepton number violation must be much heavier than both the neutrinos and the typical energies of 0nu2beta. In this talk I will describe a family of effective operators that violate lepton number involving gauge and Higgs bosons but without the participation of quarks. These operators had been mostly ignored in the literature, but they can lead to the suppression of neutrino masses by one or two loops respect to 0nu2beta. As such, these effective interactions might produce a signal in current or forthcoming 0nu2beta experiments without the need for the neutrino masses to be in the degenerate regime. I will show also that these operators can be isolated from other LNV-ing sources by appropriately selecting the chirality of the involved leptons, which happens to be a useful feature when designing models that may underlie the effective interactions. I will describe one such models in which the new particles can be as light as the TeV scale and still provide a signal in 0nu2beta searches and small neutrino masses. In this model the neutrino mass matrix presents strong hierarchies that require the theta_13 angle to be nonzero, in a range compatible with the measurements of Daya Bay, RENO and Double Chooz.
Séminaire précédant Séminaire suivant 14ème séminaire : jeudi 24 avril, 15h45
Histoire de la mesure de l’unité astronomique
Maurice Gabriel (AGO)

On retracera l’histoire des mesures de l’unité astronomique depuis l’antiquité jusqu’à la nouvelle définition adoptée par l’UAI lors de l’assemblée générale de Pékin. La première mesure est évidemment celle d’Aristarque de Samos. Sa méthode sera encore utilisée au 17ème siècle. Mais elle sera abandonnée au profit des mesures de la parallaxe de Mars. Nous verrons ce qui motivera les Français à organiser l’expédition à Cayenne (un point généralement passé sous silence). Ensuite, Mars sera délaissée au profit des passages de Vénus. Nous verrons ce qui permettra d’améliorer la précision de l’observation de ces deux planètes et ce qui la limitera. Elles seront délaissées au 20ème siècle en faveur des passages périhéliques d’Eros. Nous verrons que d’autres méthodes ont aussi été utilisées et qu’elles ont parfois fournis des résultats plus précis. Finalement elles seront toutes abandonnées en faveur des mesures par radar de la distance de Vénus et cela jusqu’à la décision de 2012.
Mois précédent Mois suivant Mai 2014
Séminaire précédant Séminaire suivant 15ème séminaire : jeudi 08 mai, 16h00
Joint Colloquium :

IceCube and the Discovery of High-Energy Cosmic Neutrinos
Francis Halzen (Wisconsin IceCube Particle Astrophysics Center and Department of Physics, University of Wisconsin, Madison)

The IceCube project has transformed one cubic kilometer of natural Antarctic ice into a neutrino detector. The instrument detects 100,000 neutrinos per year in the GeV to PeV energy range. Among those, we have recently isolated a flux of high-energy cosmic neutrinos. I will discuss the instrument, the analysis of the data, and the significance of the discovery of cosmic neutrinos.
Séminaire précédant Séminaire suivant 16ème séminaire : jeudi 15 mai, 15h45
Athena, ESA's next mission to study the Hot and Energetic Universe
Gregor Rauw (AGO)

In November 2013, the European Space Agency (ESA) has chosen the science theme for its second large mission in the framework of the Cosmic Vision plan. The theme that was selected, among more than 50 proposals, is "The Hot and Energetic Universe". To implement this theme, the X-ray astrophysics community is proposing a mission concept called Athena which will provide the most sensitive X-ray observatory ever flown. In this seminar talk, I will briefly review the motivation to observe the sky in the X-ray domain and describe the drivers of the hot and energetic Universe science theme. I will then present the design of the Athena project and highlight some of its unique capabilities.
Séminaire précédant Séminaire suivant 17ème séminaire : mardi 20 mai, 15h45
Mathematical and phenomenological aspects of Generalized Parton Distributions
Dieter Müller (Institute for Theoretical Physics II, Ruhr-Universität Bochum, Germany)

Generalized Parton Distributions (GPDs) are considered as a tool to resolve the partonic content of hadrons, in particular the nucleon, and nuclei in three dimensions and to decompose the nucleon spin in terms of the fundamental degrees of freedom. In the last decade enormous experimental efforts have been spend to measure exclusive processes in the deeply virtual regime. On the other hand GPDs are intricate functions that possesses specific mathematical properties. In this talk some mathematical aspects of GPD representations are discussed and it is explained how they are used in the phenomenological description of experimental measurements.
Mois précédent Mois suivant Juin 2014
Séminaire précédant Séminaire suivant 18ème séminaire : jeudi 05 juin, 15h45
Geometrical CP violation with a complete fermion sector
Ivo De Medeiros Varzielas (University of Basel, Switzerland)

