Séminaires à l'Observatoire en 2009/2010

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


Liste des Séminaires en 2009/2010


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Année 1996/97
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Année 1994/95

Le projet NOEMA.

Vendredi 4 Septembre 2009, 10h30

Roberto NERI
IRAM, Grenoble

Je tracerai un bref aperçu historique de l'interféromètre du Plateau de Bure (PdBI) avant de développer une vision d'extension à moyen terme. Je présenterai ainsi NOEMA, le projet actuellement proposé à la communauté et je me baserai sur les derniers résultats, les plus marquants, obtenus avec le PdBI pour promouvoir l'intérêt et l'impact que NOEMA devrait avoir sur le plan scientifique.


Emission gamma haute énergie des binaires compactes

Vendredi 11 Septembre 2009, 10h30

Guillaume DUBUS
LAOG, Grenoble

L'astronomie gamma des hautes et très hautes énergies bénéficie actuellement d'un contexte observationnel très favorable avec, depuis 2002, la mise en service de plusieurs instruments Cherenkov au sol et, en juin 2008, le lancement du télescope gamma spatial Fermi. Le ciel gamma haute énergie (au-delà de la centaine de MeV) se révèle d'une grande richesse non seulement en nombre de sources mais aussi dans la physique associée. Un des évènements marquants est la découverte d'émission gamma associée à certaines binaires X massives. L'émission gamma est modulée à la période orbitale et leur luminosité gamma est comparable ou supérieure à la luminosité X. Je présenterai les contraintes que ces observations apportent sur la géométrie et l'origine du rayonnement haute énergie. Il s'agit vraisemblablement de pulsars jeunes interagissant avec leur étoile compagnon de type O ou Be, ce qui ouvre de nouvelles perspectives pour comprendre la physique des pulsars sur des échelles spatiales qui étaient inaccessibles auparavant.


The Cygnus Loop: the weakest SN in the Galaxy?

Vendredi 16 Octobre 2009, 10h30

Andrea PREITE MARTINEZ
INAF/IASF, Roma; CDS/Observatoire de Strasbourg

Le Cygnus Loop est un reste de supernova bien connu et bien observé en toutes les longueurs d'ondes. Il est censé être le résultat de l'explosion " core-collapse " d'une étoile d'environ 13-15 masses solaires, à l'intérieur d'une cavité formée par l'action de la perte de masse stellaire. Ce scenario est testé à l'aide d'un modèle hydrodynamique simple (en symétrie sphérique) qui utilise les données observationnelles comme contraintes pour le modèle. Les résultat sont globalement compatibles avec les hypothèses de départ, mais mettent à jour aussi quelques problèmes. D'autre modèles, avec des paramètres en entrée différents, donnent des résultat en meilleur accord avec les données observationnelles. En tout cas, tous les modèles concordent sur la faible intensité de l'explosion de la SN qui est à l'origine du Cygnus Loop (E= 0.08-0.1x1051 erg). On discute de la possibilité que la SN du Cygnus Loop puisse faire part d'une sous-classe de SN " core-collapse " de faible énergie, comme SN2008ha ou même SN203cx.


Multiplanet Systems Residing in Debris Disk Clearings.

Vendredi 6 Novembre 2009, 10h30

Alice QUILLEN
Associate Professor à l'Université de Rochester

All observed dusty circumstellar debris disks have central clearings. Recently planets have been discovered inside clearings in 2 extra solar systems with debris disks. I will discuss interactions between planets and debris disks and constraints on multiple planet systems likely to reside insidethe debris disk clearings. I will introduce some possible scenarios for recent evolution of the Fomalhaut and HR8799 systems.


A new galactic scaling relation.

