Séminaires à l'Observatoire en 2011/2012

Salle de Cours, Grande Coupole, 10h30
11 rue de l'Université
67000 STRASBOURG


Liste des Séminaires en 2011/2012

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Formation d'étoiles dans les galaxies chimiquement jeunes du Groupe Local

Vendredi 09 Septembre 2011, 10h30

Pierre GRATIER
IRAM, Grenoble

La variété de galaxies dans le Groupe Local rend possible l'étude du milieu interstellaire et de la formation d'étoiles dans des conditions différentes de celles trouvées dans la Voie Lactée, tout en conservant une grande résolution spatiale grâce à leur proximité. Je présenterai des résultats issus de la combinaison d'observations du milieu moléculaire, du milieu atomique et de l'émission des poussières dans l'infrarouge lointain de deux galaxies sous-métalliques du Groupe Local: M33 et NGC6822. Je mettrai en avant deux particularités de ces environnements : un facteur de conversion NH2/ICO plus élevé et une plus grande efficacité de formation d'étoile. Je présenterai également des résultats sur les propriétés des nuages moléculaires géants de ces galaxies : variation de la fonction de masse des GMC avec le rayon galactocentrique de M33 et classification des GMC par type de formation d'étoile.


X-ray and multiwavelength study of LINER 1s

Vendredi 23 Septembre 2011, 14h00

George YOUNES
Observatoire de Strasbourg

This talk focuses on the faint end of the active galactic nuclei (AGNs) luminosity function in the nearby universe, mainly populated by Low Ionization Nuclear Emission-line Regions (LINERs). LINERs are characterized by nuclear optical spectra dominated by emission lines from low-ionized species (e.g., O I). These LINERs exhibit bolometric luminosities at least two orders of magnitude lower than classical luminous AGNs (Seyferts and quasars). The accretion mechanism and radiative processes responsible for the bulk of energy from radio to X-rays in LINERs remain a riddle. Both a geometrically thin optically thick accretion disk, similar to the one believed to exist in AGNs, and a radiatively inefficient accretion flow (RIAF) have been invoked as source of power in LINERs. I will tackle the problem of accretion and radiative processes in a well defined optically selected sample of LINERs showing a definite detection of broad H(alpha) emission (LINER 1s). I will give the X-ray and multiwavelength properties of these LINER 1s and show how they compare to AGNs and to there less massive counter-parts, galactic black-hole X-ray binaries.


Un siècle de mesure de l'aplatissement solaire

Vendredi 30 Septembre 2011, 10h30

Jean-Pierre ROZELOT
OCA, Nice

Les mesures exactes de la forme du soleil ont une longue histoire qui plonge ses racines dès le 19eme siècle et qui se sont poursuivies dans la seconde moitié du XXeme siècle par des expériences ballon sur SDS (Yale-US) ou au sol notamment au Pic du Midi. Les observations sont cependant très délicates, tout comme celles faites sur SOHO ou RHESSI, deux missions spatiales, en raison de la grande précision recherchée. C'est pourquoi de nouveaux instruments embarqués (sur SDO par exemple) permettront de déterminer des paramètres globaux solaires (moments gravitationnels, luminosité) et il est espéré que ceux-ci donneront alors des résultats définitifs pour l'aplatissement solaire. Au cours de cette présentation, nous montrerons quelques aspects historiques de l'aplatissement solaire, nous passerons rapidement en revue les missions en cours (SDO, RHESSI) ou futures (DYNAMICCS), et surtout nous rappelons la physique de base sous-jacente. Les mesures d'aplatissement ont en effet des implications profondes concernant la détermination des orbites planétaires, la théorie de la relativité générale, la rotation du coeur et les effets de cisaillement (au niveau de la leptocline) par exemple. Nous terminerons en montrant comment ces résultats peuvent être transposés aux étoiles.


