Accrétion | EPFL Graph Search

L’accrétion est, en astrophysique, en géologie, en médecine et en météorologie, la constitution et l'accroissement d'un corps, d'une structure ou d'un objet, par apport et/ou agglomération de matière, généralement en surface ou en périphérie de celui-ci. Astronomie L'accrétion désigne la capture de matière par un astre sous l'effet de la gravitation. L'accrétion a lieu dans de nombreux contextes astrophysiques, lorsqu'un objet compact est situé dans un environnement de matière diffuse, notamment :

les étoiles en formation ;
les planètes en formation ;
les novae ;
les trous noirs, en particulier dans les noyaux actifs de galaxies.

L'accrétion transforme l'énergie mécanique du système objet-matière en énergie thermique (chaleur) et, par voie de conséquence, en rayonnement. Deux phénomènes concourent à libérer cette énergie :

la matière orbitant autour de l'étoile est freinée par des processus dissipatifs (par ex. viscosité turbulente) et finit par tomber sur l'obje

On the Powder Formation in the Plasma Enhanced Chemical Vapor Deposition Process for the Deposition of SiOx Barrier Coatings

Marina Ricci

Nowadays thin films play an important role in everyday life and in industries. Thin film technology has been developed primarily for the growing demand for development of smaller and smaller devices that require advanced materials and new processing techniques suitable for micro and nano technology. One of the methods of producing thin films is by means of plasma technology. This is a technique by which plasma, also known as 'the fourth state of matter', is used to generate ions, electrons and radicals which deposit at a surface where they will form a thin film. The source of these depositing species could be either gaseous, liquid or solid in origin. Most of the time a plasma is generated by means of an electrical discharge. Plasma technologies are used for various applications. One of these is Plasma Enhanced Chemical Vapor Deposition (PECVD). PECVD systems have widespread applications in the electronic sector, because of their exibility in depositing different films such as silicon oxide thin films (SiOx). These films, currently deposited by organosilicon compounds, such as hexamethyldisiloxane (HMDSO), have dielectric and good barrier properties. The aim of the work at CRPP consists in optimizing PECVD processes for the deposition of these films on polymers. In this study, the species in gaseous phase and the powder produced in oxygen-diluted HMDSO plasmas were experimentally characterized in a radiofrequency (RF) capacitively-coupled reactor at 13.56 MHz. The gaseous phase of these plasmas and the particle formation were studied by in-situ infrared absorption spectroscopy and optical emission spectroscopy. The study of the powder formation is important because the particle deposition imposes an upper limit on the deposition rate. Particle formation was firstly studied as a function of HMDSO/O2 ratio. Analysis of the time evolution of the infrared absorption spectra gives the development of the size, the number density, the structure and the particle composition. At high oxygen dilution the formation of large particles is observed. The analysis of HMDSO/O2 plasma at low O2 content suggests that at the beginning of the process very small particles are formed by polymerization; the same particles do not grow much and keep sizes around 50 nm. The admixture of inert or non-reactant gases, such as Ar or N2, into the plasma promotes polymerization of the monomer; a high value of one of these inert or non-reactant gases leads to small powder particle size, since only polymerization takes place in this case. Particle size around 50 nm is also found in this case. At the same time we see a decrease of the carbon content and a weak reduction of the stoichiometry in SiOx indicated by the shift of the SiOSi asymmetric stretching vibration. This reduction also influences the deposited layers, less oxidized and, thus, quite far from a stoichiometric character. Hexamethyldisilazane (HMDSN) and tetramethylsilane (TMS) are other reagents that have been identified as alternatives to HMDSO. The in-situ infrared absorption spectra show that HMDSO/O2 and HMDSN/O2 plasmas are quite similar, but in the second case a lower SiOSi stretching peak intensity, due to NH and SiN peaks, is seen. TMS/O2 and HMDSN/O2 plasma spectra show a low SiOSi bending and stretching peak intensity, due also to the oxygen atom absence in their chemical structure. Two separate chapters are respectively devoted to the silicon nitride thin films (SiNx) and plasma crystal formation. SiNx layers are deposited by three different organosilicon compounds, such as HMDSN, TMS and bis(dimethylamino)dimethylsilane (BDMADMS). No powder formation is visualized in this contest. The ex-situ infrared absorption spectra show the difficulty to produce stable SiNx films; they present a low SiN peak intensity and are quite hygroscopic. Furthermore, in HMDSO/O2 plasmas at high oxygen dilution and high pressure, later in time into the discharge, accretion leads to larger and larger particles which can finish up in particles forming a plasma crystal. The study of RF harmonics shows the presence of instabilities, clear indications of the passage from small particulate to plasma crystal structure.

