par- 19 janvier 2009
2010 February 5th , Observatoire : Andrea Cattaneo (Observatoire de Lyon)
Galaxy formation and evolution in cosmology
2010 February 8th , Observatoire : Morgan Le Delliou ()
Separating expansion from collapse and generalizing TOV condition in spherically symmetric models with pressure, with Lambda-CDM examples
2010 February 12th , ENS : Heloise Méheut (Unv. Paris 7)
Rossby wave instability and 3D vortex in accretion discs : A path to planet formation
2010 February 26th , 11:00 Observatoire : Frank Steiner (Univ. Ulm & Obs. Lyon)
The cosmic microwave background and the topology of the Universe
2010 February 26th , 14:00 Observatoire : Boud Roukema (Torun Centre for Astronomy, Nicolaus Copernicus University)
Some spaces are more equal than others
2009 December 11th , Observatoire : Frédéric Bournaud (CEA Saclay)
Disk and spheroid formation in the cosmological context
Abstract : Star forming galaxies above redshift one have increasingly clumpy morphologies, often taking the appearance of chain galaxies and clump clusters. A large fraction of their mass is gathered into a few kpc-sized blobs. While the morphology of such systems could resemble groups of proto-galaxies in a hierarchical merging process, I will show evidence that these are actually massive, gas-rich disks, that fragmented by gravitational instabilities. Numerical models are used to study the evolution of such systems, and suggest that today’s spiral galaxies were shaped by the internal, clump-driven evolution of primordial disks. Clumps redistribute the disk material in an exponential profile. When massive enough, clumps can reach the center of the system and coalesce to form a bulge. The thick disks of today’s spiral galaxies are likely leftovers that attest of this past evolutionary processes. The ISM of such z 2 clumpy galaxies, which are the progenitors of today’s Milky Way-like spirals, is observed to be starbursting over the whole disk with high pressures and efficiencies. Major and minor mergers do not appear to be the main drivers of galactic mass assembly and star formation history. The clumpiness of high-redshift disk implies that they are strongly self-gravitating, which constrain the role of mergers and smooth gas infall in the mass assembly of galaxies. Whether or not the standard L-CDM paradigm can account for galaxy properties at high and low redshift is still largely unknown, and I will show recent progress in modelling the properties of galaxies at high resolution in the cosmological context.
2009 December 17th , ENS : Luc Dessart (Laboratoire d’Astrophysique de Marseille)
Explosions and eruptions in massive stars
Abstract : Observations of transient phenomena in the Universe reveal a spectrum of mass-ejection properties associated with massive stars, covering from Type II/Ib/Ic core-collapse supernovae (SNe) to giant eruptions of Luminous Blue Variables (LBV) and optical transients. Recently we have proposed that a large fraction of these phenomena may have an explosive origin, the distinguishing ingredient being the ratio of the prompt energy release to the envelope binding energy.
In this seminar, I will first review recent observations of such transient phenomena. Then, using radiation-hydrodynamics simulations for both neutrino or photon transport, I will present the various mechanisms that may lead to the observed diversity of explosions and eruptions in massive stars. In particular, I will discuss magneto-rotational effects and neutrino energy deposition in and above the collapsed core of massive stars, but also the potential for energy release from unstable shell burning.
2009 November 6th , Observatoire : Jerome Petri (Observatoire de Strasbourg)
Pulsars and their electromagnetic environment : an approach by numerical PIC simulations
Abstract : Neutron stars are end products of stellar evolution. Within the last few years, compact object astrophysics has greatly benefited from terrestrial based and space telescopes in the X- and gamma-ray ranges. However, these compact stars remain badly understood, especially in the high and ultrahigh-energy bands where particle acceleration, relativistic shocks and pulsed emission (MeV, GeV) occur.
In this talk, in the first part, I will briefly review some problems related to our understanding of pulsars and then focus on their magnetosphere and wind.
