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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)
Belgique

Tel.: 04.366.9774
Fax: 04.366.9729
Recul rapide Année précédente Séminaires : Archives 2016 Année suivante  
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Mois précédent Mois suivant Janvier 2016
Séminaire suivant  1er séminaire : mercredi 13 janvier, 15h45
Variation de composition chimique à l'intérieur des étoiles : effet de la diffusion microscopique.
Olivier Richard (Université de Montpellier)

Grâce aux réactions nucléaires qui se produisent dans leurs régions internes, les étoiles sont le moteur de l’évolution chimique des galaxies. Dans ce contexte il est important de savoir que les abondances chimiques observées à la surface des étoiles ne sont pas toujours les abondances originelles. Les étoiles ne sont en équilibre gravitationnel et radiatif qu’en première approximation. L’effet de la gravité sur les ions du plasma stellaire dépend de leur masse individuelle et l’effet du rayonnement dépend des détails de leur structure atomique. L’ensemble crée un processus sélectif provoquant une migration des éléments vers le centre ou la surface suivant l’intensité respective des deux effets. Cette migration dépend entre autres de l’état d’ionisation, et le même élément peut migrer vers la surface dans certaines régions de l’étoile et vers le centre dans d’autres, ce qui conduit à des accumulations ou des diminutions d’abondances locales. Ce processus de diffusion est un phénomène fondamental qui se produit dans toutes les étoiles, et pas seulement dans le cas extrême des étoiles « chimiquement particulières ». Du fait de la présence de ces processus de transport dans les zones radiatives des étoiles les abondances de surface (celles qui sont déterminées à partir des observations) ne reflètent plus les abondances d’origine (celles du milieu interstellaire), et la structure interne ainsi que l’évolution peuvent en être fortement affectées. Des exemples illustrant les effets de la diffusion microscopique sur la structure interne et l’évolution des étoiles au travers du diagramme HR, allant des étoiles « chimiquement particulières » aux étoiles vieilles pauvres en métaux, seront présentés.
Séminaire précédant Séminaire suivant 2ème séminaire : lundi 18 janvier, 15h45
Active galactic nuclei observed through a set of infrared polarized eyes
Enrique Lopez Rodriguez (University of Texas at Austin)

The new generation of sub-arcsecond resolution infrared (IR) polarimetric instruments, CanariCam on the 10.4-m Gran Telescopio CANARIAS (GTC), and MMT-Pol on the 6.5-m MMT, have opened new windows to reveal the cores of active galactic nuclei (AGN). Using both instruments, we have been able to 1) discover a highly polarized synchrotron core in Cygnus A, 2) estimate the magnetic field strength and geometry of the torus of NGC 1068 and to characterize the torus as a hydromagnetical outflow wind, and 3) study the dust composition of the ionization cones of NGC 1068 and its interaction with the jet. This talk will introduce the polarimetric modes of CanariCam and MMT-Pol, what can be learnt from polarimetric observations, and the latest results on the study of AGN using IR polarimetric techniques.
Mois précédent Mois suivant Février 2016
Séminaire précédant Séminaire suivant 3ème séminaire : jeudi 11 février, 15h45
Planet formation mechanisms: constraints from directly imaged exoplanets
Maddalena Reggiani (STAR Institute)

In the past 20 years the discovery of over 2000 extrasolar planets and the variety of their properties have increasingly attracted astronomers to investigate planet formation mechanisms with both observations and theories. In this talk I will present how directly imaged exoplanets can help addressing such questions. First I will show the results of a statistical approach to the problem, which takes advantage of the outcomes of the largest directing imaging surveys to date. Then I will present the particular case of some exoplanets imaged at the early stages of their formation, where state of the art coronagraph (e.g. the annular groove phase mask (AGPM) vector vortex coronagraph) are crucial. This talk will highlight how both approaches are necessary and complementary, especially in the light of the advent of the next generation of ELTs.
Séminaire précédant Séminaire suivant 4ème séminaire : jeudi 18 février, 15h45
Les progrès de l'astérosismologie: de la physique de laboratoire aux modèles d'évolution stellaire
Sébastien Salmon (STAR Institute)

