Résumé (Abstract):
Recent observational instruments give now access to many features for a huge amount of different stars. These observations help us to improve our understanding of stellar magnetic fields, including the physical mechanisms controlling the topology or the dynamics of these fields. For most stars, it is accepted that turbulent motions in deep interior inducing electric currents are responsible for the presence of the stellar magnetic field (``dynamo effect''). For low-mass stars such as the Sun, it is accepted that magnetic activity mainly results from convection motions in the outer layer (the convective zone). For more massive stars, the origin of the magnetic field does not seem to be related to convection motions since their convective zone can be very small and it is located very deep at the center below the radiative zone which is stably-stratified. However, fields of various amplitudes and topologies are observed on the surface of these massive stars. Under these conditions, the mechanism responsible for the origin of the magnetic activity of massive stars remains largely misunderstood. In particular, the ability of a stably-stratified flow to generate magnetism has received little attention despite the recent theoretical and numerical advances. \\ During this numerical and theoretical internship, we will try to improve our understanding on the origin of the magnetic field in massive stars, in particular, we will study how stratification influences the dynamo effect generated by shear of a spherical Couette flow. From a simplified stellar interior model, the student will work on a complete three-dimensional numerical modeling of the problem, in order to identify the different mechanisms controlling the dynamical regimes within the star: the role of turbulence, stratification, differential rotation, global rotation, or dissipative effects.      

Proposition de stage de M2 pour 2018 : Compressibility influence on dynamo effect in stellar envelopes

Résumé (Abstract):
The density of astrophysical objects such as galaxies, molecular clouds or stars strongly depends on space and time, which affects the internal dynamics of the object. For instance, the presence of a density wave in the form of spiral arms in gas-rich galaxies strongly influences the rate of star formation. More global variations of density correspond to different stellar phases from birth to death. The magnetic field, like other observables, can be strongly affected by such variations of density. For most stars, it is commonly believed that turbulent motions inducing electric currents in stellar interior are responsible for the presence of the stellar magnetic field (`` dynamo effect''). For low-mass stars such as the Sun or for planets, it is accepted that magnetic activity is mainly induced by convective motions. Although the role of compressibility and density stratification can strongly influence the dynamo effect, these compressible effects are still poorly understood. In stars, the field of a magnetic flux tube is amplified/reduced during a compression/dilatation of the fluid. Under these conditions, it seems necessary to study the influence of fluid compressibility on the dynamo effect using theoretical and numerical tools. During this numerical and theoretical internship, we will try to understand the origin of the magnetic field in low-mass stars, in particular, we will study how compressibility influences the dynamo effect induced by convection motions in spherical shell. From a simplified stellar interior model, the student will work on a numerical modeling of the problem by seeking to parameterize the influence of compressibility.\\