A New Window of Exploration in the Mass Spectrum: Strong Lensing by Galaxy Groups in the SL2S

Vendredi 7 Octobre 2011, 10h30

Marceau LIMOUSIN
OAMP, Marseille

The existence of strong lensing systems with Einstein radii covering the full mass spectrum, from ∼1-2" (produced by galaxy scale dark matter haloes) to larger than 10" (produced by galaxy cluster scale dark matter haloes) have long been predicted. Many lenses with Einstein radii around 1-2" and above 10" have been reported but very few in between. By looking automatically for gravitational arcs in the CFHTLS, we have build the Strong Lensing Legacy Survey (SL2S). Thanks to the unsurpassed combined depth, area and image quality of the CFHTLS, we have uncovered this new population of lenses, with Einstein radii between ∼3" and ∼8", which translates into a projected mass enclosed within this radius around 1013 M, i.e. characteristic of group scale dark matter haloes. This new population effectively bridges the gap between single galaxies and massive clusters, opening a new window of exploration in the mass spectrum. I will present the first representative sample of such lenses and the main results obtained so far.


The Ionised Gas of Early-Type Galaxies

Vendredi 14 Octobre 2011, 10h30

Marc SARZI
University of Hertfordshire, UK

Once considered as simple and rather dull stellar systems, early-type galaxies have recently received a lot of attention as new data have uncovered a rather more complex picture for their stellar structure and dynamics, their star-formation history and their gas content. In my talk I will focus on this last aspect, and show how the SAURON integral-field data have added to our understanding of the origin, the fate and the source of ionization of the gas in early-type galaxies.


The Center of the Galaxy and its Activities

Vendredi 21 Octobre 2011, 10h30

Farhad YUSEF-ZADEH
Northwestern University, USA

The processes occurring in the nuclear disk of our own Galaxy are interesting because this region can potentially provide a template for the study of more distant galactic nuclei. There are several sources of activity that take place in this unique region of the Galaxy. One is the underluminous supermassive black hole (Sgr A*) at the dynamical center of the Galaxy. Sgr A* exhibits flares which occur a few times a day, arising from the inner ten Schwarzschild radii of a 4 million solar mass black hole. The other is the nuclear stellar cluster which consists of a population of evolved stars as well as a young population of stars in a disk orbiting within 0.5 pc Sgr A*. On a larger scale, another source of high energy activity is evidenced by the distribution of 6.4 keV line emission from neutral iron tracing dense, cold giant molecular clouds in the inner 200pc of the Galaxy. In this talk, I will give an overview of we have learned from studying the center of our Galaxy, namely, flares, massive star formation in a steep potential an (time permitting) X-ray emitting molecular clouds.


GOODS-Herschel : à la recherche du mode dominant de croissance des galaxies

Vendredi 28 Octobre 2011, 10h30

David ELBAZ
CEA, Saclay

Les relevés profonds du satellite Herschel offrent un accès unique à la mesure la plus fiable du taux de formation d'étoiles des galaxies distantes. En combinant les observations dans l'infrarouge lointain avec une panoplie d'autres longueurs d'ondes, il devient possible d'étudier l'histoire de la formation des galaxies sur 80 pourcents de l'histoire de l'univers. Si la formation stellaire est un processus mettant en jeu une physique complexe (turbulence, gaz multiphase, champs magnétiques, chocs, fusions/effets d'environnement, etc.), une convergence d'observations récentes démontre qu'une fois intégrée à l'échelle des galaxies, elle semble respecter des lois d'échelles relativement simples. Il résulte de ce comportement paradoxal une remise en question du rôle de certains de ces paramètres supposés jouer un rôle de premier plan par le passé, comme celui des interactions de galaxies.


The HALOGAS survey: looking for cold gas accretion onto spiral galaxies

Vendredi 18 Novembre 2011, 10h30

Gianfranco GENTILE
Université de Gand, Belgique

I will present the first results of the the HALOGAS (Westerbork Hydrogen Accretion in LOcal GAlaxieS survey, which uses very deep neutral hydrogen (HI) data to investigate the global characteristics of cold gas accretion onto spiral galaxies in the local Universe. The HALOGAS survey is the deepest HI survey of a sizeable sample of spiral galaxies. The HI data are analysed by modelling the observed data cube: I will show the outcome of the models of some of the HALOGAS galaxies, revealing extraplanar gas in various amounts.