Avec l'avènement des grandes campagnes d'observation depuis le sol et des missions spatiales CoRoT et Kepler, la détection des pulsations que peuvent présenter les étoiles a connu des progrès significatifs. L'astérosismologie, qui consiste en l'interprétation de ces pulsations, nous a conduit devant de nouvelles énigmes dans notre compréhension des étoiles. Ainsi, l'étude des étoiles pulsantes de type β Cephei et sdB, mais également de notre propre Soleil (via l'héliosismologie) semblent pointer un défaut dans notre connaissance de l'opacité de la matière au sein des plasmas stellaires.

Pour pallier cette incertitude, des efforts ont été entrepris via la tentative de mesurer expérimentalement cette opacité, et par de nouveaux calculs numériques. La création de plasmas reproduisant les conditions stellaires nécessite l'emploi de grandes installations laser. Ces dernières sont peu nombreuses et dans les prochaines années, seules deux, le NIF aux Etats-Unis, et le laser MégaJoule en France, devraient permettre de produire des plasmas reproduisant les conditions de densité et température solaires d'intérêt. De plus, le calcul théorique des opacités nécessite un savoir-faire et des moyens de calcul numérique que seules quelques équipes possèdent actuellement. Lié à mon séjour post-doctoral au CEA, je ferai un état des lieux des expériences menées par mon équipe d'accueil, dont un projet sur le laser MégaJoule, et des premiers résultats que nous avons obtenus avec les nouvelles opacités théoriques -OPAS- calculées au CEA.

Dans un tout autre contexte, grâce aux observations accumulées sur plusieurs années par le satellite Kepler, des structures régulières jusqu'alors indétectables se sont fait jour au sein des spectres de pulsation des étoiles de type γ Dor. Il a été établi que ces régularités devraient présenter une information sur le profil de rotation interne et les processus de mélange au sein de ces étoiles. A cet effet, je vous présenterai les derniers diagnostiques sismiques portant sur ces aspects de la physique stellaire, que nous avons établis avec ma collègue R-M. Ouazzani d'un point de vue théorique pour ces étoiles.
Séminaire précédant Séminaire suivant 5ème séminaire : mercredi 24 février, 10h45
The geometry of the mass-loss process of evolved AGB stars
Claudia Paladini (ULB)

The mass-loss process from evolved stars is a key ingredient for our understanding of many fields of astrophysics, including stellar evolution and the chemical enrichment of the interstellar medium via stellar yields. How the mass loss affects the atmosphere of late-type stars, particularly asymptotic giant branch (AGB) stars, is still argument of discussion since the 70's. The standard model pictures the AGB stars being round with a spherically symmetric stellar wind. However this is in contrast with observations of post-AGB and planetary nebulae. Taking advantage of the recent results of the Herschel mass loss of Evolved StarS (MESS) program, we initiated a coordinated effort to study the same sample of AGB stars with different techniques. Our aim is to characterise the geometry of the mass-loss process in AGB stars at different spatial scales. In this contribution I will show our recent images of surface structures on the photosphere of AGBs obtained with VLTI/PIONIER. I will then move to the dust forming region in the mid-infrared presenting the results of our VLTI/MIDI Large program. I will conclude describing the link between the inner (5-10 stellar radii) and the outer spatial scales (>100 stellar radii) that will be covered by VLT/VISIR.
Séminaire précédant Séminaire suivant 6ème séminaire : jeudi 25 février, 15h45
The temperature and chronology of heavy-element nucleosynthesis in low-mass stars
Sophie Van Eck (ULB)

