Juan Pablo Hidalgo | Searcharxiv

arXiv2026

Global 3D simulations of convection and dynamos in Red Giants

Understanding the internal dynamics of red giants is essential for studying angular momentum transport and magnetic field generation during stellar evolution. We present three-dimensional magnetohydrodynamic simulations of a $1M_{\odot}$ red giant at the bottom of the red giant branch using a star-in-a-box setup with the Pencil Code, together with ongoing efforts to refine the numerical model. By varying the rotation rate, we explore different Rossby numbers and identify systematic changes in the convective dynamics and large-scale flows. As the rotation increases, convective motions become more anisotropic, showing clear signatures of rotational constraint and a shift in the dominant convective scales. These changes are accompanied by variations in the differential rotation profile, reflecting the redistribution of angular momentum within the convective envelope. The impact of rotation is also evident in the growth rate and saturation level of the magnetic field, indicating a strong dependence of dynamo efficiency on the underlying flow structure. Together, these results highlight the interplay between convection, rotation, and large-scale flows in shaping magnetic activity in red giant stars.

astro-ph.SR↗

arXiv2023

Survival of fossil fields during the pre-main sequence evolution of intermediate-mass stars

Chemically peculiar Ap and Bp stars host strong large-scale magnetic fields in the range of $200$~G up to $30$~kG, which are often considered to be the origin of fossil magnetic fields. We assess the evolution of such fossil fields during the star formation process and the pre-main sequence evolution of intermediate stars, considering fully convective models, models including a transition to a radiative protostar and models with a radiative core. We also examine the implications of the interaction between the fossil field and the core dynamo. We employ analytic and semi-analytic calculations combined with current observational constraints. For fully convective models, we show that magnetic field decay via convection can be expected to be very efficient for realistic parameters of turbulent resistivities. Based on the observed magnetic field strength - density relation, as well as the expected amount of flux loss due to ambipolar diffusion, it appears unlikely that convection could be suppressed via strong enough magnetic fields. On the other hand, a transition from a convective to a radiative core could very naturally explain the survival of a significant amount of flux, along with the presence of a critical mass. We show that in some cases, the interaction of a fossil field with a core dynamo may further lead to changes in the surface magnetic field structure. In the future, it will be important to understand in more detail how the accretion rate evolves as a function of time during the formation of intermediate-mass protostars, including its impact on the protostellar structure. The latter may even allow to derive quantitative predictions concerning the expected population of large scale magnetic fields in radiative stars.

astro-ph.SR↗