Dissertations / Theses on the topic 'Accrétion stellaire'
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Chaulagain, Uddhab Prasad. "Radiative shocks : experiments, modelling and links to astrophysics." Thesis, Paris 6, 2015. http://www.theses.fr/2015PA066734.
Full textRadiative shocks are strong shocks which are characterized by a plasma at high temperatures emitting an important fraction of its energy as radiation. Radiative shocks are found in many astrophysical systems, including stellar accretion shocks, supernovae remnants, jet driven shocks, etc. Recently, radiative shocks have also been produced experimentally using high energy lasers. Thus opening the way to laboratory astrophysics studies of these universal phenomena.In this thesis we discuss the results of an experiment performed on the Prague Asterix Laser System facility. Shocks are generated by focusing the PALS Infrared laser beam on millimetre-scale targets filled with xenon gas at low pressure. The shock that is generated then propagates in the gas with a sufficiently high velocity such that the shock is in a radiative flux dominated regime. We used different diagnostics to characterize these shocks. The two main ones include a radiography of the whole shock structure using sub-nanosecond Zn X-ray laser at 21.2 nm, which is able to penetrate the dense post-shock layer, and a space-and-time resolved plasma self-emission using high speed diodes.The experimental results show, for the first time, an unambiguous shock structure which includes both the post-shock and the precursor, and we also obtained multiple shock velocity measurements from the different diagnostics. The experimental results are compared to simulations, and show good agreement with the numerical results
Bouvier, Jerome. "Rotation et activité dans les étoiles T Tauri." Phd thesis, Université Paris-Diderot - Paris VII, 1987. http://tel.archives-ouvertes.fr/tel-00725122.
Full textAlbuquerque, Rachel. "Régions d’accrétion-éjection autour des objets stellaires jeunes." Thesis, Université Paris sciences et lettres, 2020. http://www.theses.fr/2020UPSLO001.
Full textDuring the last decades, observations and numerical simulations are helping us to characterize young star-disk systems. For such matter, the challenging goal of this project is to reinforce the synergy between observational and theoretical perspectives. On one hand, observations help us to constrain numerical simulations. On the other hand, the latest are useful the better understand the evolution along time of the physical mechanisms responsible for some spectroscopic features we can detect in YSOs (Young Stellar Objects) spectra
Bouvier, Jérôme. "Rotation et activité dans les étoiles T Tauri." Paris 7, 1987. http://www.theses.fr/1987PA077021.
Full textCastro, Matthieu. "Etude des éléments chimiques et tests sismiques de la structure interne du Soleil et des étoiles." Phd thesis, Université Paul Sabatier - Toulouse III, 2006. http://tel.archives-ouvertes.fr/tel-00128196.
Full textLes deux premières parties de ce manuscrit présentent de manière théorique les processus de transport et les principes de l'astérosismologie utilisés dans les modèles stellaires.
La troisième partie s'interesse à la signature astérosismique de la diffusion de l'hélium dans les étoiles de type F tardives et à son évolution. Nous montrons que le gradient créé par la diffusion de l'hélium sous la zone convective conduit à un pic dans le transformée de Fourier des secondes différences de spectre de fréquences d'oscillations. Plus le gradient est important, plus l'amplitude du pic est grande.
Le quatrième chapitre étudie la destruction du lithium dans les étoiles avec planètes. De récentes observations de Israelian et al. (2004) montrent que les étoiles froides avec planètes présentent une destruction du lithium importante, contrairement aux étoiles sans planètes. Nos modèles surmétalliques ont permis de montrer d'une part que le gradient de µ pouvait stabiliser le mélange dans les étoiles sans planètes, empêchant la destruction du lithium, et d'autre part que cette destruction dans les étoiles avec planètes pouvait provenir d'instabilités de cisaillement dues à la migration des planètes vers leur étoile centrale.
Enfin, la cinquième partie présente un travail sur les modèles solaires avec les nouvelles abondances de Asplund et al. (2005), qui présentent un désaccord avec les déductions héliosismiques. Nos modèles simulant une accrétion sous-métallique au début de la séquence principale améliorent la situation mais ne réussissent pas à rétablir l'accord avec l'héliosismologie, malgré l'introduction d'un overshooting et d'un mélange rotationnel sous la base de la zone convective.
Malbet, Fabien. "Environnement circumstellaire des étoiles jeunes." Phd thesis, Université Paris-Diderot - Paris VII, 1992. http://tel.archives-ouvertes.fr/tel-00003233.