Asymptotic giant branch (AGB) stars are essential contributors to the heavy-element enrichment of the interstellar medium, because they are responsible for the production of roughly half of the elements heavier than iron and because they eject their envelope through strong stellar winds at the end of their lives. Extrinsic AGB stars are binaries, they are not located on the AGB but bear the unaltered signature of an ancient AGB nucleosynthesis that once took place in their companion star. Detailed abundance determinations in both AGB and extrinsic AGB stars allow to put strong new constrains on the s-process nucleosynthesis taking place in evolved stars. The s-process builds up heavier elements that are subsequently brought to the stellar surface by convection. Two neutron sources, activated at distinct temperatures, have been proposed: 13C and 22Ne. To explain the measured stellar abundances, stellar evolution models invoking the 13C neutron source (which operates at temperatures of about one hundred million kelvin) are favored. Isotopic ratios in primitive meteorites, however, reflecting nucleosynthesis in the previous generations of stars that contributed material to the Solar System, point to higher temperatures (more than three hundred million kelvin), requiring at least a late activation of 22Ne. We determine the s-process temperature directly in evolved low-mass giant stars, using zirconium and niobium abundances, independently of stellar evolution models. The radioactive pair 93Zr-93Nb used to estimate the s-process temperature also provides, together with the pair 99Tc-99Ru, chronometric information on the time elapsed since the start of the s-process.
Mois précédent Mois suivant Mars 2016
Séminaire précédant Séminaire suivant 7ème séminaire : jeudi 03 mars, 15h45
Instabilities induced by element accumulation and accretion of planetary matter in stars: Their internal structure and evolution revisited
Morgan Deal (Université de Montpellier)

Standard studies of the internal structure and evolution of stars, even when they include atomic diffusion, forget an important hydrodynamical process induced by local element accumulation or accretion of planetary matter, namely “fingering convection”. This is a double-diffusive instability similar to the “thermohaline convection” which occurs in the ocean. Computing this effect in stars needs precise computations of the radiative accelerations on each element, coupled with the induced hydodynamical instabilities. Some results will be showed in the case of element accumulation in A type stars and in the case of planetary matter accretion on main sequence stars and white dwarfs.
Séminaire précédant Séminaire suivant 8ème séminaire : mardi 08 mars, 15h45
Gauged Two Higgs Doublet Model and The LHC 750 GeV Diphoton Anomaly
Wei-Chih Huang (TU Dortmund)

A novel model embedding the two Higgs doublets in the popular two Higgs doublet models into a doublet of a non-abelian gauge group SU(2)H is presented. The Standard Model SU(2)L right-handed fermion singlets are paired up with new heavy fermions to form SU(2)H doublets, while SU(2)L left-handed fermion doublets are singlets under SU(2)H. Two of distinctive features of this anomaly-free model are: (1) Electroweak symmetry breaking is induced from spontaneous symmetry breaking of SU(2)H via its triplet vacuum expectation value; (2) One of the Higgs doublet can be inert, with its neutral component being a dark matter candidate as protected by the SU(2)H gauge symmetry and Lorentz invariance instead of an ad hoc Z2 symmetry. I will discuss the model implications on collider constraints, Higgs physics, bounds from the electroweak precision data and dark matter relic density. In addition, this model can account for the LHC 750 GeV diphoton anomaly with the help of the heavy fermions and an SU(2)H scalar doublet.
Mois précédent Mois suivant Avril 2016
Séminaire précédant Séminaire suivant 9ème séminaire : jeudi 14 avril, 15h45
An overview of the mid-infrared spectro-interferometer MATISSE: science, concept and current status
Alexis Matter (Laboratoire Lagrange, OCA)

MATISSE is the second generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances by opening new avenues in various fundamental research fields: studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L & M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 μm, exploring angular scales as small as 3 mas (L/M band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range between R~30 to ~5000. Here, I introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performances. I also present the potential of MATISSE for the study of the inner regions of protoplanetary disks, which is one of the main science cases that has driven the instrument design and motivated several VLTI upgrades (GRA4MAT & NAOMI). Finally, I discuss the current status of the instrument, which is entering its testing phase, and the foreseen schedule for the next two years that will lead to the first scientific exploitation in 2018.
Séminaire précédant Séminaire suivant 10ème séminaire : mercredi 27 avril, 15h15
Jets at the LHC, from Run I to Run II and beyond
Gregory Soyez (IPhT, Saclay)