Spiral galaxies : high redshift progenitors and formation histories

Vendredi 25 Novembre 2011, 10h30

Marie MARTIG
Swinburne University of Technology, Australie

I present a suite of 33 cosmological re-simulations following the formation and evolution of Milky Way-mass isolated galaxies from z=5 to z=0. Our sample at z=0 comprises galaxies with a broad range of Hubble types, from nearly bulgeless to bulge-dominated galaxies. This talk is mainly focused on spiral galaxies, and what are their progenitors and formation histories. An important result is that we do not find any correlation between the morphology at z=1 and at z = 0, with a whole range of possibilities for the z=1 progenitors of spiral galaxies (interacting galaxies, bulge-dominated systems, pure disks, unstable disks ...). However, we find a correlation between the bulge fraction at z=0 and the mass ratio of the largest merger undergone after z=2, as well as a correlation with the gas accretion rate at z > 1. We find that the most disk-dominated galaxies both have an extremely quiet merger history (with in most cases only minor mergers after z=2) and an extremely quiet gas accretion history (gas accreted at a low and constant rate, with an angular momentum always in the same direction). By contrast, more violent merger or gas accretion histories give birth to galaxies with more prominent bulges. We find that the galaxies with the highest bulge Sersic index at z=0 (generally referred to as ``classical bulges") are not those with mergers but those with intense gas accretion at z=1 and either early bar formation or other disk instabilities.


Gaia: aujourd'hui, bientôt, dans 10 ans

Vendredi 2 Décembre 2011, 10h30

Dimitri POURBAIX
Université Libre de Bruxelles, Belgique

A l'heure ou l'assemblage de Gaia, mission astrometrique de l'ESA, va bon train, alors que certains mettent la derniere touche a leurs codes de traitement, d'autres reflechissent deja a comment donner acces a la communaute a cette quantite gigantesque de donnees et d'autres a ce qu'ils pourront en tirer sur le plan scientifique. Je presenterai l'etat d'avancement de la construction tant du satellite que de l'infrastructure dans laquelle les observations seront traitees, sans oublier les possibilites offertes a l'ensemble de la communaute scientifique pour exprimer la forme des ses futures requetes. Un possible calendrier de mise a disposition publique des resultats sera egalement evoque.


Highly Ionized Emitters in Active Galactic Nuclei

Vendredi 9 Décembre 2011, 10h30

Stefano BIANCHI
Università di Roma Tre, Italie

An Active Galactic Nucleus (AGN) is powered by accretion of mass by the supermassive black hole (BH) at the center of the host galaxy. Although the basic physics at work in the engine of AGN is believed to be the same for all the objects, the observed phenomenology is very rich. In particular, the phenomena related to the interaction of the nuclear radiation with the environment of the BH on all scales allow us to to extract a wealth of information about the physical processes occurring in AGN. Emission lines from highly ionized species have become common features in X-ray spectra of radio-quiet AGN. In this review, I will show how high resolution spectroscopy and imaging, together with a proper theoretical modelling, can be used as effective tools to investigate the physical conditions and the geometrical properties of the materials responsible for the production of these emission lines, and their relation to the circumnuclear gas observed in other wavelengths. These results will be put in the framework of generalized Unified Models, taking into account not only the line of sight to the object, but also the role of the host environment, and of fundamental parameters of AGN, such as the accretion rate and the BH mass.


PHAT or The Panchromatic Hubble Andromeda Treasury: a new vision of M31

Vendredi 16 Décembre 2011, 10h30

Morgan FOUESNEAU
University of Washington, USA

The Panchromatic Hubble Andromeda Treasury (PHAT) is a Hubble Space Telescope Multi-cycle program to map roughly a third of M31’s star forming disk, using 6 filters covering from the ultraviolet through the near infrared. With HST's exquisite high-resolution, sensitivity, and multi-wavelength imaging of M31, we have begun to assemble one of the largest and most complete samples of resolved stars and stellar clusters in a disk galaxy to date. In this talk, I provide an overview of the current status of the PHAT observations and the wide range of scientific endeavors that the survey provides. I will in particular focus on the star cluster content. This sample includes ∼600 high-quality objects, representing the first installment of a ∼2500 object sample expected from the full dataset. This represents after only one year a factor of 4 increase in the number of known objects within the survey region. We identified hundreds of new clusters that push well into the low luminosity/mass regime, down to M(V) ∼-2. I will present luminosity functions and other derived properties of the Year 1 clusters, compare our results to previous cluster studies in M31, and highlight interesting future science possible with this incredible dataset.


