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Modélisation analytique et numérique de l'évolution des restes de supernova en phase radiative

Abstract : At the end of a massive (typically more than 5 to 6 solar masses) star life, the star becomes a supernova. After the explosion, the strong shock begins to propagate with the ejecta of the star in the circumstellar medium and later in the interstellar medium (ISM). This object is called a supernova remnant (SNR). During its expansion, the SNR accumulates matter behind the shock front to form a shell. Theory describes three stages of the SNR. The first stage corresponds to a ballistic expansion with a radius R of the SNR proportional to its age t. In the second stage, called the Sedov-Taylor (ST) regime, the evolution is adiabatic (energy conservation) and R increases like R(t) ∝ t^2⁄5 . In this PhD thesis, we present an analytical and numerical modeling of the SNR evolution in its third stage (radiative late stage) where the energy is not anymore conserved. The remnant loses its energy by radiative emission processes or by ionisation of the ISM. In this last stage, the radius is given by R(t) ∝ t^n where the exponant n satisfies 1/4 < n < 2/5. First, we study the structure of the shock which ionizes the ISM and show that the compression ratio can become high (of the order of 40 compared to 4 for a adiabatic shock in a monoatomic gas). Indeed, a portion of the shock energy is used to ionize the gas and is no longer available to heat the postshock medium, so the gas will be compressed. Then, we apply this property to the modeling of a SNR in late stage and show that the SNR shell can become dense and thin. In a second part, we perform the self-similar analysis of a SNR where energy losses are described by a spatially uniform cooling function. This study demonstrates that there are two types of hydrodynamic solutions : ST-type solutions which are smooth up to the center of the remnant and shell-type solutions where the shell is bounded by a contact discontinuity that separates it from the hot and rarefied interior (bubble) of the SNR. Also, we show that the shell becomes dense and thin when the energy loss is strong enough. Finally, we complete the study of the transition to the radiative stage of the SNR for more realistic radiative losses using the numerical simulation. We show that although the formation process of the thin and dense shell is complex (catastrophic cooling of Falle and secondary shocks), the evolution of SNR in the radiative stage shares common properties for a wide variety of cooling functions. In particular, we show that in almost every cases the SNR radius follows the law R(t) ∝ t^n where n is close to 2/7. This behavior is constistent with theory, because this value has been found previously by other autors and we highlight it also in our self-similar analysis.
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Antoine Gintrand. Modélisation analytique et numérique de l'évolution des restes de supernova en phase radiative. Astrophysique [astro-ph]. Université Paris sciences et lettres, 2019. Français. ⟨NNT : 2019PSLEO012⟩. ⟨tel-02884251⟩

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