"Jets" are proxies to the quarks and gluons produced in high-energy collisions. They are ubiquitous objects, used in the majority of the analyses at the LHC. I will first give a brief introduction of the concept of a jet and the basic framework for their reconstruction. I will then discuss two additional challenges that have to be faced at the LHC: pileup, where additional soft collisions contaminate the reconstruction of the hard collisions, and "boosted jets" where objects different from quarks and gluons can mimic jets. In both cases, I will present the situation for Run I of the LHC, argue that improvements are needed for Run II and beyond, and discuss possible ways to achieve these improvements.
Mois précédent Mois suivant Mai 2016
Séminaire précédant Séminaire suivant 11ème séminaire : jeudi 12 mai, 15h45
The peculiar fast-rotating star 51 Oph probed by VEGA/CHARA
Narges Jamialahmadi (School of Astronomy, Institute for Research in Fundamental Sciences (IPM), Tehran)

I wished to spatially resolve the photosphere and gaseous environment of 51 Oph, a peculiar star with a very high v sin i of 267km s−1 and an evolutionary status that remains unsettled. It has been classified by different authors as a Herbig, a β Pic, or a classical Be star. I used the VEGA visible beam combiner installed on the CHARA array that reaches a submilliarcsecond resolution. Observation were centered on the Hα emission line. I derived, for the first time, the extension and flattening of 51 Oph photosphere. I found a major axis of θeq = 8.08±0.70 R⊙ and a minor axis of θpol = 5.66±0.23 R⊙. This high photosphere distortion shows that the star is rotating close to its critical velocity. Finally, using spectro-interferometric measurements in the Hα line, I constrained the circumstellar environment geometry and kinematics and showed that the emission is produced in a 5.2±2 R⋆ disk in Keplerian rotation. From the visible point of view, 51 Oph presents all the features of a classical Be star: near critical-rotation and double-peaked Hα line in emission produced in a gaseous disk in Keplerian rotation. However, this does not explain the presence of dust as seen in the mid-infrared and millimeter spectra, and the evolutionary status of 51 Oph remains unsettled.
Séminaire précédant Séminaire suivant 12ème séminaire : jeudi 19 mai, 15h45
Exoplanet science with the LBTI and prospects with the VLTI
Denis Defrère (STAR Institute)

The Large Binocular Telescope Interferometer (LBTI) is a strategic instrument of the LBT designed for high-sensitivity, high-contrast, and high-resolution infrared (1.5-13 microns) imaging of nearby planetary systems. To carry out a wide range of high-spatial resolution observations, it features various single-pupil imaging modes (e.g., AO imaging, coronagraphy, integral field spectroscopy, non redundant aperture masking) and can combine the two AO-corrected 8.4-m apertures for Fizeau or nulling interferometry. In this talk, I present the two key science surveys and review the latest scientific highlights. I also describe recent instrumental milestones such as first-light images with the integral field spectrograph and record-setting interferometric nulling observations. I end this presentation by describing the preliminary instrumental concept of a possible high-contrast VLTI instrument operating in the thermal near-infrared (3-5 microns), a sweet spot to image and characterize young extra-solar planetary systems.
Mois précédent Mois suivant Juin 2016
Séminaire précédant Séminaire suivant 13ème séminaire : mardi 07 juin, 15h45
A l’écoute des pulsations de Saturne
Benjamin Palmaerts (Max Planck Institute for Solar System Research (Göttingen) & LPAP-ULg)

L’exploration des magnétosphères de Jupiter et Saturne par différentes sondes spatiales a révélé plusieurs phénomènes périodiques, dont certains ne sont pas liés à la période de rotation planétaire. En particulier, dans la magnétosphère de Saturne, plusieurs études ont mis en évidence des pulsations avec une périodicité d’environ une heure dans le flux de particules chargées, dans les émissions radio, dans l’intensité du champ magnétique, ou encore au niveau de la brillance des émissions aurorales. Sur base des mesures récoltées pendant plus de 10 ans par la sonde Cassini en orbite autour de Saturne, je présenterai les résultats de l’analyse des pulsations quasi-périodique d’une heure dans le flux d’électrons énergétiques (100 keV-10 MeV). L’étude des pulsations simultanées dans les émissions radio et le champ magnétique, ainsi que leur empreinte ionosphérique au niveau des émissions aurorales nous guident dans la compréhension des processus magnétosphériques qui leur donnent naissance.
Séminaire précédant Séminaire suivant 14ème séminaire : mardi 21 juin, 10h30
The evolution of interacting massive binaries: theory vs. recent observations
Nicki Mennekens (Vrije Universiteit Brussel)

