Dynamique des zones radiatives stellaires en rotation différentielle

Doctorant : Bastien GOUHIER

Directeur de thèse : Laurène JOUVE, François LIGNIERES

Début de thèse : Octobre 2018

Groupe thématique : PS2E

Understanding how the rotation of stars influences their evolution is a major issue for modern stellar physics. In the coming years, important progress are expected from the confrontation between new observational constraints (from asteroseismology, astrometry, spectropolarimetry and interferometry) and stellar evolution models as they include better descriptions of the transport processes driven by the rotation. The transport of angular momentum and chemicals is particularly efficient when mechanisms such as core contraction, envelope expansion or stellar winds force different layers in the star to rotate at different rates. This thesis aims at improving the physical description of transport processes in a differentially rotating star.

For this purpose, the canonical flow driven by two spheres rotating at different rates, the so-called spherical Couette flow (studied in detail by Wicht 2014 in the geophysical context), will be investigated in a stellar context. The physical conditions encountered within stellar radiative zones are characterized by an important effect of the restoring buoyancy force, a large thermal diffusivity of radiative origin, and the possibility to generate magnetic fields through dynamo action. Recent works have demonstrated that numerical simulations of spherical Couette flows including all these ingredients can reach parameter regimes suited to address rotationally driven transport processes by large scale motions. In particular crucial assumptions used in evolutionary models, such as the shellular rotation hypothesis (rotation rate dependent on the radial coordinate only, Zahn 1992), can now be studied numerically in physically relevant conditions. Axisymmetric and stationary solutions of the spherical Couette flow will be first obtained, their stability to non-axisymmetric perturbations and the non-linear evolutions of the unstable cases will then be considered. For this last part, a focus will be made on the redistribution of angular momentum and mixing induced by these instabilities. The ability of the flow to act as an efficient dynamo (as studied by Guervilly & Cardin 2010 in the geophysical context) will also be investigated. The open-source MAGIC code (   http://magic-sph.github.io/) and super-computing facilities of the CALMIP regional center and of French national centers will be used.

This PhD subject combines 2D and 3D numerical modelling, understanding of fundamental physical processes in astrophysical fluid dynamics, comparison to theoretical arguments and to very recent high quality observations in stellar physics. It will benefit from the vibrant observational context driven by the scientific exploitation of Kepler and the preparation of Plato.