EPFL2011

Towards Efficient and Accurate Numerical Simulations of Galaxies using Task-based Parallelism and Application to Dwarf Galaxies

Loïc Hausammann

Numerical simulations are of a tremendous help to understand the growth of non-linear cosmological structures and how they lead to the formation of galaxies. In recent years, with the goal of improving their prediction power, new hydrodynamical techniques and physical models have been developed. Beside those improvements, the community must be ready for what could be a major paradigm change, the exascale supercomputer that will allow for the simulation of higher resolution and/or larger cosmological volume. Being ready to fully benefit from those upcoming facilities requires an important effort in adapting existing codes.Among the domains that will benefit from the exascale computing are dwarf galaxies and their low mass end, the ultra faint dwarf galaxies (UFD). Since twenty years, dwarfs have become an important research topic in astrophysics due to recent observing facilities allowing a large number of unprecedented observations. Simulations of such galaxies require extremely high resolution down to the individual star level and, thus, require the adaptation of existing physical models.In the first part of my thesis, I extended a previous work performed with the code GEAR on understanding the formation of dwarf galaxies in a cosmological context. I studied the ram pressure stripping mechanism due to the hot halo of the Milky Way and how it modifies the observed properties of dwarf galaxies. I demonstrated that the thermal pressure due to the hot halo on the dwarf's gas can strongly impact the effect of ram pressure.To ensure the quality of our simulations, I have participated in the AGORA project that aims at comparing the predictions of different simulation codes and understanding the underlying differences. In the last AGORA paper, we have performed the first comparison of cosmological simulations using common models for the radiative cooling and star formation.In the second part of this thesis, an important effort has been dedicated to migrate our physical models from our code GEAR towards SWIFT. Thanks to the improved performance of SWIFT (speedup of 7.65x) and the successful migration, our code is now fast enough to simulate UFDs at the resolution of individual stars. On the physics side, our star formation and stellar feedback methods need to be further improved due to the high resolution required in UFDs. To solve this issue, I designed and started the implementation of a new star formation scheme based on a new type of particles called "sink particles".Going towards the exascale also requires a better design of the output system previously based on snapshots of the simulation saved at regular time intervals. In this purpose, I introduced the "Continuous Simulation Data Stream" (CSDS) specially designed for simulations where gravity is producing strong differences in timescale between particles or cells. The CSDS reduces the disk space used by a single simulation while improving the time resolution of the output and making the analysis easier and flexible.Finally, while the simulation code and the associated physical models have a tremendous impact on the quality of the simulations, the initial conditions need to be carefully designed as they can deeply impact the results. Using a Bayesian approach, I have developed my own code that generates constrained initial conditions that reproduce the Local Group.

EPFL2021

Measuring accretion disk sizes of lensed quasars with microlensing time delay in multi-band light curves

Vivien François Bonvin, Hung-Hsu Chan, Frédéric Courbin, Martin Raoul Robert Millon, Karina Alexandra Rojas Olate

Time-delay cosmography in strongly lensed quasars offers an independent way of measuring the Hubble constant, H-0. However, it has been proposed that the combination of microlensing and source-size effects, also known as microlensing time delay, can potentially increase the uncertainty in time-delay measurements as well as lead to a biased time delay. In this work, we first investigate how microlensing time delay changes with assumptions on the initial mass function (IMF) and find that the more massive microlenses produce the sharper distributions of microlensing time delays. We also find that the IMF has a modest effect on the magnification probability distributions. Second, we present a new method to measure the color-dependent source size in lensed quasars using the microlensing time delays inferred from multi-band light curves. In practice, the relevant observable is the differential microlensing time delays between different bands. We show from a simulation using the facility as Vera C. Rubin Observatory that if this differential time delay between bands can be measured with a precision of 0.1 days in any given lensed image, the disk size can be recovered to within a factor of 2. If four lensed images are used, our method is able to achieve an unbiased source measurement within an error on the order of 20%, which is comparable with other techniques.

EDP SCIENCES S A2021