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

¹ Dipartimento di Fisica, Sapienza, Università di Roma, Piazza le Aldo Moro 5, 00185 Roma, Italy
² Institut für Sonnenphysik (KIS), Georges-Köhler-Allee 401a, 79110 Freiburg, Germany
³ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Institute of Space Sciences (ICE-CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain

^⋆ Corresponding author.

Received: 14 April 2025
Accepted: 30 May 2025

Abstract

Context. Large-scale magnetic fields of Ap/Bp stars are stable over long timescales and typically have 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.

Aims. Our aim was 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.

Methods. We used numerical 3D star-in-a-box simulations of a 2.2 M_⊙ A-type star, where the core encompasses 20% of the stellar radius. As an initial condition, we imposed two purely poloidal configurations, both with a surface dipolar strength of 6 kG, and we explored different obliquity angles β (the angle between the magnetic and rotational axes), ranging from 0° to 90°.

Results. 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 five 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 β. 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 β = 90°. In the rest of the cases, the core dynamos are enhanced and the surface magnetic field survives, keeping simple topologies as in the observations.