Radial differential rotation leading to dipole collapse in pre-main-sequence stars

Context. 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. During the pre-main-sequence (PMS) phase, the inner layers of these stars 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 a 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. Aims. We aim to study the stability of dipolar magnetic fields inherited from the proto-stellar phase 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. Methods. We performed 3D convective dynamo simulations of rotating spherical shells with an imposed differential rotation (shear) between the bottom and top boundaries. We used anelastic approximation, which allowed us to consider background density and gravity profiles and convective zone thicknesses close to those predicted in PMS low-mass stars by the 1D stellar evolution code Cesam2k20. We then carried out a parameter study by systematically varying the shear amplitude. Results. Radial differential rotation can induce dipole collapse leading to weaker and oscillatory magnetic fields. We highlight that the stability of dipolar dynamos mainly depends on the relative importance of shear measured by a shear-based Rossby number compared to the vigor of convective motions as measured by the convective Rossby number. We show that the stability criterion depends on the field strength and the size of the radiative core. Differential rotation seems to perturb the α 2 dynamo mechanism responsible for dipolar magnetic fields by shearing poloidal field lines and affecting turbulent magnetic transport processes. Conclusions. The PMS phase can represent a critical period for the magnetic properties of stars, as the development of shear layers can perturb the stability of strong initial dipoles. Applying the stability criterion in PMS stellar evolution models, we qualitatively reproduced 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.