I consider the concept of geometrical CP violation focusing on Delta(27) models. I discuss the scalar potential responsible for the spontaneous CP violation with calculable phases and give details on the issues that arise when introducing fermions compatible with geometrical CP violation. The minimal model with viable quark masses will be introduced as well as structures that lead to the observed lepton masses and mixing, which combined provide an existence proof of geometrical CP violation models accounting for the masses and mixing of all fermions.
Séminaire précédant Séminaire suivant 19ème séminaire : mardi 10 juin, 15h45
Classically Scale Invariant Extensions of the Standard Model
Gunnar Ro (Institute for Particle Physics Phenomenology, Durham University, UK)

Classically Scale Invariant Extensions of the Standard Model, where the weak scale is generated via the Coleman-Weinberg mechanism, have been proposed to address the Standard Model naturalness problem. I will show how these models include viable dark matter candidates, can generate the observed matter anti-matter asymmetry of the universe and improve the stability of the Higgs potential.
Séminaire précédant Séminaire suivant 20ème séminaire : vendredi 13 juin, 14h00
Towards 3D-Mapping of Exoplanet Atmospheres
Julien de Wit (MIT)

Within two decades, the field of exoplanetology has broadened our perspective on planetary systems with more than 1800 planets found in over 1100 systems different from ours. Research priorities are now moving from planet detection to planet characterization. During this seminar, I introduce a method to map exoplanet atmospheres—while their host stars cannot be resolved. I discuss the degeneracies behind exoplanet mapping and present a framework to mitigate consistently their effects. I introduce two applications to the hot Jupiters HD189733b and Kepler-7b in the infrared and in the visible, respectively. Finally, I discuss the potential of the method to yield constraints on the 3D temperature, composition, and circulation patterns of a planet’s atmosphere in the near future. Before wrapping up, I briefly introduce a second method, MassSpec, that aims to determine the atmospheric properties of transiting exoplanets and their masses solely from their transmission spectra—their star’s light filtered by their atmospheres during transits. Within the next decade, MassSpec will be applicable to exoplanets as small as Earth, and hence will play a pivotal role in the identification of the first habitable exoplanet.
Séminaire précédant Séminaire suivant 21ème séminaire : jeudi 19 juin, 15h45
Inflation after Planck and BICEP2
Sébastien Clesse (Université de Namur)

After an introduction on inflation (theoretical motivations, dynamics, mechanism to generate the primordial density perturbations, observational predictions), I will examine the consequences for inflation models of the Planck measurements of the Cosmic Microwave Background (CMB) temperature anisotropies and of the observation by BICEP2 of a B-mode polarization in the CMB on large scales.
Séminaire précédant Séminaire suivant 22ème séminaire : vendredi 20 juin, 15h45
Partonic Orbital Angular Momentum
Abha Rajan (University of Virginia)

With the EMC experiment in the late 1980s which showed the quark spin to have a very small contribution to the proton spin, the hunt has been on for the source of the 'missing spin'. I focus on the angular momentum contribution generated by the motion of quarks and gluons inside the proton. The Generalised Parton Distributions play a key role in our understanding of this study. By making a connection to helicity amplitudes we can have a physical motivation for the picture described by the GPDs. Most importantly, it helps us understand how we can connect orbital angular momentum to an observable that can be measured experimentally.
Mois précédent Mois suivant Août 2014
Séminaire précédant Séminaire suivant 23ème séminaire : jeudi 28 août, 15h45
Minimal anomaly-free chiral fermion sets beyond the standard model and gauge coupling unification
Catarina Simoes (Centro de Física Teórica de Partículas (CFTP), Instituto Superior Técnico, Lisbon)

We look for minimal chiral sets of fermions beyond the Standard Model that are anomaly-free and, simultaneously, vector-like particles with respect to colour SU(3) and electromagnetic U(1). We then study whether the addition of such particles to the Standard Model particle content allows for the unification of gauge couplings at a high energy scale, above 5.0 × 1015 GeV so as to be safely consistent with proton decay bounds. The possibility to have unification at the string scale is also considered. Inspired in grand unified theories, we also search for minimal chiral fermion sets that belong to SU(5) multiplets. Restricting to representations up to dimension 50, we show that some of these sets can lead to gauge unification at the GUT and/or string scales.
Mois précédent Mois suivant Septembre 2014
Séminaire précédant Séminaire suivant 24ème séminaire : mardi 16 septembre, 15h45
What does gravity do with axions?
Sacha Davidson (IPN Lyon)

