Thesis title
Magnetohydrodynamic instabilities in stellar radiative regions.
Composition of the examination board
- Marie-Christine ANGONIN (University Professor, LTE) : Examiner
- Laurence REZEAU (University Professor, LPP) : Examiner
- Geoffroy LESUR (Research Director, IPAG) : Supervisor
- François LIGNIERES (Research Director, IRAP) : Rapporteur
- Ludovic PETITDEMANGE (Research Director, LIRA) : Thesis Supervisor
- Kevin BELKACEM (Research Fellow, LIRA) : Co-supervisor
- Anna GUSEVA (Lecturer, UPC) : Guest
Abstract
Recent observations from the Kepler and CoRoT missions have revealed that our understanding of the evolution of stellar interiors remains incomplete. In particular, the mechanisms responsible for the transport of angular momentum from the inner regions to the outer regions remain poorly understood.
The work carried out in this thesis is devoted to the study of the development of magnetohydrodynamic (MHD) instabilities in radiative regions. These can develop when the shear or magnetic gradient is sufficiently large. Their amplification could give rise to turbulence capable of efficiently transporting angular momentum in these particularly stable regions.
Three instabilities are presented : the GSF (centrifugal) instability, the magneto-rotational instability (MRI, weak field and shear) and the Tayler instability (deformation of a toroidal magnetic field). Their linear development is analysed using both a local and a global approach, and their non-linear development is investigated using direct numerical simulations (DNS).
In particular, this research has highlighted the existence of a magnetostratified regime, in which stratification and magnetic stress alter the properties of each instability. GSF modes are stabilised, MRI modes are slowed down, whilst new Tayler modes, specific to stratified regions, emerge. These results are validated by a global stability analysis, which enables the identification of the mode structure within a flow, and thus their location. The application of these results to models of the interiors of subgiants and young red giants reveals numerous configurations conducive to the development of these instabilities. Finally, preliminary results from numerical simulations are presented to assess their impact on the dynamics of the internal regions.