In a second part, I will show how to face some of these problems with help on numerical simulations like Particle-In-Cell codes (PIC) in order to model the plasma flow around those stars. I will show the existence of a peculiar structure of the magnetosphere called electrosphere, an example of non-neutral plasma in astrophysics. A linear analysis has demonstrated that this geometry is unstable against the diocotron and magnetron instability. These results are then confirmed and extended by the 2D electrostatic PIC simulations in cylindrical coordinates.
Farther away from the neutron star, its magnetised wind can be subject to magnetic reconnection in the relativistic regime when crossing the termination shock. Reconnection is studied via 1D relativistic PIC simulations. This opens up a new way to solve the problem of magnetic energy conversion into kinetic energy for the particles feeding the nebula.
I will finish with some possible future improvements to be done on my PIC code : geometry flexibility (cartesian, cylindrical or spherical), implicit or explicit time integration method, spatial discretisation with finite differences or (pseudo)spectral expansion onto a base like Fourier-Legendre-Chebyshev polynomials for instance.
2009 November 27th , Observatoire : Arthur Vigan (Laboratoire d’Astrophysique de Marseille)
Detection and Characterization of Exoplanet by Direct Imaging with IRDIS
Abstract : Since the discovery of the first exoplanet orbiting a main sequence star in 1995, a large number of these objects have been found mainly with indirect methods such as radial velocity measurements and transits. The wide adaption of adaptive optics systems and coronagraphs in large telescope instruments led to the very first image of an exoplanet around a young brown dwarf in 2005, followed by a handful of similar objects in the recent years. The main difficulty for direct imaging of exoplanets is to overcome the large contrast difference between the star and the planet, which can reach 12 to 18 magnitudes for a Jupiter mass planet in a young planetary system. The next generation of planet finding instruments currently under development will combine : extreme adaptive optics systems with a large number of actuators to reach very high turbulence correction in the near-infrared and high effciency coronagraphs to obtain optimal star extinction. SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) is the dedicated planet imager for the VLT that will start operation in 2011. It aims at detecting exoplanets down to the Jupiter mass around nearby young stars at angular separations of 0.1as from the star in either one of its two near-infrared science instruments. One of them is the differential imager IRDIS which will offer several observing modes for exoplanet detection and characterization. I will present the performance analysis of the two main IRDIS modes : the Dual Band Imaging mode dedicated to exoplanet detection with spectral and angular differential imaging (SDI and ADI) data analysis methods and the long slit spectroscopy mode (LSS) in low and medium resolution. These modes are both studied in terms of detection capabilities of faint objects in speckle-dominated data, and in terms of characterization of the detected objects. I will also present the impact of calibration errors on the performances for calibration procedures which I developed for SPHERE : calibration of the distortion, calibration of the field rotation center, and calibration of the large structures in the flat field and thermal background.
2009 Oct. 16th, Observatoire : François Hénault (OCA, Grasse)
Cophasing segmented optical surfaces in polychromatic light. Application to the VLTI and ELTs
Abstract : We explain the principle of some of the multispectral methods used or envisaged for cophasing segmented optical surfaces, such as sparse apertures interferometers, future Extremely Large Telescopes (ELTs) or spaceborne hyper-telescopes. Following a brief reminder of the problem (cophasing consists in accurately aligning different, separated mirror segments or telescopes so that they behave together as a single optical system), we examine the "dispersed fringes" method (Koechlin et al), the "dispersed speckles" method (Borkowski et al) and the multiple-wavelength methods (Wyant, Creath...). We show some examples of their application to the CHARA interferometer, to the SIRIUS testbench developed in UMR Fizeau, and to the second-generation VLTI fringe tracker (POPS). We conclude by describing a new method based on a multispectral, phase-shifting piston sensor, applicable for cophasing the future ELTs.
