Séminaires au LIRA

Despite significant progress in the observational characterization of stellar magnetic fields, the physical processes that govern their intensity and topology, which could certainly result from their formation history, remain poorly understood. Protostars are embedded in their accretion disk and the accretion of gas could affect the stellar magnetism. During the pre-main-sequence (PMS) phase, the inner layers tend to contract and a radiative core gradually develops. In contrast, the convective envelope is gradually braked through the magnetic interactions with the accretion disk and winds, thus slowly developing differential rotation inside the star. It is likely during this PMS phase that the dynamo processes that efficiently generated strong dipolar magnetic fields through vigorous convective motions in protostars become highly perturbed, leading to the observed diversity in the magnetism on the main sequence (MS). We aim to study the stability of dipolar magnetic fields inherited from the proto-stellar phases by considering the emergence of a large-scale radial differential rotation resulting from the combined actions of contraction and of the interactions with the surrounding medium for PMS stars and. For protostars, we consider in our 3D models the entropy associated with the accretion shocks at the surface of the stars. We perform three-dimensional convective dynamo simulations of rotating spherical shells suited for the stellar phases under consideration in order to track the evolution of the star. We use the anelastic approximation that allows us to consider background density and gravity profiles and convective zone thicknesses close to those predicted low-mass stars by the one-dimensional stellar evolution code Cesam2k20. We then carried out parameter studies by systematically varying the shear amplitude for PMS stars. We qualitatively reproduce the trends observed in the magnetic topologies of low-mass stars when assuming an efficient internal angular momentum redistribution process. This suggests that stellar magnetic properties are intimately related to the PMS angular momentum evolution.