Magnetic fields of Ap stars as a result of an instability

TL;DR

This paper proposes that the diverse magnetic fields observed in Ap stars result from an instability of buried toroidal magnetic fields, rather than stable fossil fields, explaining their complexity and surface emergence within a few years.

Contribution

It introduces a novel instability-based mechanism for Ap star magnetism, challenging the traditional fossil field hypothesis and explaining observed magnetic diversity.

Findings

Magnetic field remainders are nonaxisymmetric, matching observed Ap star diversity.

Surface magnetic fields can emerge within a few years due to the instability.

Normal A stars may host stable, yet invisible, internal toroidal magnetic fields.

Abstract

Ap star magnetism is often attributed to fossil magnetic fields which have not changed much since the pre-main-sequence epoch of the stars. Stable magnetic field configurations are known which could persist probably for the entire main-sequence life of the star, but they may not show the complexity and diversity exhibited by the Ap stars observed. We suggest that the Ap star magnetism is not a result of stable configurations, but is the result of an instability based on strong toroidal magnetic fields buried in the stars. The highly nonaxisymmetric remainders of the instability are reminiscent of the diversity of fields seen on Ap stars. The strengths of these remnant magnetic fields is actually between a few per cent up to considerable fractions of the internal toroidal field; this means field strengths of the order of kGauss being compatible with what is observed. The magnetic fields emerge at the surface rather quickly; rough estimates deliver time-scales of the order of a few years. Since rotation stabilizes the instability, normal A stars may still host considerable, invisible toroidal magnetic fields.