Vendredi 13 Novembre 2009, 10h30

Benoit FAMAEY
Observatoire Astronomique de Strasbourg

The universality of the mean dark matter surface density within one dark halo core radius is known to hold for galaxies spanning a luminosity range of 14 magnitudes and the whole Hubble sequence. I will show that a similar relation exists for the baryonic matter, namely that the mean baryonic surface density is also universal within the halo core radius. This means that the gravitational acceleration generated by the baryonic component in galaxies is always the same at this radius. While the total baryonic-to-dark matter ratio is not universal in galaxies, this new result, together with the universality of the dark matter surface density, implies that the baryonic-to-dark matter ratio is universal inside the halo core radius. I will show that this finding can be interpreted as a close correlation between the enclosed surface densities of - and thus the gravitational accelerations generated by - baryonic and dark matter in galaxies. This argues in favor of a key role played by the interactions between baryonic and dark matter in the process of galaxy formation and evolution, or even in favor of modified gravity.


Galactic dynamics and galaxy interactions in modified gravity.

Mercredi 18 Novembre 2009, 16h00, exceptionnellement

Olivier TIRET
SISSA, Trieste

I use numerical simulations to explore an alternative to the Newtonian gravity: MOND, where the modification of the gravity follows an acceleration scaling law, without invoking any dark matter. This gravity is non-linear and needs more sophisticated methods than those which are used for Newtonian gravity with dark matter. I will present the results of N-body simulations for isolated galaxies and interacting galaxies, in both frameworks of modified gravity and Newtonian gravity with dark matter. The stability of the discs, the angular momentum transfers and the dynamical friction are studied to bring some constraints and to test the different scenarios.


X-ray emission of comets and planets.

Vendredi 20 Novembre 2009, 10h30

Konrad DENNERL
Max-Planck-Institut fuer Extraterrestrische Physik, Garching

The history of X-ray astronomy is closely related to solar system research, starting with observations of the Sun and an attempt to detect X-rays from the Moon. In the recent decade, this field has considerably expanded. We know today that our solar system is full of X-ray sources, including all planets from Venus to Saturn and additional sources there, and even comets. This is remarkable because X-ray emission, usually associated with matter at extreme conditions, was not expected to be generated at so many locations in our immediate astronomical neighborhood.

Perhaps the most striking example is the ROSAT discovery of X-ray emission from comets, which came as a big surprise to many scientists. It revealed the importance of a process for producing X-rays which was overlooked before: charge exchange, an interaction that occurs when a highly ionized atom in a hot plasma (the solar wind) captures an electron from cool gas and attains an excited state, from which it may de-excite by emitting an X-ray photon. With their extended atmospheres, comets have now been recognized as the ideal targets for investigating the physics of charge exchange interactions.

The occurrence of charge exchange, however, is by far not restricted to comets. Chandra and XMM-Newton have discovered X-ray signatures of charge exchange also in the exospheres of Venus and Mars, where its high cross section makes this interaction a novel tool for measuring planetary outgassing rates, remotely and on a global scale. In planetary atmospheres also scattering of solar X-rays is important. Planets possessing a magnetic field, like Earth, Jupiter, and Saturn, may generate X-rays by yet another process: auroral activity.

The discovery of charge exchange at comets has led to implications which go far beyond our solar system. It is now understood that charge exchange in the Earth's exosphere and in the heliosphere contributes to the soft X-ray background radiation which was attributed before solely to the Local Hot Bubble in the interstellar medium. This new insight calls into question the very existence of the Local Hot Bubble.

The talk will give an overview about our current knowledge of X-ray emission of solar system objects and will present recent developments in this field of high energy astrophysics.


Relativistic formulations of modified Newtonian dynamics.

Vendredi 4 Décembre 2009, 10h30

Jean-Philippe BRUNETON
Chercheur associé à l'Institut d'Astrophysique de Paris

We will discuss the various attempts made so far to implement a theoretically consistent model of modified Newtonian dynamics. Moreover we investigate the possibility that a non-standard interaction between baryonic and dark matter could reproduce the successes of cold dark matter at extragalactic scales while making baryonic matter effectively obey a modified field equation in spiral galaxies. We may also have time for some comments about the Pioneer anomaly and its relation to such models.


