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

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.

12 pages, 10 figures. Article accepted for publication in A&A

References in corpus (26)

• Modules for Experiments in Stellar Astrophysics (MESA): Pulsating Variable Stars, Rotation, Convective Boundaries, and Energy Conservation
• Global-Scale Turbulent Convection and Magnetic Dynamo Action in the Solar Envelope
• Weak magnetic fields in Ap/Bp stars: Evidence for a dipole field lower limit and a tentative interpretation of the magnetic dichotomy
• How mergers magnetise massive stars
• Magnetic field generation in fully convective rotating spheres
• Asteroseismic measurement of surface-to-core rotation in a main sequence A star, KIC 11145123
• The MiMeS survey of magnetism in massive stars: Magnetic analysis of the O-type stars
• Magnetic Cycles and Rotation Periods of Late Type Stars from photometric time series
• The Pencil Code, a modular MPI code for partial differential equations and particles: multipurpose and multiuser-maintained
• Evolutionary state of magnetic chemically peculiar stars
• Simulations of core convection in rotating A-type stars: Magnetic dynamo action
• On non-axisymmetric magnetic equilibria in stars
• Searching for links between magnetic fields and stellar evolution III. Measurement of magnetic fields in open cluster Ap stars with ESPaDOnS
• Powering Stellar Magnetism: Energy Transfers in Cyclic Dynamos of Sun-like Stars
• Asteroseismic inference of the near-core magnetic field strength in the main sequence B star HD 43317
• Origin and evolution of magnetic fields in PMS stars : influence of rotation and structural changes
• Spin-down by dynamo action in simulated radiative stellar layers
• Star-in-a-box simulations of fully convective stars
• Three-dimensional magnetic and abundance mapping of the cool Ap star HD 24712 II. Two-dimensional Magnetic Doppler Imaging in all four Stokes parameters
• Magnetic field topology and chemical spot distributions in the extreme Ap star HD 75049
• Single-hemisphere dynamos in M-dwarf stars
• Mean-Field Modeling of ${\alpha }^2$-Dynamo Coupled with Direct Numerical Simulations of Rigidly Rotating Convection
• Orbital advection with magnetohydrodynamics and vector potential
• The complex magnetic field topology of the cool Ap star 49 Cam
• Analytic solution of an oscillatory migratory alpha^2 stellar dynamo
• Magnetohydrodynamic simulations of A-type stars: Long-term evolution of core dynamo cycles

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