Nonlinear simulations explaining Ap star magnetic fields by instability remnants

Abstract

The question of the origin of magnetic fields of Ap/Bp stars is still regarded as an interesting puzzle of stellar astrophysics. We investigate the possibility that the randomness and relative complexity of these fields are remnants of a magnetic instability. In the studied scenario it is assumed a priori that the surface of an Ap star is slowed down in its early evolutionary stage more than its analogous A star. This leads to a significant differential rotation in its interior, making it possible to generate a strong toroidal magnetic field in the radiative zone. Under such circumstances the kink-type Tayler instability is likely to set in. The presented numerical simulations in a compressible, spherical domain show that the instability can produce large surface magnetic fields, even of the order of 0.01−1 of the internal toroidal component (depending on the setup). The resulting magnetic fields can then serve as “initial conditions” evolving into a stable magnetic configuration (however, the matter of long-term stability is not addressed here). This theory naturally supports the fact that Ap stars rotate typically slower than normal A stars (the Tayler instability is suppressed when rotation is too fast), it also qualitatively explains the dependence of the apparent obliquity of the main magnetic axis on the rotation period, as well as the existence of the minimum field threshold (no Ap stars have been observed with fields weaker than ≈10 2 G). Given that the generation of the initial differential rotation and initial poloidal fields are not discussed here, the results have a speculative nature and can be viewed as a possible step toward a full understanding of Ap star magnetism.