Some spaces are more equal than others.

Vendredi 11 Décembre 2009, 10h30

Boudewijn ROUKEMA
Torun Centre for Astronomy, Nicolaus Copernicus University

The standard Concordance Model of cosmology is supported by many different types of cosmological observations. However, it allows many different possibilities for three-dimensional comoving space of different curvatures and different topologies. We have a theory linking curvature to matter-energy density (general relativity), but we don't yet have a theory relating topology to other physical properties of the Universe. A demonstration of how gravity, geometry, topology and inhomogeneity together give one way of comparing different spaces will be presented, firstly in a simple example, and then in some more general cases. This theoretical result will be compared to observational analyses of WMAP data.


X-shooter: the new UV to near-IR intermediate resolution spectrograph at the VLT.

Vendredi 15 Janvier 2010, 10h30

Joël VERNET
European Southern Observatory

X-shooter is the first second-generation instrument newly commissioned at the VLT. It is a high efficiency single target intermediate resolution spectrograph covering the range *300 - 2500 nm in a single shot*. I will give an overview what you need to know to propose, obtain and exploit X-shooter data: - overview of the instrument - key *performances* as measured during commissioning and the first months of science operations - the *observing modes* - overview on how to *calibrate and reduce* your data with the pipeline I will also highlight some of the *first science results* obtained with this new instrument.


Chimie et Dynamique du Gaz Moléculaire dans le Centre Galactique.

Vendredi 29 Janvier 2010, 10h30

Nemesio RODRIGUEZ FERNANDEZ
Institut de Radio Astronomie Millimétrique, Grenoble

Dans les 500 pc centraux de la Voie Lactée se trouvent presque 10 Galaxie sous forme moléculaire. Ce gaz présente une dynamique complexe, des conditions physiques extrêmes (forte turbulence, haute densité et température) et une riche chimie. Cette chimie se caractérise par une forte abondance de molécules organiques complexes et de molécules avec éléments réfractaires qui normalement se trouvent dans les grains de poussières. L'origine de ces propriétés n'est pas connue à ce jour. Après avoir présenté cette problématique, je discuterai possibles explications. Plus en particulier, je discuterai le rôle des rayons-X et des chocs dus à la dynamique galactique dans l'excitation et la chimie du gaz moléculaire.


Massive Stars in Globular Clusters: Links between Chemistry and Dynamics.

Vendredi 5 Février 2010, 10h30

Thibaut DECRESSIN
Université de Bonn

Globular clusters exhibit peculiar chemical patterns where Fe and heavy elements are constant inside a given cluster while light elements (Li to Al) show strong star-to-star variations. This pattern can be explained by self-pollution of the intracluster gas by the slow winds of fast rotating massive stars. This scenario of self-pollution also brings constraints on the dynamical evolution of clusters which can be used to find the initial properties of these clusters.


Musings on faint Local Group satellite galaxies.

Vendredi 12 Février 2010, 10h30

Nicolas MARTIN
Max-Planck-Institut fuer Astronomie, Heidelberg

Thanks to large scale surveys of the Milky Way and the Andromeda galaxy, our understanding of the Local Group dwarf galaxy system has been transformed by the discovery of numerous faint dwarf galaxies, providing an avenue to solve the "missing satellite problem" (the overproduction of dark matter satellites in Lambda-CDM simulations compared to the small number of observed dwarf galaxies in the Local Group). I will present the latest discoveries that comprehensive analyses from survey data, as well as deep photometric follow-up, reveal to be complex systems. In spite of their very low luminosities, these faintest of stellar systems can show very distorted morphologies and/or extended periods of star formation. Much remains to be understood about these systems and whether they are really bound stellar systems before they can be used in solving the "missing satellite problem".


Towards a new understanding of star formation in galaxies.

