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Theses

Dynamique des zones radiatives stellaires en rotation différentielle

Abstract : Stars experience contraction and/or expansion phases at different stages of their existence. Seismic measurements of internal rotation have shown the need to improve the physical description of transport processes of angular momentum and chemical elements during these rapid phases of evolution. In addition, spectropolarimetric surveys suggest a strong-fields/weak-fields dichotomy among the intermediate-mass stars. Understanding how these fields interact with the differential rotation forced by the contraction could help us to address the origin of the magnetism of these stars. This thesis aims at making progress on the issue of the angular momentum transport induced by the contraction of a magnetised, or non-magnetised, stellar radiative zone. For this purpose, we perform axisymmetric (magneto)hydrodynamical simulations of a stably stratified rotating spherical layer undergoing contraction, having or not, a large-scale magnetic field. Contraction is modelled by a steady radial mass flux and simulations are carried out in the Boussinesq and anelastic approximations. An extensive parametric study is conducted to cover the different regimes of rotation, contraction, stable stratification and magnetic field that are appropriate for stars. In the absence of magnetic field, we determine two relevant regimes: the Eddington-Sweet regime in which the meridional circulation dominates the angular momentum transport, and the viscous regime where the viscosity plays a leading role. The Eddington-Sweet regime, relevant for pre-main sequence stars and outside the degenerate core of subgiants, naturally results in a differential rotation, both in latitude and radius. The viscous regime, relevant in the degenerate core of subgiants, leads to a spherically symmetric rotation profile. In both cases, we derive a scaling law which enables us to estimate the amplitude of the differential rotation. When a magnetic field is applied, the flow is characterised by the presence of two magnetically decoupled regions. In one of them, magnetic tension imposes solid-body rotation while in the other, called the dead zone, the Lorentz force becomes negligible and a differential rotation exists. In some cases, we show that the latitudinal shear built in the dead zones triggers a powerful axisymmetric instability able to destroy the large-scale structure of the magnetic field and to result in a flow reconfiguration. We propose that this could potentially explain the rotational evolution of subgiants as well as the observed dichotomy between strong-fields/weak-fields of intermediate-mass stars.
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Submitted on : Wednesday, July 6, 2022 - 12:04:11 PM
Last modification on : Thursday, July 7, 2022 - 6:16:08 AM

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2022TOU30025a.pdf
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  • HAL Id : tel-03715323, version 1

Citation

Bastien Gouhier. Dynamique des zones radiatives stellaires en rotation différentielle. Astrophysique stellaire et solaire [astro-ph.SR]. Université Paul Sabatier - Toulouse III, 2022. Français. ⟨NNT : 2022TOU30025⟩. ⟨tel-03715323⟩

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