Modelling magnetized accretion columns of young stars in the laboratory

Abstract : The work that is presented here has been performed in the frame of laboratory astrophysics, which consists in studying in the laboratory physical processes occurring in astrophysical objects. The main advantages in doing so are that the processes can be studied in a controlled way and that their full dynamics can be investigated. Here, we have been taking advantage of high-intensity laser facilities to perform our studies.In this manuscript, will be treated issues that include the interaction of a plasma expanding into vacuum with an ambient magnetic field. The presence of a magnetic field in a variety of astrophysical phenomena makes the inclusion of this component in the laboratory of great interest. We have used for our study a split Helmholtz coil, specifically designed in order to work in a laser environment, that allows for reaching a magnetic field strength up to 30 T.The astrophysical objects on which this study is focused are Young Stellar Objects (YSOs). Several steps of the star formation process are here investigated: (i) the generation of very long range, bright jets, (ii) the accretion dynamic involving, in the standard representation, matter falling down on the star in the shape of magnetically confined columns, and (iii) more exotic accretion channels, as the equatorial accretion that implies propagation of plasma perpendicularly to magnetic field lines.More precisely, in a first chapter, the jet formation dynamic will be discussed. A first part is dedicated to the jet formation mechanism in a poloidal magnetic field (aligned with respect to the main plasma expansion axis). A second part is dealing with the distortion of such jet formation via the interaction of the same expanding plasma with a misaligned magnetic field (i.e. presenting an angle with respect to the plasma expansion axis). Finally, a third part details the propagation of the plasma within a perpendicular magnetic field. This last part allows us to investigate exotic channels of matter accretion onto the stars, consisting of equatorial accretion from the disk to the star, through orthogonal magnetic field lines. The second chapter addresses the topic of the standard accretion dynamic via magnetically confined columns of matter, falling down onto the stellar surface. Using the same experimental setup as in the first chapter, the formed jet (in the case of the perfectly aligned magnetic field) is used to mimic the accretion column, and is launched onto a secondary target that acts as the stellar surface. The shock dynamic at the obstacle location is carefully studied and links with astrophysical accretion observations are built. A plasma cocoon, shaped around the impact region via the interaction with the magnetic field, is found to be similar to the one found in astrophysical simulations. This cocoon is an important element as a potential X-ray absorptive medium in order to explain observed discrepancies, between observed UV/Optical and X-ray emissions emitted from accreting stars.
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Guilhem Revet. Modelling magnetized accretion columns of young stars in the laboratory. Plasma Physics [physics.plasm-ph]. Université Paris-Saclay, 2018. English. ⟨NNT : 2018SACLX046⟩. ⟨tel-02100492v1⟩

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