Superluminal waves in pulsar winds: the sigma-problem revisited

Vendredi 20 Janvier 2012, 10h30

Ioanna ARKA
Grenoble

The energy lost by rotation-powered pulsars is carried by a Poynting flux dominated, relativistic electron-positron wind. However, it is still far from clear how this predominantly electromagnetic energy is converted to the kinetic energy flux that is needed to power the synchrotron emitting nebulae of young pulsars. One solution of this problem, applicable to the outer, lower-density regions of pulsar winds, is given by linearly polarized waves of superluminal phase speed. We investigate whether the particle, energy and momentum flux of a pulsar wind can be carried by such wave modes, and discuss what this possibility implies for the dynamics of pulsar winds and their termination shocks.


Observations de pulsars en rayons gamma avec le Fermi LAT

Vendredi 3 Février 2012, 10h30

Lucas GUILLEMOT
MPIfR, Bonn, Allemagne

Le lancement du satellite Fermi en juin 2008 a ouvert une nouvelle ère pour l'étude des pulsars dans le domaine des rayons gamma du GeV. En trois ans d'observation, le Large Area Telescope (LAT) à bord de Fermi a détecté plus de 100 pulsars, établissant ces objets comme les sources d'émission majoritaires dans la Galaxie à ces énergies, et permettant l'étude de leurs propriéétés temporelles et spectrales avec une sensibilité jamais atteinte. Les observations multi-longueur d'onde en soutien de la mission Fermi se sont néanmoins avérées essentielles pour l'étude des pulsars avec le LAT. Par exemple, des recherches de pulsars émetteurs radio à la position de sources non identifiées de Fermi ont permis la découverte de plus de 30 pulsars milliseconde jusqu'à présent. Je reviendrai sur les observations de pulsars en rayons gamma avec le LAT à bord de Fermi, ainsi que sur le suivi multi-longueur d'onde des pulsars découverts au sein de sources gamma non identifiées.


Escaping atmospheres of exoplanets: towards hot neptunes, super-earths, and Earth-like planets

Vendredi 10 Février 2012, 10h30

David EHRENREICH
Grenoble

Nearly 700 extrasolar planets have been detected so far and an intense characterisation effort has been undertaken to unveil the atmospheric properties of some of these distant worlds seen in transit accross their stars. A large number of transiting exoplanets are found in extreme irradiation environments, very close to their stars, and the question arise of whether the atmospheres of these planets remain stable or get blown away. Atmospheric evaporation was observed in some hot giant exoplanets or "hot jupiters", but does not significantly alter the fate of these massive objects ( 300 Earth masses). Hot neptunes, on the other hand, are a class of exoplanets with typical masses around 20x Earth. They are the link between hot jupiters and super-earths (1 to 10 Earth masses). It is surmised that the latters can be evaporation remnants, with atmospheres completely eroded by the extreme stellar irradiation. In this case, could hot neptunes be the progenitors of the hot rocky planets detected by the Corot and Kepler missions? Detecting their extended atmospheres and measuring their mass loss rates and atmospheric heating efficiencies are key steps towards the understanding of the atmospheric dynamics and properties of low-mass exoplanets. After an introduction about planetary transits, I will review the results we have obtained with HST on the atmospheric evaporation of transiting exoplanets, on both observational and theoretical sides. I will finally discuss the prospects about atmospheric characterisation for Earth-size planets in more temperate - habitable - environments, and how the upcoming transit of Venus in June 2012 could be a Rosetta stone to interpret the future transmission spectra of Earth-like exoplanets that could be obtained with JWST.


Constraining dark matter models with tidal shells

Vendredi 17 Février 2012, 10h30

Robyn SANDERSON
Groningen, Pays-Bas

Tidal shells are the spectacular products of nearly head-on collisions between two galaxies, one more massive than the other. The high symmetry of these events dramatically reduces the number of degrees of freedom in the interaction. The shells themselves are real-life instances of mathematical fold catastrophes, also known as caustics, which have an equally simple structure. This underlying simplicity makes tidal shells useful gauges for probing the dark content of galaxies, and in this talk I will describe two examples that exploit them for this purpose. The first shows how to use the extreme phase-space density achieved in the shells as a test for dark matter with a velocity-dependent cross section (Sommerfeld dark matter), using a shell system in the Andromeda galaxy as an example. The second demonstrates how the universal density and velocity profiles of the shells make it possible to measure the mass of the host galaxy using only images and line-of-sight velocities, without the need for either N-body simulation of the merger or parameterized mass modeling of the host. Together, these two examples illustrate how a relatively common event in our dynamic universe can help us better understand its dark content.