Vendredi 19 Février 2010, 10h30

Pavel KROUPA
Université de Bonn

The stellar IMF is the distribution function of stellar masses born together in one causally connected event within a spatial region of not more than a few pc in extent. It cannot be measured in any system, but statistical methods combined with corrections for dynamical bias allows one to infer the existence of a universal canonical IMF as the parent distribution from which the various simple stellar populations are drawn. There is no evidence for variation of the IMF except at the highest star-burst cluster masses above 106 Msun and in the immediate vicinity of the Galactic super massive black hole. The IMF in a whole galaxy is, in contrast, that of a composite population such that the IMFs of the individual simple populations must be added. This leads to an integral over the currently forming star cluster population and implies the integrated galactic IMF (IGIMF) to be top light. The steepness of the IGIMF for massive stars can be shown to depend on the star formation rate of the galaxy. This leads to an entirely new avenue of understanding galaxy evolution with potentially deep insights into fundamental physics issues.


Stellar populations and morphology on the red sequence at z=1.

Vendredi 5 Mars 2010, 10h30

Simona MEI
GEPI - Observatoire de Paris

We present results from a detailed study of cluster red sequence at z 1 from the ACS Intermediate Redshift Cluster Survey. Our analysis shows that the red sequence is well defined at these redshifts and elliptical and lenticular galaxies lie on similar color-magnitude relations. We analyze the parameters of the early-type color-magnitude relations - scatter, slope and zero-point - as a function of redshift, galaxy properties and cluster mass.


Top-heavy initial mass functions in ultra-compact dwarf galaxies?

Vendredi 19 Mars 2010, 10h30

Joerg DABRINGHAUSEN
Université de Bonn

Ultra compact dwarf galaxies (UCDs) are dense stellar systems at the border between massive star-clusters and small galaxies. The perhaps most remarkable finding about them is that their average optical mass-to-light (M/L) ratio cannot be explained by stellar populations with the canonical stellar initial mass function (IMF). It seems unlikely that this is due to dark matter or MOND, which leaves a variation of the IMF as an alternative. In aged stellar systems like the UCDs, a top-heavy IMF could provide unseen mass by an abundance of stellar remnants. Such a variation of the IMF can be understood if UCDs represent a case of rapid star-formation in an extremely dense environment. While top-heavy IMFs imply a much heavier mass-loss shortly after the formation of a stellar system, this process does not necessarily dissolve the UCDs. Their formation with a top-heavy IMF would therefore not contradict their existence.


Galaxy kinematics: dark matter vs. modified gravity.

Vendredi 16 Avril 2010, 10h30

Gianfranco GENTILE
Université de Gand

The dark matter particle has not been detected (yet?), so that in some astrophysical contexts even its very existence is being questioned. Therefore, it is worth investigating alternatives to dark matter in galaxies such as MOND (Modified Newtonian Dynamics). After introducing the main issues, I will review the pros and cons of dark matter and MOND to explain observational data, focussing on galaxy scales.


Universality of dark matter surface densities from a chameleon-like scalar field.

Vendredi 23 Avril 2010, 10h30

HongSheng ZHAO
Université de St Andrews

We show that, with a scalar field coupled differently to cold dark matter and baryons, some galaxy scaling relations can be recovered, such as the recently discovered universality of dark matter surface densities in galaxies. The scalar field potential and coupling function are so chosen that the scalar field acquires a heavy mass in regions with high density (i.e., it is chameleon-like). We focus on (1) how the existence of the scalar field affects the formation of nonlinear large-scale structure, and (2) how the different couplings of the scalar field to baryons and CDM particles lead to different distribution and evolution for these two matter species, both on large scales and inside galactic halos.


Direct searches for dark matter.

