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vendredi 10 janvier 2014.

Intervenant(e) : Ana Laura Serra (Univ. Torino)

Titre : Drawing dynamical properties of self-gravitating systems with the caustic technique : results and resources.

Résumé : The caustic technique uses galaxy redshifts alone to measure the escape velocity profiles of galaxy clusters to clustrocentric distances beyond their virial radius, where dynamical equilibrium does not necessarily hold. This feature allows us to estimate the mass profile and to identify the members of galaxy clusters up to 4r200.

We briefly review the caustic theoretical framework. Then we focus on the analysis of the ability of the caustic technique to measure the mass profile and to identify members in galaxy clusters. We show the application of this technique to real self-gravitating systems : galaxy clusters (for computing the mass) and dwarf spheroidal galaxies (for identifying members). Moreover, we explore the possibility of using the caustic technique to measure the accretion rate in galaxy clusters.

Finally, we present the Caustic App, a new graphical interface to apply the caustic formalism and to produce the appropriate plots. This tool is aimed at making easier the use of the technique and to shorten the time scales that writing the code from scratch would imply.


vendredi 13 décembre 2013.

Intervenant(e) : François Mignard (OCA, Lagrange)

Titre : Echelles de distance de l’Univers

Résumé : La question des dimensions dans l’Univers débute avec celle de la Terre par rapport aux repères habituels de distances et longueurs de la vie quotidienne. En 20 siècles, avec un nombre de principes fondamentaux très limité, on a successivement situé la taille du système solaire par rapport à la Terre, puis pris conscience de l’immensité qui nous séparait des premières étoiles, avant de comprendre au cours du XXe siècle que cette hiérarchie d’emboîtement s’étendait aux galaxies et amas de galaxies. Au cours de cet exposé je passerai en revu ce déroulement historique en indiquant à chaque fois les méthodes, toutes simples dans leur principe mais délicates dans leur mise en oeuvre, qui ont été appliquées pour évaluer les dimensions du cosmos. Ce séminaire est généraliste et s’adresse à tous les personnels.


vendredi 6 décembre 2013.

Intervenant(e) : Mario Flock (CEA, Saclay.)

Titre : Radiation Magnetohydrodynamics In Global Simulations Of Protoplanetary Disks.

Résumé : We present details from our first global 3D radiation magneto-hydrodynamic simulations of a stratified protoplanetary disk. Our aim is to study the thermal and dynamical evolution of protoplanetary disks in global simulations, including the physics of radiation transfer and magneto-hydrodynamic turbulence caused by the magneto–rotational instability.We develop a radiative transfer method based on the flux-limited diffusion approximation that includes frequency dependent irradiation by the central star. This hybrid scheme is implemented in the PLUTO code. The simulation parameters are chosen to approximate those of the system AS 209 in the star-forming region Ophiuchus. Starting the simulation from a disk in radiative and hydrostatic equilibrium, the magneto–rotational instability quickly causes magneto–hydrodynamic turbulence and heating in the disk. Turbulent dissipation heats the disk midplane and raises the temperature by about 15% compared to passive disk models. A roughly flat vertical temperature profile establishes in the disk optically thick region close to the midplane. The present work demonstrates for the first time that global radiation magneto–hydrodynamic simulations of turbulent protoplanetary disks are feasible with current computational facilities. This opens up the windows to a wide range of studies of the dynamics of protoplanetary disks inner parts, for which there are significant observational constraints.


Jeudi 21 novembre 2013

Intervenant(e) : Luc Gilles (TMT, Caltech.)

Titre : Adaptive Optics Control Algorithms Development and Prototyping for Next Generation Optical Telescopes

Résumé : After a cursory overview of astronomical Adaptive Optics (AO) to correct for atmospheric turbulence and of its challenges for Next Generation Optical Telescopes, we will delve in the development and analysis of wavefront control algorithms for high‐order and low‐order feedback loops, illustrated by examples from the Thirty Meter Telescope (TMT) cutting edge laser guide star (LGS) tomography AO system. In particular, we will discuss :
- The overall wavefront control architecture blending high‐order and low‐order loops
- Servo loop optimization based on transfer function analysis
- Integral and Kalman filter controllers


vendredi 27 septembre 2013

Intervenant(e) : Boud Roukema (Torun Centre for Astronomy, Nicolaus Copernicus University)

Titre : Virialisation-induced negative curvature as a physical explanation for dark energy

Résumé : By definition, the standard, homogeneous models of the Universe are likely to fail when the Universe is inhomogeneous. The virialisation fraction measures inhomogeneity. Its redshift evolution roughly matches the evolution of would-be "dark energy", if the "dark energy" is inferred from the observations by assuming a homogeneous model. This suggests that "dark energy" is a misinterpretation of the fact that we live in the inhomogeneous epoch of the Universe. A virialisation-induced approximation based on the scalar-averaging approach to defining cosmological parameters will be presented as a physically motivated, dark-energy-free approach to modelling extragalactic observations.


vendredi 6 septembre 2013

Intervenant(e) : Violeta Gonzalez (CPPM)

Titre : Modeling galaxies : from the luminosity function evolution to isolated pairs of galaxies as a cosmological test

Résumé : In this talk I will summarize the semi-analytical technique for populating with galaxies the simulated dark matter haloes. I will describe the characteristics of the model we have developed within the past year and then I will talk about two applications : i) The effect that the choice of stellar evolution models has over the predicted evolution of the luminosity function of galaxies ; ii) The study of pairwise velocity of isolated galaxy pairs as a first step toward using them as a cosmological test.


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