Evolution physico-chimique des galaxies dans la seconde partie de l'histoire de l'univers

Vendredi 24 Février 2012, 10h30

Fabrice LAMAREILLE
Toulouse

Les grands relevés spectro-photométriques comme le VVDS permettent de déterminer la distance de milliers de galaxies et d'apporter des contraintes sur les modèles cosmologiques et l'évolution des propriétés globales des galaxies (luminosité, masse, environnement, ...). Mais les observations et notamment les spectres de ces galaxies contiennent beaucoup plus d'informations permettant d'affiner plus précisément les modèles d'évolution physique et chimique. Je ferai le point sur quelques résultats récents et sur les perspectives attendues avec les nouveaux instruments (MUSE, EMIR, Euclid, ...).


How the Milky Way Disk was Assembled

Vendredi 9 Mars 2012, 10h30

Hans-Walter RIX
MPIA Heidelberg, Allemagne

What if we could see the global structure of the Milky Way's stellar disk(s) looking only at stellar populations of a given age or abundance? This could be the most direct route to delineating the build-up of a prototypical large galaxy. We have just solved the problem of how to model rigorously the complex selection functions of large ground-based spectroscopic surveys of the Galaxy (such as SDSS/SEGUE), which now enables this approach. I will show how the global (radial and vertical) structure of the Milky Way's disk changes as a function of stellar age and abundance, and discuss what this may mean for the build-up of the Milky Way disk and its evolution through internal and external processes, such as radial migration and satellite infall, respectively.


Unveiling obscured star formation in the (not so) distant Universe

Vendredi 16 Mars 2012, 10h30

Sébastien HEINIS
OAMP, Marseille

In order to understand the physical processes driving galaxy evolution, it is essential to be able to measure accurately the star formation activity. The ultraviolet range is often used as a star formation tracer over a wide range of cosmic times, as it is sensitive to the emission of young stars. The interstellar dust however absorbs and scatters a part of this energy, which is then re-emitted in the far infrared. It is then necessary to take this dust attenuation into account in order to have the full energy budget of the star formation. An approach in this context is to study the far infrared properties of ultraviolet-selected galaxies. I will present results based on a sample of ultraviolet selected galaxies at a redshift around 1.5, in combination with Herschel data, which enable to improve the estimation of the infrared luminosity at these redshifts. My results enable to discuss the validity of recipes commonly used to correct from dust attenuation, and also to quantify the contribution of ultraviolet selected galaxies to the cosmic star formation rate at these epochs.


Disc kinematic substructure beyond the solar neighbourhood

Vendredi 23 Mars 2012, 10h30

Teresa ANTOJA
Groningen, Pays-Bas

The presence of kinematic substructure in the Solar neighbourhood has been well established through the analysis of Hipparcos and other surveys. Some of the structures may be related to orbital effects of the bar and/or spiral arms of our Galaxy. The advent of new surveys will allow us to characterise these and new kinematic groups in different disc neighbourhoods, providing constraints on the properties of the non-axisymmetric components of the Milky Way. First, I'll present here some predictions of test particle simulations regarding the kinematic substructure at different disc regions. For example, the kinematic groups induced by the spiral arm models change significantly if one moves only 0.6 kpc in Galactocentric radius, but  2 kpc in azimuth. Second, I'll analyse the kinematics of disc stars observed by the RAVE survey and trace, for the first time, the kinematic substructure 1 kpc far from the Sun. For instance, we find that for regions located inside or outside of the solar circle, the known groups appear shifted in the velocity plane. Some of the trends that we detect are consistent with the models.


Why the Cosmic horizon is not a horizon

Vendredi 30 Mars 2012, 10h30

Geraint LEWIS
University of Sydney, Australie

Recently it has been suggested that our cosmological models contain a previously unknown horizon that limits our view of the universe. In this talk I will show that this Cosmic Horizon is not a true horizon. By examining the paths of photons through the universe we show that this putative horizon is nothing but a poor understanding of the actions of the Hubble sphere.