2009 Oct. 2nd, Observatoire : Bernhard Dorner (CRAL, Observatoire)
How to develop a space mission to characterize earth-type exoplanets : Report of the Alpbach Summer School 2009
Abstract : Held since 1975, the Alpbach Summer School is an annual educational event to train European students, young reasearchers and engineers in space-related mission design. It comprises a series of lectures on a dedicated science topic as well as workshops to create a space mission study. This year’s theme dealt with the discovery and characterisation of extra-solar Earths. I will give an introduction to the summer school in general, the field of exoplanetary research and present the mission proposal of our team : The Carl Sagan Space Telescope (CST), a very large IR and UV space observatory.
2009 Oct. 9th, Observatoire : Céline Reylé (Observatoire de Besançon)
Last news of the Canada-France Brown Dwarf Survey : Searching for field brown dwarfs
Abstract : 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.
2009 September 9th, 14:00, ENS salle des thèses : Guillaume Laibe (CRAL, soutenance de thèse)
Croissance des grains de poussière : premières étapes de la formation planétaire
Abstract : Le but de cette thèse est l’étude de la première étape de la formation planétaire. En effet, alors que les observations montrent l’existence de grains de poussière dans les disques protoplanétaires ainsi que la présence d’exoplanètes, le mécanisme qui permet de passer de l’un à l’autre n’est pas encore bien compris. Notamment la phase primordiale de croissance des grains qui permet la formation des premiers embryons de planètes, les planétésimaux. Pour décrire quantitativement le phénomène de croissance des grains ainsi que son couplage à la dynamique, j’ai étendu un code SPH (smoothed particle hydrodynamics) 3D développé au Centre de Recherche Astrophysique de Lyon (CRAL-ENS) permettant de traiter l’évolution du gaz et de la poussière (y compris la croissance) de façon couplée.
Dans une première partie, je resitue l’importance de l’étude de la croissance des grains dans le contexte de la formation planétaire. Je montre en particulier que jusqu’ici, des analyses physiques quantitatives sur la phase de formation des pré-planétésimaux sont restreintes. Dans une seconde partie, je développe analytiquement des résultats permettant de prédire le comportement de la phase gazeuse d’un disque protoplanétaire. J’insiste sur l’importance des gradients de viscosité dans le disque et regarde la stabilité de certaines configurations à l’ordre linéaire et non-linéaire. Dans une troisième partie, je dégage des résultats à partir d’études analytiques et numériques sur le comportement des grains en présence d’une phase gazeuse. En particulier, je développe un modèle simple couplant croissance et sédimentation des grains qui met évidence les paramètres qui contrôlent le comportement des grains dans le disque. Dans une quatrième partie, je décris le code SPH utilisé pour réaliser des analyses théoriques quantitatives. Je présente la façon dont j’y ai inclus la croissance des grains, ainsi qu’une formulation permettant de traiter plus rapidement la dynamique des petits grains. Enfin, dans une cinquième partie, je présente et interprète les résultats des simulations obtenues avec le modèle de croissance de Stepinski et Valageas (1997) pour différentes configurations initiales de disque. Je caractérise aussi la croissance des grains dans un disque où une planète est déjà formée. En conclusion, je distingue trois phases dans le mouvement des grains, qui se répartissent dans le disque selon une distribution bimodale. En particulier, j’insiste sur le fait que pour certains profils de densité bien caractérisés, les grains ne sont pas accrétés par l’étoile centrale.
2009 May 18th, Observatoire : Maxime Bois (CRAL-Observatoire de Lyon)
Numerical simulations of galaxy mergers
Abstract : Galaxies populate a bimodal colour distribution, corresponding to a blue cloud of late-type galaxies, and a red sequence of early-type galaxies. The gap between these two groups arises through a rapid evolutionary transition from blue to red, via merging of gas-rich blue-cloud galaxies and subsequent quenching of star-formation. Merging within the red sequence is also required to produce the massive end of the red sequence. The Altas3D Project combines a multi-wavelength survey of a complete sample of early-type galaxies within the local (40Mpc) volume with numerical simulations and semi-analytic dynamical modeling. This project aims to quantify the global stellar kinematics and dynamics of a statistically significant sample of objects to characterize the class of early-type galaxies, and relate this to their formation and evolution. In this context, I analyse numerical simulations of disc galaxy mergers (mass ratio from 6:1 to 1:1) and elliptical re-mergers at an unmatched resolution to make comparisons with the Atlas3D galaxy sample and to provide recipes for the semi-analytic modeling.