Full textColombo, Salvatore. "Radiation hydrodynamic and magnetohydrodynamic models of plasma flows accreting onto Classical T Tour Stars." Thesis, Sorbonne université, 2019. http://www.theses.fr/2019SORUS608.
Full textIn this work we investigated the mass accretion process in CTTSs. We studied if accretion from the disk to the star might occur as a result of a coronal activity, and we analyzed the structure and the dynamics of the accretion column plasma in the impact regions. We developed numerical models that describe: a star-disk system subject to the effects of a coronal activity in proximity of the disk surface; the impact of an accretion column onto the surface of a CTTS.We investigated if an intense coronal activity due to flares that occur close to the accretion disk may perturb the stability of the inner disk, disrupt the inner part of the disk, and possibly trigger accretion phenomena with mass accretion rates comparable with those observed in CTTSs. To this end, we modeled a magnetized protostar surrounded by an accretion disk through 3D magnetohydrodynamics simulations.As it concerns the study of accretion impacts, we analyzed the effects of radiation emerg-ing from the shock-heated plasma at the base of accretion columns on the structure of the pre- shock downfalling material. To this end, we upgraded a module handling the local thermodynamic equilibrium (LTE) radiation-hydrodynamics (RHD) in the PLUTO code , which we have extended to handle also the non-LTE regime. Then, we investigated if a significant absorption of radiation arising from the shock heated plasma occurs in the unshocked downfalling material, and if it leads to a pre-shock heating of the accreting gas
Cabrit, Sylvie. "Ejection de matière dans les objets protostellaires et les étoiles jeunes de faible masse." Phd thesis, Université Pierre et Marie Curie - Paris VI, 1989. http://tel.archives-ouvertes.fr/tel-00725199.
Full textCadoll-Bel, Marion. "Etude des émissions à haute énergie des trous noirs stellaires accrétants." Paris 7, 2006. http://www.theses.fr/2006PA077082.
Full textThe present work is dedicated to the study of various X-ray binary Systems harbouring accreting stellar mass black holes (or candidates) associated in X-ray binary Systems mainly through the spectral and timing properties of the high energy 3 keV-1 MeV emission, sometimes completed by observations performed in radio, near-infrared and optical. The first part is devoted to accretion physics phenomena and the challenges of understanding the X-ray/gamma emission produced with the modelisations of such high energy processes. Then I will define in a second part the instruments on board INTEGRAL and the way coded masked aperture is employed. In a third part, I will develop the standard data reduction analysis and my own contribution in improving the usual software before detailing the specific informatics tools I have developed for my own analysis. In the fourth part I will turn towards the deep analysis and interpretations I have performed on several black hole X-ray binary Systems chosen properly: the persistent black hole source Cygnus X-1 which has been studied since several years and surprised us by a high-energy excess detected; two new transient sources which provide interesting informations, XTE J1720-318 located in the galactic bulge and SWIFT J1753. 5-0127, probably situated in the halo. I will also detail my work on H 1743-322, recently identified by INTEGRAL as the HEAO source discovered in 1977, and on three (almost) persistent microquasars with superluminal jets, 1E 1740. 7-2942, GRS 1758-258 and GRS 1915+105. I will analyze for each source spectral parameter evolutions and their links with each other during state transitions. I will then discuss the presence of two different X/gamma-ray emitting media with a relatively changing geometry. While establishing a cyclic order for the different variability classes of GRS 1915+105 observed during ten years, I will propose an interpretation for such behaviour, compatible with the theoretical predictions of the Accretion-Ejection Instability. As a conclusion I will discuss my results in the framework of theoretical models to explain the observations presented and I will derive some caveats to the general (well understood) physical processes occurring in X-ray binary Systems. I will discuss two important phenomena recently observed. I will conclude with my perspectives of future research work
Martin, Nicolas. "A la recherche de structures stellaires du disque galactique au halo de la galaxie d'Andromède." Université Louis Pasteur (Strasbourg) (1971-2008), 2006. https://publication-theses.unistra.fr/public/theses_doctorat/2006/MARTIN_Nicolas_2006.pdf.
Full textJean-Baptiste, Ingrid. "Reconstruire l'histoire d'accrétion de la Voie Lactée par l'étude de son système d'amas globulaires et de ses étoiles de champ." Thesis, Paris Sciences et Lettres (ComUE), 2016. http://www.theses.fr/2016PSLEO004/document.