The effect of radial migration on galactic disk heating: implications to thick disk formation

Vendredi 13 Avril 2012, 10h30

Ivan MINCHEV
AIP Potsdam, Allemagne

Disk non-axisymmetric components, such as spirals and central bars, are presently known to play a major role in shaping galactic disks. An important aspect of disks' secular evolution has been shown to be the radial migration of stars, inevitably resulting from these perturbers. It has been suggested recently that migration can bring out stars from the inner disk with high vertical energies, thus populating a thick disk component. Analyzing N-body simulations, I will show that, contrary to expectation, as stellar samples migrate, on the average, their energy and velocity dispersion change in such a way as to match the non-migrating population at the radius at which they arrive. An increase of the velocity dispersions from migrators is found mostly beyond three disk scale-lengths and it is on the order of 5-10% (likely less if gas inflow is considered), giving rise to disk flaring. I will show that a large portion of the inner disk is, in fact, cooled by migrators.


Constraining the Milky Way Halo Shape Using Thin Streams

Vendredi 27 Avril 2012, 10h30

Hanni LUX
University of Nottingham, UK

Tidal streams are a powerful probe of the Milky Way halo shape. A classic example is the Sagittarius stream. Its advantages are its length and its good data quality from over a decade of observations. Its disadvantages are its thickness and complexity. Thinner tidal streams avoid these obstacles and hold a promise of simpler analysis techniques. We determine the applicability of the simple test particle orbit technique to cold streams and analyse which of the currently known streams are particularly interesting for constraining the Milky Way halo shape. In particular, we show the special constraining power of the globular cluster stream NGC 5466.


Period-luminosity relation for low-mass X-ray binaries in the near-infrared

Vendredi 11 Mai 2012, 10h30

Ivan ZOLOTUKHIN
Observatoire de Paris

I will show that in many cases spectral energy distributions of low-mass X-ray binaries (LMXBs) in the NIR can be adequately described by a simple model of illuminated accretion disk and companion star reprocessing X-ray emission of central compact object. As a consequence, one can reliably estimate orbital period of a binary in the frame of this model using merely its X-ray luminosity and absolute NIR magnitude. For a set of well-known LMXBs we have constructed such correlation of LX, Porb and MK with a scatter of less than 0.3 magnitude. This relation, similar to the one of van Paradijs & McClintock (1994) in the optics, can be applied for LMXBs population studies in the forthcoming surveys of the Galaxy in X-ray and NIR (in particular, using ongoing VISTA surveys data).


Grouped Star Formation

Vendredi 25 Mai 2012, 10h30

Thomas MASHBERGER
Grenoble

Stars do usually not form each one on its own, but in smaller or larger groups, such as Taurus, Orion or even 30Dor in the LMC. Observations, e.g. with the Herschel satellite, and large hydrodynamical simulations provide now sufficiently large data sets so that the properties of the formation of a whole group of stars can be statistically analysed. One aspect, for example, is dynamical or primordial mass segregation of a young star forming region. Similarly, the spatial distribution of the stars, centrally concentrated or in filaments, gives clues about the course of star formation. Related to this is the question whether stars form in star clusters. As a last point I will discuss the initial mass function and potential variations of it.


Our Galactic supermassive black hole Sgr A*: the ideal testbed for theories of accretion and black hole life cycles

Mardi 29 Mai 2012, 10h30

Sera MARKOFF
Amsterdam, Pays-Bas

Sgr A* is the weakest accreting black hole we have ever observed, yet it is not a particularly unique object. We know that the majority of galaxies are more like Sgr A* than active galactic nuclei (AGN), so our Galactic center represents a dominant stage in the "typical" life cycle of a supermassive black hole. I will present a variety of different studies focusing on Sgr A*, and how we can place it into context with both nearby low-luminosity AGN as well as quiescent black holes at the stellar-mass scale. Along the way I will discuss our current understanding of accretion around this source based on semi-analytical models, and compare these to results from recent general relativistic magnetohydrodynamical simulations that include radiative cooling. In particular I will focus on the questions surrounding the disk/jet connection and the role of particle acceleration in the jets at low accretion rates.