The QCD axion is a curious Dark Matter candidate, having a mass like the neutrino, but behaving as Cold Dark Matter. I will review how this occurs, and discuss the interesting question of whether WIMPs could be distinguished from axions with Large Scale Structure data. Sadly I do not have an answer, which would require simulating galaxy formation in the presence of classical field dark matter.
Mois précédent Mois suivant Octobre 2014
Séminaire précédant Séminaire suivant 25ème séminaire : jeudi 02 octobre, 15h45
Frequency and Continuous Time-Frequency Analysis under Unevenly Sampled Time Series
Guillaume Lenoir (UCL)

Spectral analysis is a common approach for the interpretation of astronomical or climate time series. In particular, the discrete Fourier transform, the Gabor transform and the continuous wavelet transform are widely used. The discretization procedure is traditionally done on a regular grid, however, numerous data are only available with unevenly spaced time steps. Although a pragmatic solution may be to interpolate the data on a regular grid, this process may significantly affect the analysis. An alternative approach is to use the Lomb-Scargle periodogram to estimate the Fourier spectrum, but it has two main limitations:

- The spectrum of classical stochastic processes needs to be estimated by Monte-Carlo simulations, which may be time expensive.

- There is no rigorous theoretical equivalent for the continuous time-frequency transforms.

The objective here is to generalize the current framework for the analysis of unevenly sampled time series. Specifically, we extend the Lomb-Scargle periodogram to some continuous time-frequency transforms. Then, we estimate the analytical spectrum of some stochastic processes under those transforms. The latter offers a significant gain on the computing cost associated with Monte-Carlo simulations.

Based on the previous results, we propose a rigorous framework to estimate the significance of wavelet spectrum given unevenly sampled data. We proceed as follows:

- Define the model: Significance testing is always based on a model, that needs to be properly defined.

- Define the parametric hypotheses H0 (null case) and H1 (alternative case), and attempt to find an appropriate statistical summary.

- Consider the correlations between neighbouring coefficients in the continuous transform.

- Reject some parts of the transform due to the Shannon-Nyquist Sampling Theorem.

Some examples of astronomical and paleoclimate time series analysis are given. Finally, we investigate whether considering the interpolated data to get a constant time step and using the traditional tools may lead to significant errors.
Séminaire précédant Séminaire suivant 26ème séminaire : mercredi 08 octobre, 16h00
Joint Colloquium :

Recent Developments in Micromagnetism
Riccardo Hertel (Institut de physique et chimie des matériaux de Strasbourg (IPCMS), Université de Strasbourg)

The properties of ferromagnetic materials can depend strongly on the length scales that are involved. Modern concepts of magnetic data storage devices aim at exploiting the unique magnetic features unfolding on the sub-micron scale. Nanosized shift-registers are designed where magnetic domain walls act as units of information that can be written and displaced along nanowires. From a physical perspective, the sub-micron length scale is particularly complex due to inhomogeneous fundamental magnetic structures, like magnetic vortices or domain walls. Many simplifying assumptions commonly used in condensed matter physics, such as translational invariance or periodic boundary conditions, cannot be applied to study these systems in which finite-size effects are crucial and where domain walls are about as large as the sample size. The theory of micromagnetism provides the theoretical framework to describe accurately the magnetization on the sub-micron scale. Excellent agreement with high-resolution experimental studies has repeatedly confirmed its reliability and predictive power. This continuum theory aims at calculating the vector field of the magnetization. Its basic equations include the equations of motion for the magnetization dynamics on the picoseconds time scale. Although the theory has been outlined more than fifty years ago, it took the advent of high-performance computers and advanced simulation methods to solve these equations reliably. A major technical leap in micromagnetic modelling was achieved a few years ago by exploiting the computational power of graphical processing units. Recent progress in computational nanomagnetism includes multiscale approaches bridging the gap between atomistic and continuum theory. This allows studying micromagnetic singularities – exotic micromagnetic structures that often govern magnetic switching processes. In this seminar I will emphasize the importance of simulations in this field of research and present a few theoretical predictions that await experimental verification.
Séminaire précédant Séminaire suivant 27ème séminaire : jeudi 09 octobre, 15h45
Neutrinos as a probe of new physics effects
Davide Meloni (Dipartimento di Matematica e Fisica, Universita di Roma Tre)

In the recent years neutrino oscillation experiments have provided a huge amount of data and the values of the neutrino mixing angles and mass differences are now known with high accuracy. In this seminar I show that, using the precise data from reactor and accelerator neutrino experiments, stringent bounds can be put on new physics models involving, for example, sterile neutrinos, extra dimensions and non-standard neutrino interactions.
Séminaire précédant Séminaire suivant 28ème séminaire : mardi 14 octobre, 16h00
Joint Colloquium :