2009 Apr 17th, ENS, grande salle du CECAM : Serena Arena (Universität Tübingen)
Porosity models and the formation of planetesimals
One of the major problems in the theory of planet formation concerns the growth mechanism leading from pre-planetesimals, agglomerates of the size between centimetres and metres, to planetesimals, objects of the size of kilometres and precursors of terrestrial planets and cores of giant planets. Different scenarios have been proposed, among them collisional growth is one of the possible mechanism. It requires a clear understanding of the outcome of collisions among pre-planetesimals given the typical material composition, size and relative velocity distribution expected in the solar nebula. In the first part of this talk, after introducing the basic concepts of solid body mechanics, I will show how collisions can be modelled and studied through numerical simulations performed using the Smoothed Particle Hydrodynamics (SPH) scheme. Then, I will summarize the finding obtained so far modelling pre-planetesimals as brittle solids and explain the increasing interest in porous materials with the consequent effort in modelling their complex behaviour. Therefore, the second part is devoted to the explanation of the properties, the differences and the related problems of three porosity models used in this context and to the presentation of a proposed improvement. Finally, I will shortly introduce a new fully thermodynamical based model that can potentially lead to studies where temperature effects, thought to be crucial in the collision process, are considered.
2009 March 13th, : Laure Fouchet (Univ. Zurich)
Migration and growth of giant planets in self-gravitating disks with varied thermodynamics
Abstract : I will present the results of novel global high-resolution three-dimensional simulations of disk-planet interaction which incorporate simultaneously realistic radiation physics and the self-gravity of the gas, as well as allowing the planet to move in a viscous disk. We find that both self-gravity and thermodynamics have a significant effect on the character and magnitude of migration of Jupiter mass planets as well as on the amount of gas available for accretion. Self-gravity increases migration rates by about 50%. In simulations with radiative transfer adopting flux-limited diffusion, inward migration can be decreased by about 30% relative to the isothermal case, while in adiabatic runs migration nearly shuts off after a few tens of orbits. If cooling is completely inhibited, such as in the adiabatic runs, migration slows down by more than an order of magnitude. Similar strong effects of thermodynamics on planet migration have been recently found for the case of low mass planets. Gas feeding rates on the planet can be reduced by more than an order of magnitude going from isothermal to radiative transfer and adiabatic simulations. A circumplanetary disk always forms except in adiabatic runs. With radiative transfer the disk is sub-keplerian (v_rot/v_kep 0.7) owing to significant pressure support, but large boulders with sizes larger than 10-100 m should drift slow enough to allow the formation of rocky satellites.
2009 March 16th, ENS small CECAM room : Sven Van Loo (University of Leeds)
The influence of dusty, weakly ionised plasmas on star formation
Abstract : Dense cores are the progenitors of young stars and were first identified in ammonia emission, which traces material with number densities exceeding 104 cm-3. There is still a debate about the mechanisms responsible for dense core and star formation. At one extreme the most important steps in formation are induced by turbulent motions or waves, whereas in the opposite case the evolution is quasistatic. The former picture is supported by observations of extended flows towards dense cores at speeds exceeding those expected in ambipolar diffusion regulated collapse. The latter is supported by the success of models of ambipolar diffusion regulated collapse in matching interferometric observations of the earliest circumstellar discs and their envelopes. However, numerical simulations show that ambipolar diffusion is important for wave-induced core formation just as it is for quasistatic core evolution. Ambipolar and Hall diffusion also influence the development of the magnetorotational instability (MRI) in protoplanetary discs. Regions where the MRI does not develop may be necessary for planetary systems to survive.