L'émission haute énergie des pulsars : zones d'ombres et perspectives à l'aube du démarrage du télescope gamma HESS-II

Vendredi 1 Juin 2012, 10h30

Arache DJANNATI-ATAI
APC Paris

Alors qu'on ne connaissait jusqu'à récemment qu'une poignée de pulsars aux hautes énergies (au-delà de 100 MeV), plus de 100 pulsars ont été détectés par l'instrument LAT depuis le lancement du satellite Fermi à l'été 2008. Et la variété est au rendez-vous: pulsars jeunes –déjà connus en radio ou alors seulement visibles en gamma– pulsars milliseconde, et même un membre de l'amas globulaire NGC 6624. La voie est donc ouverte aux tests quantitatifs des modèles qui cherchent à déterminer la géométrie, les zones et les mécanismes d'émission de ces objets compacts fantastiques. La présence d'une coupure exponentielle dans les mesures spectrales autour du GeV pour la plupart des pulsars a eu comme première conséquence la mise en défaut du modèle de la calotte polaire (qui prédisait une coupure super-exponentielle) au bénéfice des modèles préconisant les gaps externes comme zone d'émission à haute énergie. Les découvertes récentes d'émission pulsée du Crabe dans la bande 25-400 GeV par les télescopes Tcherenkov au sol ont mise en lumière la difficulté des mesures de forme spectrale par Fermi au-delà de quelques GeV et, par là, mis à mal les modèles de gap externe (lesquels en appuyant sur une émission par rayonnement de courbureprédisent une coupure exponentielle franche) alors que le modèle dit de vent strié semble encore résister. Après un bref rappel du contexte de cette recherche, en nous appuyant sur une analyse de données spécifique, nous ferons l'état des lieux sur les mesures spectrales du Crabe avec le LAT, pour nous interroger sur sa forme (exponentielle, sous-exponentielle, loi depuissance brisée), voire sur l'existence de deux composantes commecela a été supposé dans une tentative de modélisation récente. Nous examinerons également la forme spectrale au-delà du GeV de quelques autres pulsars et verrons que pour tenter de répondre à la question de savoir si le Crabe est un objet exceptionnel –autrement dit pour l'examen de la dépendance de l'émission dure aux paramètres du pulsars–, des mesures plus précises autour de quelques dizaines de GeV seraient nécessaires. Les perspectives ouvertes par la mise en fonctionnement prochaine du télescope Tchérenkov HESS-II de 28m de diamètre en Namibie, doté d'un seuil de détection à 30 GeV, seront discutées dans ce cadre.


X-ray accretion versus radio ejection in AGN

Vendredi 22 Juin 2012, 10h30

Francesca PANESSA
Rome, Italie

The accretion-ejection mechanism acting in Active Galactic Nuclei (AGN) is one of the main astrophysical open issues, being connected to the role of AGN feedback in galaxy formation evolution studies. The X-ray emission in AGN, associated with the accretion flow, is strongly coupled with the radio emission, associated with a jet. Strong correlations between the X-ray and the radio luminosities are found both in radio-loud (RL) and in radio-quiet (RQ) AGN, despite the fact that in RQ AGN jets are often absent or very weak. To explore the origin of radio emission in radio-quiet AGN, we have surveyed the sub-parsec scale radio cores of a complete sample of well known local AGN, mapping the faintest and least luminous nuclei at a level of sub-mJy flux densities. I will present the wide variety of sub-parsec cores and structures observed, not always easy to interpret within a common physical scenario. Finally, I will discuss the lack of the X-ray and radio emission at milli-arcsecond resolution.


Birth and death of star clusters

Vendredi 29 Juin 2012, 10h30

Florent RENAUD
CEA Saclay

From galaxy clusters to individual stars, star clusters seem to be at the convergence point of many topics of modern astronomy. Their formation relies on the large-scale environment and their evolution imprints the smaller-scale medium. However, both ends of the life of star clusters remain relatively unknown: in which conditions do they form? how do they become old globulars or get dissolved into the field? I will first introduce a simulation of the Milky-Way at sub-parsec resolution, which helps us to probe the "birth" of stars in molecular clouds in a fully-consistent galactic context. The "death" of the clusters will be addressed thanks to a new survey of star-by-star simulations accounting for the time-varying tidal field of galaxy mergers. Implications of such complex environments on the evolution and dissolution of the clusters will be discussed.


Observatoire Astronomique de Strasbourg

Benoit Famey