Vendredi 30 Avril 2010, 10h30

Jocelyn DOMANGE
CSNSM Orsay/SPP CEA Saclay

Dark matter is thought to play an important role in the formation of structures in our Universe and should represent about 23 per cent (85 per cent of the matter) of its energetic content. Many theoretical frameworks beyond the Standard Model of particle physics contain one or more dark matter candidates. In particular, Weakly Interactive Massive Particles (WIMPs) are a class of candidates particularly well-motivated in models in which the lightest supersymmetric particle - plausibly the neutralino - is the thermal relic from the early Universe. If neutralinos indeed constitute the major part of dark matter, a substantial flux of these particles coming from the Milky Way dark halo should produce an elastic scattering on nuclei in a terrestrial detector. Such scattering events deposit energy in the detector by recoiling nucleus, and can be observed by different channels : phonons (heat), ionization (charge) or scintillation (light). Nowadays, a lot of experiments all around the world are using these techniques in order to directly detect dark matter. In this talk, I will present the state-of-the-art for different direct detection techniques and experiments, focusing on some characteristic experiments such as CDMS-II ans EDELWEISS-II (massive bolometers), XENON 100 (liquid xenon) and DAMA /LIBRA (NaI). I will also give the latest results for WIMPs hunting.


Last news of the Canada-France Brown Dwarf Survey: searching for field brown dwarfs.

Vendredi 7 Mai 2010, 10h30

Céline REYLE
Observatoire de Besançon

Since the first discoveries of brown dwarfs in 1995, several hundreds of brown dwarfs have been identified, most of them thanks to large scale surveys. We undertook such a survey, the Canada-France-Brown-Dwarf Survey (CFBDS), based on deep multi-colour MegaCam optical imaging obtained at the Canada-France-Hawaii Telescope (CFHT). Candidate brown dwarfs and quasars are initially identified on the MegaCam optical images as objects which have very red colours (high i'-z' colours). Near-infrared J-band imaging is thus performed to separate high redshift quasars and brown dwarfs. Besides pinpointing the few high redshift quasars that bring important clues on the reionization of the Universe, the J-band photometry very effectively rejects any remaining observational artefacts, as well as the more numerous low-mass M-stars scattered into the brown dwarf/quasar colour region by large noise excursions. We expect that complete J-band imaging follow-up of all our candidates will yield about 100 T dwarfs and over 400 L or very late-M dwarfs, approximately doubling the number of known brown dwarfs. Follow-up spectroscopy of the T dwarfs candidates with the reddest z ? J colour was then carried out. Up to now, 41 T dwarfs are spectroscopically confirmed T dwarfs, with spectral type ranging between T0 and T/Y. Here I present the status of the survey and the latest results drawn from the survey, including very late T dwarfs, T subdwarf, T dwarf-main sequence star binary, as well as the space density of field brown dwarfs in the Milky Way.


Neutron stars and their differential rotation.

Vendredi 21 Mai 2010, 10h30

Loïc VILLAIN
LMPT - Université de Tours

Compact astrophysical objects (such as neutron stars and black holes) are among the most exotic entities in the Universe, but luckily enough for those who wish to study them, they are also among the most multilingual ones. Indeed, one can observe them through the electromagnetic spectrum (in radio, gamma, X-ray, etc.), one can sometimes get material particles from them (neutrinos or cosmic rays), and in the near future, one even hopes to determine how they make spacetime swing by the direct observation of gravitational waves. However, even if the relations between compact objects and those different kinds of signals are now taken for granted, the abundance of the latter relies on the complexity of the former, meaning that many questions regarding them remain opened. This talk will focus on material compact objects, the so-called neutron stars. After giving a brief overview of the many phenomena that need to be taken into account in their study, one shall address the simple issue of the calculation of their inner structure. The emphasis will be put on the implication of rotation and on the difficulties related to general relativity, with as a goal to illustrate the information one can still get without going into the most complicated numerical simulations.

XMM-Newton and Chandra Observations of Accreting White Dwarfs.

Vendredi 11 Juin 2010, 10h30

Vadim BURWITZ
Max-planck-Institut fuer Extraterrestrische Physik, Garching

The new X-ray observatories XMM-Newton  and Chandra with their high resolution grating spectrographs have opened up  a new window for studying the accretion phenomena in Cataclysmic Variables and Supersoft Sources. The high resolution X-ray spectra not only enable the use of "classical" line diagnostic tools to constrain the density and temperatures of the plasma but also show the complexity especially of white dwarf atmospheres.