2009 March 16th, ENS small CECAM room : Christoph Olczak (University of Cologne)
Star-Disc Encounters in Young Star Clusters - Environmental Effects on the Evolution of Protoplanetary Discs
Abstract : According to current knowledge, star formation occurs preferentially in clustered environments. As a byproduct of the star formation process young stars are found to be surrounded by accretion discs that are potential birth places of planets and planetary systems. Located in the hosting cluster, these protoplanetary discs are potentially subject to stellar interactions. These interactions give rise to a fundamental question of clustered star formation : How far does the cluster environment affect the evolution of protoplanetary discs and the formation of planets ? The question is addressed in terms of the effect of stellar encounters on stars and their discs in young clusters by combining numerical simulations of isolated star-disc encounters and stellar dynamics of young star clusters. Using a dynamical model of the Orion Nebula Cluster (ONC) and additional cluster models it is found that even in clusters four times sparser than the ONC the effect of encounters in terms of disc destruction is non-negligible. Moreover, it is shown how the mechanism of encounter-induced disc-mass loss could be traced observationally.
2009 March 9th, : Jakob Walcher (ESA)
Black holes, nuclear star clusters and the low-mass end of M-σ
Abstract : It has become clear in the last decade that the nuclei of galaxies do not only host massive black holes (BH), but also massive star clusters, commonly called nuclei or nuclear star clusters (NC). Concerning black holes, the discovery of a number of global-to-nucleus relations has lead astronomers to realize that the evolution of galaxies must be closely linked to their nuclear properties. This realization obtains additional weight when considering that NCs follow similar relations, prompting the idea that BHs and NCs should be grouped together into "central massive objects". I will ponder some evidence for a link or a disjunction between NCs and BHs and review the current status at the low-mass end of global-to-nucleus relations.
2009 Feb 20th, : Matthew Hayes (Observatoire de Genève)
Lyman alpha emission in distant galaxies
Abstract : The last decade has shown the Lyman-alpha (Lya) emission line to be a powerful and competitive tool for uncovering the galaxy population at the highest redshifts. In addition, Lya promises the opportunity for astrophysical studies of the host galaxies and galaxy population as a whole. But can it deliver on this promise ? Lya is resonance line and as such undergoes a complex radiative transport through the interstellar medium, and as such its physics and escape probability becomes very sensitive to conditions in the ISM. I will : (1) present some recent results from HST observations of local star-forming galaxies and extrapolate conclusions to high-z (2) demonstrate the wealth of inferences that can be made from modeling the line profile (3) present a new, unique study to examine the Lya-emitting galaxy population at z 2.
2009 January 29th 16:00, Observatoire : Ferréol Soulez (Obs. Lyon)
Remake of thesis defense, in French : Inverse problems for multidimensional data
Abstract : This talks presents an ``inverse problems’’ approach for reconstruction in two different fields : digital holography and blind deconvolution.
The "inverse problems" approach consists in investigating the causes from their effects, i.e. estimate the parameters describing a system from its observation. In general, same causes produce same effects, same effects can however have different causes. To remove ambiguities, it is necessary to introduce a priori information. In this work, the parameters are estimated using optimization methods to minimize a cost function which consists of a likelihood term plus some prior terms.
After a brief description of this approach, I will present in a second part its application to digital holography for particles image velocimetry (DH-PIV). Using a model of the hologram formation, we use this "inverse problems" approach to circumvent the artifacts produced by the classical hologram restitution methods (distortions close to the image boundaries, multiple focusing, twin-images). The proposed algorithm detects micro-particles in a volume 16 times larger than the camera field of view and with a precision improved by a factor 5 compared with classical techniques.
Finaly in a third part, I will show the use of this framework to address the problem of heterogeneous multidimensional data blind deconvolution. Heterogeneous means that the different dimensions have different meanings and units (for instance position and wavelength). For that, we have established a general framework with a separable prior which have been successfully adapted to different applications : deconvolution of multi-spectral data in astronomy, of Bayer color images and blind deconvolution of bio-medical video sequences (in coronarography, conventional and confocal microscopy).
