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Calendrier des évènements

Mars,2017

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Jeudi, 2 Mars, 2017 - 11:30
Bât. 121, salle IDOC
I. Carucci (SISSA, Trieste, Italie)

The LCDM model experiences small scale problems that could be solved by allowing dark matter to have intrinsic thermal velocities, i.e. warm dark matter (WDM). In this talk I discuss the impact of WDM on the 21cm power spectrum, by means of high resolution hydrodynamical N-body simulations of different dark matter (DM) scenarios and different models of neutral hydrogen (HI) spatial distribution. I forecast the bounds that the Square Kilometre Array (SKA) will place on the DM particle mass. A major drawback of the 21cm observations is foreground contamination, expected to be orders of magnitudes higher than the signal. To address this problem, I propose to cross-correlate the Lyman-alpha forest with the 21cm maps. I show how the two fields are completely anti-correlated on large scales and I use the anisotropy of the power spectra in redshift-space to determine the values of the bias parameters of both fields. My results point out that linear theory is capable of reproducing the shape and amplitude of the cross-power up to rather non-linear scales. Finally, I show how the 21cm-Lya cross-power spectrum can be detected by combining data from a BOSS-like survey together with 21cm intensity mapping observations by SKA1-MID with a S/N ratio higher than 3 in k range of [0.06, 1] h/Mpc.

 
 
 
 
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Jeudi, 9 Mars, 2017 - 11:30
Bât. 121, salle 123
M. Morfouli (SYRTE, Paris)

According to Westfall “The very heart of the new natural philosophy was mechanics, the science of motion. Mechanics required the measurement of a third dimension, time. The creation of the new world of precision was intimately connected to the success of science in learning to measure time.”

The study of local motion and the free fall in particular, was in the heart of the 17th century research on Natural Philosophy. The establishment of mathematical relations between time, distance, velocity and acceleration was a great challenge, a task carried out by Galileo with fruitful results used later by scholars, including Newton.  

In his major work Philosophiae Naturalis Principia Mathematica (1687) Newton presents his theory of universal gravitation. The title of this book reveals an important aim of its author: to introduce mathematical certainty in Physical studies.

 In the 3rd book called De Mundi Systemate Newton demonstrates his theory. He compares two free falls: that of a heavy body near the Earth’s surface and that of the Moon towards the Earth’s center, considering only the force of gravity, the so-called Moon test (Prop. IV, Theorem IV). A little further in the same book (Prop. XIX & XX) Newton proposes the application of universal gravitation in order to solve several astronomical and physical problems, such as determining the Earth’s shape. If the theory is correct (the inversed square law) then the force of gravity must be different to different latitudes of the Earth hence the shape of the Earth is not a sphere but an oblate spheroid. One way to confirm this hypothesis is to compare the different lengths of Seconds Pendulums (T=2sec) in different latitudes.

In order to realise these mathematical demonstrations Newton makes use of a number of data, either from observations, essentially astronomical, or from results of calculus. Among the data used, certain concerns time measurement. In the case of the Moon Test one needs to be able to measure or calculate the time of the fall for a certain distance. In the case of the Shape of the Earth one needs to be able to determine the length of a time- measuring instrument. Time measurement thus is well implicated in this affair.

This presentation aims at giving a response to the following question: Is the emerging, increasing precision in time measurement a crucial element for the creation and the confirmation of Newton’s Universal Gravity?

 
 
 
 
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Jeudi, 16 Mars, 2017 - 11:30
Bât. 121, salle 123
F. Vazza (IRA, Bologne, Italie)

On large scales cosmic matter is distributed in a web consistent of clusters, filaments, walls and voids. While the dark-matter skeleton of the cosmic web is closely traced by galaxies and galaxy clusters, the large-scale gaseous distribution is more hardly detected. In particular, the warm-hot intergalactic component (T~10^5-10^7K) where nearly half of the "missing" cosmic baryons should be located, has yet to be firmly detected. This situation may change within the next decade, thanks to the new generation of telescopes that will soon survey the radio sky: LOFAR, MWA, Meerkat, ASKAP and the Square Kilometer Array. Non-thermal components, relativistic particles and magnetic fields are thought to have a spatial distribution that is broader than that of thermal baryons. Based on advanced cosmological simulations of extragalactic magnetic fields, I will discuss the capabilities of the new generation of radio telescopes in detecting the synchrotron emission from the cosmic web. Detecting the very elusive and large-scale signal from the cosmic web will also enable us to better understand the origin of extragalactic magnetic fields, which still is an astrophysical puzzle.