Caractérisation multibande de galaxies par hiérarchie de modèles et arbres de composantes connexes.

Vendredi 18 Juin 2010, 10h30

Benjamin PERRET
LSIIT, Strasbourg

Le travail présenté consiste à étudier la représentation multibande des galaxies sous forme d'une composition de structures astrophysiques (bulbe, disque, barre stellaire, bras spiraux, anneau, ?`) combinée à un modèle observationnel destiné à s'affranchir des effets perturbateurs de l'atmosphère terrestre et des sources de bruits. La description spectro-morphologique obtenue doit permettre d'appréhender les diverses caractéristiques spatiales (extension, aplatissement, rapport bulbe/disque, importance de la barre stellaire, structure spirale plus ou moins développée) et d'évaluer simultanément les comportements spectraux sur une bande étendue de fréquences. Cette approche, bénéficiant d'une caractérisation plus complète que les études précédentes, pourra alors servir de base au développement de nouveaux schémas de classification spectro-morphologique des galaxies.

La méthodologie retenue cherche à tirer le meilleur parti des méthodes de modélisation paramétrique et non paramétrique. D'un côté les méthodes de modélisation paramétrique fournissent une description de haut niveau sémantique de l'objet, donnant accès à une interprétation plus aisée des résultats en terme astronomique. D'un autre côté, les techniques de modélisation non paramétrique basées sur la morphologie mathématique offrent un outil puissant permettant de gérer avec précision et robustesse les étapes de pré- et post-traitements ainsi que la mise en place de filtres adaptatifs intégrés au modèle paramétrique.

Les méthodes développées sont appliquées à un jeu de données réelles de plusieurs milliers d'images extrait de la base EFIGI (Extraction de Formes Idéalisées de Galaxies en Imagerie) fournie par l'Institut d'Astrophysique de Paris. La comparaison des résultats obtenus avec les annotations des experts permettent de valider le bon comportement de nos algorithmes et leur applicabilité au sein de l'Observatoire Virtuel.

Brown dwarfs in the UKIDSS and PanSTARRS1 surveys.

Vendredi 25 Juin 2010, 10h30

Bertrand GOLDMAN
MPIA, Heidelberg

Brown dwarfs are substellar objects not massive enough to sustainably burn hydrogen, and they cool down with time after their initial contraction phase. The first brown dwarfs have been discovered 15 years ago, and the coolest ones have temperatures of about 600K, bridging (almost!) very low mass stars to the Solar giant planets. Indeed some irradiated exoplanets have temperatures typical of brown dwarfs. With a new generation of on-going and soon-to-start surveys such as PanSTARRS1, UKIDSS/VISTA and WISE, the prospects to find brown dwarfs in larger number, various flavours and cooler temperatures have never so good. In this talk I'll review (some of) our knowledge about brown dwarfs and describe our recent discoveries from the near-infrared UKIDSS and optical PanSTARRS1 surveys.


Nuclear Star Clusters - Clues to Black Hole Formation.

Vendredi 2 Juillet 2010, 10h30

Nadine NEUMAYER
ESO Garching

Nuclear star clusters appear to follow the same scaling relation as supermassive black holes in that their mass is proportional to the mass of their host galaxy. It has therefore been proposed that nuclear star clusters are an alternative incarnation of "central massive objects" in lower mass galaxies. However, unlike black holes, nuclear star clusters provide a visible record of the accretion of stars and gas into the nucleus. I will present our ongoing programme to obtain high spatial resolution kinematic observations of the nearest nuclear star clusters using adaptive optics-assisted integral-field spectroscopy. These observations provide important information on the formation mechanism of nuclear star clusters, and allow us to estimate nuclear star cluster and potential black hole masses and examine scaling relations with their host galaxies.


Observatoire Astronomique de Strasbourg

Benoît Famey