PhD subjects

10 sujets IRFU

Dernière mise à jour : 17-12-2017


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• Astrophysics

 

Central engine of extreme explosions: magnetic field amplification in proto-neutron stars

SL-DRF-18-0298

Research field : Astrophysics
Location :

Service d'Astrophysique (SAp)

Laboratoire de Théorie et de Modélisation

Saclay

Contact :

Jérôme Guilet

Thierry FOGLIZZO

Starting date : 01-10-2018

Contact :

Jérôme Guilet

CEA - DRF/IRFU/SAp/LTM

01 69 08 04 37

Thesis supervisor :

Thierry FOGLIZZO

CEA - DRF/IRFU/SAp/LTM

01 69 08 87 20

Personal web page : http://wwwmpa.mpa-garching.mpg.de/~jguilet/

Laboratory link : http://irfu.cea.fr/Sap/Phocea/Vie_des_labos/Ast/ast_groupe.php?id_groupe=1250

More : http://irfu.cea.fr/Sap/Phocea/Vie_des_labos/Ast/ast_technique.php?id_ast=4201

The collapse of the iron core of massive stars gives rise to some of the most violent explosions of the universe. The physical mechanism driving these explosions is, however, not well understood and its theoretical description is one of the big challenges of modern astrophysics. The most extreme of these explosions - in terms of kinetic energy or luminosity - suggest the likely presence of a rapid rotation as well as a strong magnetic field which can efficiently extract this large kinetic energy reservoir. They could indeed correspond to the birth of the most magnetised neutron stars, called magnetars, which dipolar magnetic field of the order of 10^15 G is the most intense known in the present universe. This PhD project will endeavour to answer one major open question: the origin of this extreme magnetic field. The process widely considered as the most probable source of this magnetic field is the development of a magnetohydrodynamic instability called the magnetorotational instability (MRI). Numerical simulations of a small patch of a forming neutron star have demonstrated an efficient amplification of the magnetic field (e.g. Guilet & Müller 2015). This PhD project aims at determining for the first time the efficiency at generating a large-scale magnetic field coherent over the whole neutron star. This is crucial both for the launch of the explosion and to explain the properties of galactic magnetars. The project will consist primarily in developing numerical simulations of a global model of a proto-neutron star with the code MagIC. These simulations will allow to study the development of the magnetorotational instability with a focus on the generation of a large-scale magnetic field. These results will then be used to develop an analytical prescription of the magnetic field amplification that can be used in a model of the full explosion.

Cosmological constraints from the large scale quasar surveys of eBOSS and DESI

SL-DRF-18-0294

Research field : Astrophysics
Location :

Service de Physique des Particules (SPP)

Groupe Bao

Saclay

Contact :

Etienne Burtin

Starting date : 01-10-2018

Contact :

Etienne Burtin

CEA - DRF/IRFU/SPP

0169085358

Thesis supervisor :

Etienne Burtin

CEA - DRF/IRFU/SPP

0169085358

Laboratory link : http://irfu.cea.fr/Phocea/Vie_des_labos/Ast/ast_groupe.php?id_groupe=3428

The observation of the acceleration of the expansion of the Universe has triggered a wide research program in order to identify and understand the phenomenon of 'dark energy'.



For the last ten years, the imprint of baryonic acoustic oscillations (BAO) on the galaxy spatial distribution has been used as a 'standard ruler' to probe the geometry of the Universe and to constrain the cosmological parameters. At present, the community os converging towards the use of the full anisotropic clustering of tracers of matter to fully constrain the expansion rate of the Universe and to test possible modifications of gravity through the measurement of the growth rate of cosmic structures.



To perform these measurements, we use the data from the large spectroscopic surveys eBOSS (2014-2019) and DESI (2019-2025). These surveys allow for the measurement of the redshift (z) of millions of astrophysical objects for which we can build a 3D map of the matter distribution in the Universe. In the proposed subject, we will use the quasars, the brightest sources of light, to probe the Universe in an almost unexplored redshift domain 0.8


Furthermore, the correlation between quasars and other cosmological probes such as the lensing of the cosmic microwave background is a way to access new observables. This is a promising technique to test potential modifications of general relativity.