 
 
 
Vendredi, 17 Mars, 2017 - 11:00
Bât. 121, salle 45
Helen Mason (DAMP, Cambridge)

Spectroscopic Observations with Hinode/EIS and IRIS provide us with the opportunity to measure plasma diagnostics (DEM, electron density and flows). These data can be used with imaging data (SDO, Hinode/XRT) to study the temperature distribution of small energetic solar features. Such parameters are crucial for distinuishing between theoretical models. Here we discuss blue-shifted emission seen during small flares by IRIS  with the FeXXI (1354A) spectral line, together with electron densities measurements obtained using IRIS OIV and SIV spectral lines (Polito et al, 2016). We also investigate the relationship between cool and hot emission in solar jets (Mulay et al, 2016).

 
 
 
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Jeudi, 23 Mars, 2017 - 11:30
Bât. 121, salle 123
P. Petit (IRAP, Toulouse)

La dernière décennie a marqué un tournant dans l'étude du magnétisme des étoiles. Avec ESPaDOnS au CFHT et NARVAL au TBL, une nouvelle génération de spectropolarimètres stellaires a vu le jour, affichant pour la première fois des performances suffisantes pour s'engager dans des mesures systématiques de champs magnétiques dans la quasi-totalité du diagramme de Hertzsprung-Russell, élargissant ainsi radicalement la gamme d'objets accessibles à ces études. Dans cet exposé, je détaillerai les principaux résultats obtenus dans ce cadre, pour différentes classes d'étoiles, mettant en évidence différentes propriétés physiques liées à des objets pris à des masses et des âges différents. Dans un premier temps, j'illustrerai les progrès dans l'observation des étoiles froides de séquence principale, permettant l'observation directe du produit de la dynamo globale de jumeaux solaires de faible activité, et le suivi à long terme de leurs cycles magnétiques. Dans un deuxième temps, je détaillerai les résultats récents liés au magnétisme des étoiles froides évoluées, qui offrent de nombreuses variétés de comportements actifs en fonction de leur passé sur la séquence principale et de leur stade évolutif. Enfin, je présenterai les prolongements possibles de ces études dans les années à venir, dans le contexte de l'arrivée de SPIRou au CFHT fin 2017, et de Neo-NARVAL au TBL courant 2018.

 
 
 
 
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Lundi, 27 Mars, 2017 - 11:30
Bât. 209, salle 67
M. Cheung (LMSAL, Palo Alto, USA)

In part one of this seminar, we present a validated method to perform differential emission measure (DEM) inversions on extreme ultraviolet imaging observations of the solar corona taken by the Atmospheric Imaging Assembly onboard NASA’s Solar Dynamics Observatory. We begin with a description of the method and proceed to discuss test cases used for validation. We then present applications of the method to a number of science cases, including the (1) thermal structure of active regions and emerging flux regions, (2) magnetic reconnection outflows and (3) chromospheric evaporation in solar flares. In part two, we present results from a 3D radiative MHD model of a solar flare and compare synthetic remote sensing diagnostics with flare observations (e.g. GOES X-ray light curve, temperature dependence of footprint evaporation flows and x-ray spectra).

 
 
 
 
Jeudi, 30 Mars, 2017 - 11:30
Bât. 121, salle 123
A. Bracco (CEA, Saclay)

Investigating the physics of the interstellar medium (ISM) is key to understand how our Galaxy works and evolves. The ISM is the fuel of the Galactic engine, the matter reservoir of the Milky Way to allow for new star formation. This interstellar plasma is a melting pot of cosmic rays, multiphase gas, and dust particles, all tightly coupled with magnetic fields. It is through their interactions that a complex cycle, involving gravity, several phase transitions, and magneto-hydrodynamic turbulence, leads diffuse/warm matter to condense into denser/colder regions, where stars eventually form. However, the detailed processes of this matter cycle are still unclear. For decades, one of the most difficult challenges of observational astrophysics has been the characterization of magnetic fields along this evolutionary sequence.
Today, thanks to the technological breakthrough of new experiments, such as the Planck satellite, we are now entering a new era to probe the magnetic properties of the ISM.

In this talk, after reviewing the state-of-the-art investigation of magnetic fields in the Milky Way, I will give an overview of the recent Planck results on the magnetized ISM. Using all-sky maps of linear polarization at sub-millimeter wavelengths, for the first time, we were able to trace the magnetic field structure of our own Galaxy with unprecedented statistics. I will focus on several aspects of the data analysis to show the relevance of magnetic fields in the Galactic environment, from the diffuse medium to the regions where early star formation takes place.

I will conclude with interesting perspectives for the future to study the magnetic properties of the Milky Way by combining multiple probes of the ISM with existing and upcoming experiments, such as Planck, LOFAR, and SKA.

 
 
 
 
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