First an overview is presented of the general scheme of massive binary evolution, as well as some interesting types of events they can be responsible for. Special attention is given to those aspects which are still shrouded in uncertainty and the parameters that describe them. This scheme is then applied to attempt the reconstruction of the evolution of some recently observed systems, such as past mass-exchange massive binaries, but also GW150914 At the same time, it is investigated whether these observations can help constrain still uncertain aspects of massive binary evolution.
Mois précédent Mois suivant Septembre 2016
Séminaire précédant Séminaire suivant 15ème séminaire : jeudi 15 septembre, 15h45
Extended gauge groups and the Standard Model fermions
Renato M. Fonseca (IFIC, University of Valencia)

The Standard Model is a chiral theory where three families of left and right handed fermions transform differently under the SU(3) x SU(2) x U(1) gauge group. One must preserve such SM chirality in models with an extended gauge group, and this turns out to be a constraint which is far from trivial to satisfy. In my talk, I will explain further this idea and explore its far reaching consequences for models based on simple gauge groups, as well as those based on the SU(3) x SU(3) x U(1) gauge group.
Mois précédent Mois suivant Octobre 2016
Séminaire précédant Séminaire suivant 16ème séminaire : jeudi 20 octobre, 15h45
New instrumentation and facilities at Paranal
Alain Smette (ESO)

Following the commissioning of KMOS, MUSE, SPHERE, and of the re-furbished VLTI laboratory, ESO will complete the implementation of its VLT/VLTI 2nd generation instruments in the near future with the commissioning of GRAVITY, ESPRESSO and MATISSE. In parallel, the installation of the Deformable Secondary Mirror on Yepun (UT4) - taking place as I speak - is the most critical element of the deployment of the Adaptive Optics Facility, which has already seen the installation of the 4-Lasers Guide Star Facility and the GRAAL, the adaptive optics module for HAWK-I; it will be completed by the installation of GALACSI, the adaptive optics module for MUSE. These modules will allow Ground Layer Adaptive Optics assisted observations for HAWK-I, and MUSE in Wide Field mode as well as Laser Topographic Adaptive Optics for MUSE in Narrow Field mode. A short description of these instruments and their scientific capabilities will be given.
Mois précédent Mois suivant Novembre 2016
Séminaire précédant Séminaire suivant 17ème séminaire : mardi 08 novembre, 15h30
High precision photometry of exoplanets using TROBAR Telescope
Kevin Alabarta Jativa (University of Valencia)

We present the results of a project which aim is to explore the possibility of carrying out physical studies of exoplanets by differential photometry, obtained with the TROBAR Telescope, located in the Aras de los Olmos Observatory. We analyze some photometric series using an algorithm designed to improve accuracy of photometric reduction. First we reduce the images taken by the telescope. Secondly we perform an astrometric calibration of the images, in order to remove errors introduced by the alignment of the images. Then we perform the aperture photometry of the principal star and the auxiliary stars and we create the 'mean comparison star'. Finally we perform the differential photometry with the principal star and the mean comparison star. With this process we obtain light curves and we check if their accuracy arrives to be on the order of a hundredth, from which they can be used to make physical studies.
Séminaire précédant Séminaire suivant 18ème séminaire : jeudi 10 novembre, 15h45
Asteroseismology of massive stars: high degree mode instability and macroturbulence
Mélanie Godart (STAR Institute)