Sparse semi-parametric recovery of astronomical radio-images

SL-DRF-18-0561

Research field : Astrophysics
Location :

Service d'Astrophysique (SAp)

Laboratoire CosmoStat (LCS)

Saclay

Contact :

Jean-Luc STARCK

Starting date : 01-10-2018

Contact :

Jean-Luc STARCK

CEA - DSM/IRFU/SAp/LCS

01 69 08 57 64

Thesis supervisor :

Jean-Luc STARCK

CEA - DSM/IRFU/SAp/LCS

01 69 08 57 64

Personal web page : http://jstarck.cosmostat.org

Laboratory link : http://www.cosmostat.org

Radio-astronomy is entering the golden age, with new very sensitive instruments such as LOFAR, LWA and SKA). This allows us to reinvestigate the radio emission from ground. At these frequencies, both the close and distant universe can be studied. Radio instruments require however to reconstruct first images from a subset of Fourier components. Very nice progress have been made in the recent years in this domain, thanks to recent breakthrough in applied mathematics such sparse theory and compressed sensing theories [1,2]. Once the images are restored, the second phase can start, consisting generally in detecting sources and deriving their electromagnetic spectrum. The challenge is also to be able to process very large images of 10k x 10k pixels and to take at the same time into account Direction Dependent Effects (DDE).

The goal of this PhD thesis is to propose a new framework for analysing radio data set, where restoration and interpretation steps are done jointly. This involves a semi-parametric approach where regularisation is performed in the parameters space.





References:

H. Garsden, J. Girard, J.-L. Starck, S. Corbel, C. Tasse et al, "LOFAR Sparse Image Reconstruction", Astronomy and Astrophysics, 575, A90, 2015.

M. Jiang, J. Bobin and J.-L. Starck, "Joint Multichannel Deconvolution and Blind Source Separation", SIAM Journal on Imaging Sciences, 10, 4, pp. 1997-2021, 2017.

The physics of giant star-forming regions in primordial galaxies from a synergy of observations and simulations.

SL-DRF-18-0323

Research field : Astrophysics
Location :

Service d'Astrophysique (SAp)

Laboratoire de Cosmologie et d'Evolution des Galaxies (LCEG)

Saclay

Contact :

Emanuele DADDI

Frédéric BOURNAUD

Starting date : 01-10-2018

Contact :

Emanuele DADDI

CEA - DRF/IRFU/SAp

Thesis supervisor :

Frédéric BOURNAUD

CEA - DRF/IRFU/SAp/LCEG

01 69 08 55 08

The physics of giant star-forming regions in primordial galaxies from a synergy of observations and simulations --





The morphology of star forming galaxies in the distant Universe at redshift z=1-3, is remarkably different from that of nearby spirals. They have irregular morphologies, often dominated by giant star forming regions dubbed « giant clumps ». This is thought to be driven by their very large gas fractions and strong turbulence: gas-rich galaxies can become violently unstable and fragment in such giant clumps.



However, a quantitative understanding of the nature and physical properties of these clumps is still lacking Hotly debated topics of contending remain, regarding the distribution of clumps masses, star formation rates, sizes, clumps formation rates and lifetime. It also remains unknown whether they survive stellar feedback processes, in which case they can drive the growth of galactic bulges and the fueling of supermassive black holes at the center of galaxies.



We propose a PhD project to explore a state of the art approach to this topic, tracking 3 complementary lines of research on:

1) the physical characterization of clumps properties from observations,

2) systematic and quantitative comparison of observations to simulations,

3) running suites of very high resolution numerical simulations with an improved modeling of star formation and feedback.



These complementary lines will bring a robust understanding of the nature of the giant clumps and their role in shaping disks, bulges and central black holes. They will also provide a new way to constrain the gas fraction in galaxies at various epochs, which is another highly debated issue in galaxy formation. The proposed work will ideally prepare future developments with the use of JWST, and comparison to forthcoming ALMA data. The proposed thesis will also employ numerical simulations performed on the largest supercomputers in France and Europe.

Cosmological analysis of the XXL cluster survey

SL-DRF-18-0565

Research field : Astrophysics
Location :

Service d'Astrophysique (SAp)

Laboratoire de Cosmologie et d'Evolution des Galaxies (LCEG)

Saclay

Contact :

Marguerite PIERRE

Starting date : 01-09-2018

Contact :

Marguerite PIERRE

CEA - DRF/IRFU/SAp/LCEG

0169083492

Thesis supervisor :

Marguerite PIERRE

CEA - DRF/IRFU/SAp/LCEG

0169083492

Personal web page : htt://irfu.cea.fr/xxl

Laboratory link : http://irfu.cea.fr/Sap/

Clusters of galaxies are – along with supernovae, CMB and baryonic acoustic oscillations – are a major probe for the various cosmological scenarios. In particular, cluster number counts are very sensitive to the Dark Energy equation of state that depends on the volume (geometrical effects) and on the growth rate of the structures (gravitational effect). Currently, several cluster studies show a puzzling tension with the cosmological parameters derived from the CMB.