Planetary system formation and evolution in the golden age of high contrast imaging
Dimitri Mawet (European Southern Observatory & Caltech)

In this talk, I first briefly review the approaches used so far to tackle open questions about the formation and evolution of extra-solar planetary systems: the 'top-down' brown dwarf and the 'bottom-up' disk studies, both bracketing exoplanet detection and characterization efforts. As examples of direct imaging applications, I present the status of our ambitious survey of Spitzer/WISE debris disk stars at Keck, VLT, and Palomar, and recent results on famous brown dwarf and disk-bearing systems using innovative high contrast imaging techniques, polarimetry, and data mining in the VLT and HST archives. Despite vigorous efforts, direct imaging has only scratched the surface of a huge parameter space, complementary to the very successful, yet indirect methods. With modern coronagraphic and wavefront control technologies on the ground and in space, facilities such as GPI, SPHERE, P3K, SCExAO, and later on JWST and WFIRST-AFTA, are poised to break through the current contrast and inner working angle (IWA) limits. However, first-generation ground-based instruments and the HST still have a lot of untapped potential that savvy hardware and data reduction upgrades can unleash. As a case in point, I present the development, deployment, and science results of small IWA vector vortex coronagraphs (VVC) at Palomar, VLT, and LBT, and the status of on-going Palomar and Keck upgrades, paving the way towards future (extremely) large ground and space-based on-axis and/or segmented telescopes. Finally, I discuss the results and implications of the successful 2-year VVC test campaign on the JPL high contrast imaging testbed performed within NASA's technology demonstration for exoplanet missions program.
Séminaire précédant Séminaire suivant 29ème séminaire : jeudi 16 octobre, 17h00
Joint Colloquium (with SRSL) :

L'univers mathématique de Georges Lemaître
Dominique Lambert (U. Namur)

Nous nous proposons de mettre en évidence l'originalité et la cohérence des travaux mathématiques que Georges Lemaître a menés, non seulement pour soutenir ses recherches en cosmologie, en mécanique céleste ou en théorie des rayons cosmiques, mais aussi "pour l'honneur de l'esprit humain"! Ceci nous permettra d'aborder certaines facettes moins connues, mais très fécondes du travail scientifique du fondateur de l'hypothèse de l'atome primitif.
Séminaire précédant Séminaire suivant 30ème séminaire : mercredi 22 octobre, 16h00
Joint Colloquium :

Spin currents: from cold-atom systems to magnetic insulators
Rembert Duine (Utrecht University)

Control of spin currents is important for applications in the field of spintronics, and is also fundamentally interesting as such currents, and their decay mechanisms, are different from charge currents. In this talk I will discuss different forms of spin currents, their fundamental aspects, and some of their applications. At the fundamental end of the spectrum, I will discuss spin currents in cold-atom systems and theoretical and experimental results on their relaxation via the so-called spin-drag mechanism. Other examples of spin currents are those carried by electrons in ferromagnetic metals. Using the interaction of these latter spin currents with the magnetization is an important avenue towards building novel magnetic memories. Finally, I will discuss magnetic insulators. Spin transport in these latter systems has very low dissipation which may eventually lead to spin superfluidity, the spin analogue of superconductivity.
Mois précédent Mois suivant Novembre 2014
Séminaire précédant 31ème séminaire : jeudi 27 novembre, 16h00 
Joint Colloquium :

GRAVITY: towards the central galactic black hole
Guy Perrin (Observatoire de Paris)

GRAVITY is one of the two second generation instruments built for the Very Large Telescope Interferometer. Its main driver is the observation of the supermassive compact object at the Galactic Center to investigate its nature and study gravitation in the strong field regime. GRAVITY will combine the four 8m-telescope of the VLT to reach an angular resolution of 4 mas (to get access to the closest stars orbiting the central object) and an astrometric accuracy of order 10 µas (matching the Schwarzschild radius). GRAVITY will carry out the ultimate empirical test to show whether or not the Galactic Center harbours a black hole of four million solar masses and will finally conclude on the nature of the near-infrared flares from Sgr A*. Measuring the innermost stellar orbits, GRAVITY may measure the fundamental parameters defining a black hole and test the no-hair theorem . If the current hot-spot interpretation of the near-infrared flares is correct, GRAVITY will directly determine the spacetime metric around the black hole and test the theory of general relativity in the presently unexplored strong field limit. GRAVITY will also be a general-purpose near-infrared interferometer and will address more classical science cases in stellar physics or extragalactic astronomy. The main characteristics of the instrument will be presented in the talk as well as some of the expected observations.
Université de Liège > Faculté des Sciences > Département d'Astrophysique, Géophysique et Océanographie : CoWebAGO, Juin 2009.