2009 January 30th 11:00, Observatoire : Yue Wu (Obs. Lyon & NAOC Beijing)
Determination of the atmospheric parameters of stars
Abstract : The exploration of the formation and evolution of the Milky Way proceeds through the study of its stellar populations. This notably requires the measurement of the atmospheric parameters of large samples of stars. Large spectroscopic databases, like SDSS, RAVE and LAMOST are available or in the course of acquisition. I will present the methods used or foreseen to analyse these samples. I will in particular describe my work to develop and use ULySS, University of Lyon Spectroscopic analysis Software (http://ulyss.univ-lyon1.fr/) to measure automatically the temperature, surface gravity and metallicity of stars. ULySS implements a full spectrum fitting method and performs a non-linear minimization between an observed spectrum an a model.
2009 Feb. 6th, : Georges Meynet (Obs. Geneve)
The massive stars in the Universe : From the first stars to the gamma Ray Bursts
Abstract : The evolution of massive stars is deeply affected by mass loss and rotation. After a brief presentation on how these two processes interact at various metallicities, we shall show how rotation might affect, in a very important manner, the evolution of the first stellar generations in the Universe. Consequences for the early phases of the chemical evolution of galaxies, for explaining the C-rich Ultra Metal Poor stars, the origin of the chemical anticorrelations and of He-rich stars in globular clusters will be discussed. The question of the nature of the GRB progenitors will be also evoked.
2009 January 22th, Observatoire : Mina Koleva (Obs. Lyon & Obs. Sofia)
Stellar populations in dwarf elliptical galaxies
Abstract : The past of the galaxies is imprinted in their stellar populations. The most numerous galaxies in the Universe are the dwarf ellipticals (dEs), left-over of the hierarchical mass-assembly. Consequently, the evolution of the Universe can be read from the stellar populations of the dwarf elliptical galaxies. In this thesis I present and validate an accurate and eﬃcient method to study the age and metallicity evolution in stellar systems using spectra integrated along the line-of- sight. It was extensively tested and validated on a library of 40 Galactic globular clusters and applied to a sample of 16 dwarf elliptical galaxies in cluster and group environments and to NGC 205. The comparison between the integrated light measurement and CMD estimates of the clusters age and metallicity shows that : (1) The metallicity estimations of the old stellar populations are accurate up to 0.15 dex ; (2) the models have to be improved to account for the blue-horizontal branchs and the blue stragglers stars, but this problem can be presently solved by adding ad’hoc blue stars to the models ;(3) the diﬀerent synthesis models give similar results providing large libraries are used for the synthesis. Further, encouraged by the good results, I applied the full spectrum ﬁtting to dwarf elliptical galaxies observed with FORS at the VLT. The most striking results are : (1) The small galaxies start to form stars in the early Universe (at similar ages like the ages of Galactic globular clusters) and their star formation history is in agreement with the down-sizing scenario of galaxy evolution. 40% of the stellar mass of dEs was formed before z=1 ; (2) The dwarf ellipticals have in general decreasing metallicity from the centre by typically 0.5 dex in one half- light radius. These gradients are already present in the old population. The numerical simulations predict them, but need a longer time to construct them. The new observations will allow to improve the models. The study of NGC 205, galaxy of the Local Group of a similar mass, indicate similar characteristics, suggesting that they do not depend on the environment.
2009 January 16th, Observatoire : Syksy Räsänen (Univ. Genève)
Explaining late-time acceleration with structure formation
Abstract : I will discuss the possibility that the observed accelerated expansion of the universe could be explained by accounting for the effects of the inhomogeneities related to structure formation, without introducing a new matter component or modifying gravity. I will present a semi-realistic model where the observed timescale of around 10 billion years emerges from the physics of structure formation. I will also show how the average expansion rate is related to the observable redshift and luminosity distance in a clumpy universe.