The thesis will take place within the framework of the XXL project, which is the largest extragalactic survey performed by XMM (50 deg2), the X-ray satellite of the European Space Agency. The ultimate goal is toe constrain the dark energy equation of state using the some 500 clusters of galaxies newly discovered in the survey. In addition to the X-ray band, numerous observations are available in other wavebands (infrared, optical, millimetre, radio) as well as high-resolution numerical simulations. The XMM observations were performed from 2011 to 2013 and the first series of 14 publications involving limited bright samples was published in 2016.

The proposed thesis is particularly timely because cluster counts from two different surveys, with very different detection methods and mass ranges (Planck S-Z 2015 article XXIV ; XXL 2016 article II) appear to be incompatible with CMB cosmology. The work will take place during the final phase of the project and consists in a detailed study of the parameters impacting on the cosmological analysis. This regards especially the evolution of the cluster physical properties that influence their detection as well as their mass estimates. The final goal will be to adequately model these factors in the global cosmological analysis, extending the current results with 100 clusters to the complete sample of some 500 clusters.



Tools that will be used during the thesis work:

Cluster evolutionary models; X-ray pipeline; multi-wavelength observations of clusters; results from numerical simulations.

Cosmological analysis package; it follows an original method developed at Saclay, based on the forward modelling of the X-ray observable parameters.



All is available and the student will have to get rapidly acquainted to them.



Working context: worldwide consortium gathering some 100 scientists and organised in well-defined sub-projects.

http://irfu.cea.fr/xxl

See page ‘publications’ for detailed presentations of the XXL early results at international conferences.

References for the cosmological code :

https://arxiv.org/abs/1609.07762

https://arxiv.org/abs/1710.01569



We are seeking excellent candidates and having a good practice of English.

A search for gamma ray bursts with CTA

SL-DRF-18-0269

Research field : Astrophysics
Location :

Service d'Astrophysique (SAp)

Laboratoire d'Etudes des Phénomènes Cosmiques de Haute Energie (LEPCHE)

Saclay

Contact :

Thierry STOLARCZYK

Starting date : 01-09-2018

Contact :

Thierry STOLARCZYK

CEA - DRF/IRFU/DAp/LEPCHE

+33 1 69 08 78 12

Thesis supervisor :

Thierry STOLARCZYK

CEA - DRF/IRFU/DAp/LEPCHE

+33 1 69 08 78 12

More : http://neutrini.free.fr

The CTA observatory is a next-generation ground-based instrument for exploring the sky in gamma rays at very high energies, with a sensitivity ten times better than the existing and an amazing new capacity for the search of transient source counterparts. The thesis work is a contribution to the performance optimisation of the real-time search strategy for transient sources, in particular gamma ray bursts and gravitational wave counterparts.

The Univers with MeerKAT: searching for explosive and exotic high energy transients.

SL-DRF-18-0657

Research field : Astrophysics
Location :

Service d'Astrophysique (SAp)

Laboratoire d'Etudes des Phénomènes Cosmiques de Haute Energie (LEPCHE)

Saclay

Contact :

Stéphane CORBEL

Starting date : 01-09-2018

Contact :

Stéphane CORBEL

Université Paris 7 - DRF/IRFU/SAP/LEPCHE

01 69 08 45 62

Thesis supervisor :

Stéphane CORBEL

Université Paris 7 - DRF/IRFU/SAP/LEPCHE

01 69 08 45 62

Laboratory link : http://www.thunderkat.uct.ac.za/

More : https://arxiv.org/abs/1711.04132

SL-DRF-18-0550

Research field : Astrophysics
Location :

Service d'Astrophysique (SAp)

Laboratoire d’études de la formation des étoiles et du milieu interstellaire

Saclay

Contact :

Philippe ANDRE

Philippe ANDRÉ

Starting date : 01-10-2018

Contact :

Philippe ANDRE

CEA - DSM/IRFU/SAp/LFEMI

0169089265

Thesis supervisor :

Philippe ANDRÉ

CEA - DSM/IRFU/SAp/LFEMI

0169089265

Study of interstellar grains in the JWST era

SL-DRF-18-0283

Research field : Astrophysics
Location :

Service d'Astrophysique (SAp)

Laboratoire d’études de la formation des étoiles et du milieu interstellaire

Saclay

Contact :

Frédéric Galliano

Starting date : 01-10-2018

Contact :

Frédéric Galliano

CNRS - DSM/IRFU/SAp/LCEG

01 69 08 18 21

Thesis supervisor :

Frédéric Galliano

CNRS - DSM/IRFU/SAp/LCEG

01 69 08 18 21

The interstellar medium, filling the volume between the stars of a galaxy, is constituted of two main components: gas and dust. Dust grains are small solid particles, mainly composed of silicate and carbonaceous materials. They play a major role in the physics of the interstellar medium, although accounting for only one percent of its mass. Indeed, they absorb, and reemit in the infrared, an important fraction of the power radiated by stars and accretion disks. In particular, star forming regions are completely opaque in visible light. Only the infrared radiation, emitted at 99% by dust, allows us to study them. Grains are also responsible for the gas heating by photoelectric effect, in photodissociation regions (PDR). Finally, grains are catalysts of numerous chemical reactions, including the formation of dihydrogen, the most abundant molecule in the universe.



