Shaping core dynamos in A-type stars: The role of dipolar fossil fields

TL;DR

This study uses 3D simulations to explore how fossil dipolar magnetic fields influence core dynamo action in A-type stars, revealing conditions that enhance magnetic strength and stability, consistent with observed stellar magnetic topologies.

Contribution

It demonstrates that certain dipolar configurations can significantly enhance core dynamo strength and stability, aligning with observed magnetic features in Ap/Bp stars.

Findings

Dipolar fields with closed lines inside the star enhance dynamo action.

Enhanced dynamos produce magnetic fields 5 times stronger than kinetic energy.

Surface magnetic fields remain stable with simple topologies in most cases.

Abstract

Large-scale magnetic fields of Ap/Bp stars are stable over long timescales and have typically simple dipolar geometries, leading to the idea of a fossil origin. These stars are also expected to have convective cores that can host strong dynamo action. We aim to study the interaction between the magnetic fields generated by the convective core dynamo of the star, and a dipolar fossil field reminiscent of observed magnetic topologies of Ap/Bp stars. We use numerical 3D star-in-a-box simulations of a $2.2M_\odot$ A-type star, where the core encompasses $20\%$ of the stellar radius. As an initial condition, we impose two purely poloidal configurations, both with a surface dipolar strength of 6 kG, and we explore different obliquity angles $\beta$ (the angle between the magnetic and rotational axes), ranging from $0^\circ$ to $90^\circ$. The inclusion of a poloidal field where none of the magnetic field lines are closed inside the star, does not affect the core dynamo in a significant way. Dipolar configurations where all the field lines are closed inside the star can enhance the dynamo, producing a superequipartition quasi-stationary solution, where the magnetic energy is 5 times stronger than the kinetic energy. The enhanced core dynamos have typical magnetic field strengths between 105 and 172 kG, where the strength has an inverse relation with $\beta$. The strong magnetic fields produce an almost rigid rotation in the radiative envelope, and change the differential rotation of the core from solar-like to anti-solar. The only cases where the imposed dipoles are unstable and decay are those with $\beta = 90^\circ$. In the rest of cases, the core dynamos are enhanced and the surface magnetic field survives keeping simple topologies like in the observations.