Apart from the important impact of theoretical developments in asteroseismology , progress in this field has been commonly associated with the analysis of time-resolved observations. Recently, the so-called macroturbulent broadening has been proposed as a complementary and less expensive way – in terms of observational time – to investigate pulsations in massive stars. The physical origin of this feature, encountered in the line profiles, is not yet determined and one scenario is to relate the broadening to the presence of a dense frequency spectrum of pulsation modes. During this seminar, I will present an homogeneous prediction for the non-radial instability domains of massive stars for degree up to 20 and compare our pulsational stability analysis to the empirical results coming from the analysis of macroturbulent broadening in massive stars.
Séminaire précédant Séminaire suivant 19ème séminaire : jeudi 17 novembre, 15h45
Tracking the jet production and outbursts in the symbiotic object R Aqr
Joy Nichols (Harvard Smithsonian Center for Astrophysics)

The nearby interacting symbiotic binary system R Aqr is being investigated with a multi-year, multi-wavelength campaign from X-ray to radio. R Aqr, composed of a white dwarf accreting from the wind of the companion red giant, has a complex system of fossil shells and lobes from previous outbursts and collimated jet ejection, allowing us to trace the history of these events. In addition, evidence of very recent jet production implies current activity on a time scale of a few years, or even months. Analysis of R Aqr can lead us to a better understanding of all types of jet-producing objects, including AGN. Data from Chandra X-ray Observatory, XMM, HST, FUSE, GALEX, ground-based narrow band imagery, and radio are presented.
Séminaire précédant Séminaire suivant 20ème séminaire : jeudi 24 novembre, 15h45
ARGOS, the ground-layer adaptive optics facility at LBT: From commissioning to science operations
Gilles Orban de Xivry (STAR Institute)

ARGOS is the laser guide star facility of the Large Binocular Telescope (LBT). It implements a ground layer adaptive optics (AO) correction, delivering an improvement by a factor of 2 in FWHM over the 4'x4' field of view of both LUCI instruments, the two near-infrared imagers and multi-object spectrographs.

Installation of the ARGOS system started in 2013 and GLAO commissioning began in 2015. Many issues related to the design and implementation of such a complex AO system were solved in these years and now the system is approaching its science demonstration time.

In this talk, I will present the goals and overall design of ARGOS. I will then summarize the commissioning effort and the status of the project, reviewing critical aspects of the on-sky operations of such system. Finally, I will also present the early results of joint ARGOS and LUCI observations, discussing the performances and gains provided by ARGOS in term of scientific capabilities.
Mois précédent Mois suivant Décembre 2016
Séminaire précédant Séminaire suivant 21ème séminaire : jeudi 01 décembre, 15h45
On the high-energy IceCube neutrinos
Sergio Palomares Ruiz (Instituto de Física Corpuscular, University of Valencia)

The observation of the first high-energy neutrinos in the IceCube detector at the South Pole has signaled the beginning of neutrino astronomy. After four years of data taking, 53 neutrino events with energies between 20 TeV and 2 PeV have provided the first evidence for the existence of an extraterrestrial neutrino flux at more than 6 sigma. The discovery of this flux has motivated a large number of studies in the literature to unravel their origin, from different scenarios within standard cosmic-ray sources to more exotic possibilities. In this talk, I will describe the evolution of these data and their main features and I will present the results of statistical analyses of different scenarios.
Séminaire précédant 22ème séminaire : jeudi 15 décembre, 15h45 
Weakly magnetic fields in hot stars
Aurore Blazère (STAR Institute)

Magnetic fields play an important role in the evolution of hot stars (spectral type A, B and O) and contribute in important ways to the structure, dynamics and energetics of stellar atmospheres, envelopes and winds. However, the origin and even the basic properties of hot star magnetic fields are still poorly understood. Although the influence of magnetic fields on stellar evolution has been recognized for a long time, progress have been challenged due to a lack of observational constraints combined with the difficulty in modeling magnetohydrodynamics processes. I will present in the talk the result that I obtained during my PhD.
Université de Liège > Faculté des Sciences > Département d'Astrophysique, Géophysique et Océanographie : CoWebAGO, Juin 2009.