Full textThe goal of this thesis is to contribute to understand the formation history of our Galaxy, the Milky Way, using as tracers some among the oldest objects in the universe, the globular clusters, and field stars.In the LambdaCDM paradigm, one of the main mechanisms of galaxy growth is by means of satellite accretion. The Milky Way is no exception and the history of its accretions can be read also in its population of globular clusters, containing both in-situ and accreted members. While for clusters at large distances from the Galactic centre an extragalactic origin is often proposed, in the inner regions, where most of the stellar mass of our Galaxy lies, the link between globular clusters and stellar populations is still very widely debated.In this work, we use the close link between globular clusters and field stars, as well as numerical simulations, to try to remove the ambiguity on the in-situ or accreted origin of these objects and to reconstruct the accretion history of the Milky Way. As a first step, we study the spatial distribution, kinematics and chemical abundances of metal-rich ([Fe/H] > -1) globular clusters. We show that their spatial, kinematic and chemical properties are in good agreement with those of the stellar population of the thick disk of the Galaxy. We then suggest that metal-rich globular clusters and Galactic thick disk share the same epoch of formation and evolution. Secondly, we analyze the overall efficiency of kinematic diagnostics that have been proposed to identify merger debris among the field stars and discuss their application to the galactic globular cluster system. To do this, we use self-consistent numerical simulations that model the accretion of one or more satellite galaxies in a Milky Way-like potential. On the one hand, we show that the integrals of motion are not conserved during the accretion process. As a result, after the merger, accreted stars and globular clusters do not retain the initial information about the orbital properties of their progenitor satellites. On the other hand, mergers of small galaxies and tidal interactions may cause the heating of the in-situ populations (stars and globular clusters) which, in response, populate the galactic halo and the regions in the kinematic spaces preferentially occupied by objects with an extragalactic origin.In the context of the arrival of Gaia data, this work shows that the identification of debris of past accretion events experienced by the Milky Way using only kinematic diagnostics will be extremely challenging. Detailed chemical abundances and/or ages will be fundamental to disentangle the accreted or in-situ nature of the Galactic stellar populations
Trap, Guillaume. "Etude du trou noir massif central de la Galaxie et de son environnement." Phd thesis, Université Paris-Diderot - Paris VII, 2011. http://tel.archives-ouvertes.fr/tel-00862003.
Full textCadolle, Bel Marion. "Etude des émissions à haute énergie des trous noirs stellaires accrétants." Phd thesis, Université Paris-Diderot - Paris VII, 2006. http://tel.archives-ouvertes.fr/tel-00105637.
Full textCuello, Nicolas. "Mécanismes de transport dans les disques protoplanétaires et impact sur la formation des premiers solides." Thesis, Lyon 1, 2015. http://www.theses.fr/2015LYO10154/document.
Full textThe main goal of this work is to study new transport mechanisms of solids in protoplanetary disks and its implications for the composition of the first solids. The motion of solids inside the disk leads to the so-called radial-drift barrier caused by the gas aerodynamic drag, which is a severe problem for planet formation theory. In this context, it is hard to explain how sub-mm grains reach planetesimal sizes during the disk lifespan. First of all, I study the effects of photophoresis on the dust grains illuminated by the stellar radiation and quantify the efficiency of radial transport as a function of the particle properties. Then, I study the ejection of particles from the inner regions of the disk via the so-called stellar fountain model. Due to the stellar magnetic field which threads the disk, solid particles enter a jet that sends them outwards up to a few astronomical units. Both processes, photophoresis and jets, have important implications for the composition of meteorites which are discussed within the Solar Nebula scenario. In the last chapter, I study dust dynamics in multi-planetary systems through SPH simulations. The formation of particle traps in a disk with two planets is treated in detail and compared to previous work considering a single planet. Then I consider the case of HD 100546, a star with a disk which might harbor two planets according to recent observations, and study the disk evolution in different scenarios. By considering different grains sizes it is then possible to establish a link with interferometric observations of the system. We consider models with different planetary masses and radial distances in order to better constrain these quantities. The study of these mechanisms reveals that, according to particle size and composition, grains can pile up at different radial distances in the disk. This prevents the accretion by the central star by stopping the radial drift of solids, which shows that these mechanisms are good candidates to solve the radial-drift barrier. Future observations using ALMA, SPHERE and MATISSE will provide insights into the efficiency of these transport processes in protoplanetary disks
Malbet, Fabien. "Étude des conditions physiques dans les disques protoplanétaires par interférométrie. Théorie, instrumentation et premières observations." Habilitation à diriger des recherches, 2007. http://tel.archives-ouvertes.fr/tel-00189697.
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