The properties of these dust grains (abundance, chemical composition, size distribution, etc.), as well as their evolution, are, however, still poorly known. This is the direct consequence of the complexity of this component and of the lack of observations discriminating different models. These uncertainties are affecting numerous aspects of our knowledge in astrophysics: mass measurements, unreddening (i.e. correction of the extinction along the line of sight), detailed PDR models, etc. Refining our understanding of dust is also crucial to understand the interstellar lifecycle, as grains regulate several processes controlling this cycle. An accurate understanding of grain

physics is thus necessary to understand galaxy evolution.



An approach, to tackle these open questions, consists in studying the way observed grain properties vary with the physical conditions they experience. Such empirical relations, if they are precise enough, allow us to remove some degeneracies on different models. The thesis that we are proposing focuses on the detailed study of the smallest grains (radius < 10 nm) and polycyclic aromatic hydrocarbons (PAH). These components of the interstellar medium radiates out-of-equilibrium in the mid-infrared (5–40 microns). This is the wavelength domain that contains most of the solid state resonance features.



This study will focus on several nearby galaxies, including the Magellanic clouds. The interest of nearby galaxies compared to the interstellar medium of our galaxy resides in the diversity of physical conditions (metallicity, radiation field intensity, etc.)



Several studies have already been published on this topics, especially with the Spitzer space telescope. However, most have been somehow superficial. Numerous aspects remain to be studied: (i) the correlation of the main aromatic bands with the physical conditions; (ii) constraining the evolution of their size distribution; (iii) identifying and modelling several bands of solids in star forming regions. One of the originalities of this thesis will consist in developing a sophisticated method to model the data. Indeed, most previous studies have performed simple linear decompositions. We propose that the student develop a hierarchical Bayesian decomposition code to analyze infrared spectra, with constraints provided by atomic, molecular and solid-state databases. This type of code allows to physically model the sample and to statistically model the distribution of parameters, simultaneously. It allows us to remove several degeneracies and to

extract the maximum information from the data, taking into account the various sources of uncertainties, without overinterpreting the observations. We have recently developed such a code to model spectral energy distributions, and the results are convincing. This new tool and its meticulous application to the data are the warranty of a precise and original interpretation of the physical processes taking place in the studied regions.



The James Webb Space Telescope (JWST), which will be launched in 2019, will observe the mid-infrared domain with

unprecedented sensitivity and saptial resolution. The methods developed during the thesis could be applied to these new data.

Study and use of the SPICA polarimetric bolometers for submillimeter astrophysics from ground and stratospheric balloons.

SL-DRF-18-0428

Research field : Astrophysics
Location :

Service d'Astrophysique (SAp)

Laboratoire de Détection Spatiale

Saclay

Contact :

Vincent REVERET

Louis RODRIGUEZ

Starting date : 01-10-2018

Contact :

Vincent REVERET

CEA - DSM/IRFU/SAp/LSIS

01 69 08 74 02

Thesis supervisor :

Louis RODRIGUEZ

CEA -

More : http://irfu.cea.fr/Sap/

The Herschel submillimeter space observatory has revolutionized some areas of astrophysics, such as star formation, showing that stars are formed mainly in filaments of gas and dust. This rises an important question on the role of the magnetic field within these structures that cannot be solved by the current astrophysical instruments. The Safari-Pol instrument, installed on the future SPICA international space observatory and proposed by CEA Saclay, will be able to detect a wide variety of far-infrared filaments, as well as their possible associated magnetic field (through the polarization of the light coming from these regions).

Safari-Pol will contain 3 focal planes of cryogenic silicon bolometers currently developed by CEA (LETI and Saclay). These are innovative detectors, operating at 50 mK, with a very high sensitivity. Each pixel is intrinsically sensitive to the polarization of incident radiation.

For this thesis, the first step will be to model, and test the bolometers of Safari-Pol (measurement of sensitivity, time constant, cross-polarization, etc ...). Then, the candidate will propose and test one or more technological solutions to match the conditions of observation on a stratospheric balloon and on a ground-based telescope which are perfectly complementary to the space-based observations. This work will be accompanied by a study of the possible observation modes (ground-based and balloon-borne) and their expected performances.

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