Detecting deep axisymmetric toroidal magnetic fields in stars. The traditional approximation of rotation for differentially rotating deep spherical shells with a general azimuthal magnetic field

TL;DR

This paper develops a new theoretical framework to detect deep azimuthal magnetic fields in stars through asteroseismology, showing that such fields cause measurable shifts in wave period spacings detectable by space photometry.

Contribution

It generalizes the traditional approximation of rotation to include differential rotation and magnetic fields, deriving asymptotic properties of magneto-gravito-inertial waves.

Findings

Toroidal magnetic fields cause detectable shifts in period spacings.

Magnetic fields of about 10^5 G produce observable signatures.

Hemispheric magnetic configurations are more challenging to detect.

Abstract

Asteroseismology has revealed small core-to-surface rotation contrasts in stars in the whole HR diagram. This is the signature of strong transport of angular momentum (AM) in stellar interiors. One of the plausible candidates to efficiently carry AM is magnetic fields with various topologies that could be present in stellar radiative zones. Among them, strong axisymmetric azimuthal magnetic fields have received a lot of interest. Indeed, if they are subject to the so-called Tayler instability, the accompanying triggered Maxwell stresses can transport AM efficiently. In addition, the electromotive force induced by the fluctuations of magnetic and velocity fields could potentially sustain a dynamo action that leads to the regeneration of the initial strong axisymmetric azimuthal magnetic field. The key question we aim to answer is: can we detect signatures of these deep strong azimuthal magnetic fields? The only way to answer this question is asteroseismology and the best laboratories of study are intermediate-mass and massive stars. Most of these are rapid rotators during their main-sequence. Therefore, we have to study stellar pulsations propagating in stably stratified, rotating, and potentially strongly magnetised radiative zones. We generalise the traditional approximation of rotation by simultaneously taking general axisymmetric differential rotation and azimuthal magnetic fields into account in a non-perturbative way. Using this new formalism, we derive the asymptotic properties of magneto-gravito-inertial (MGI) waves and their period spacings. We find that toroidal magnetic fields induce a shift in the period spacings of MGI modes. An equatorial azimuthal magnetic field with an amplitude of the order of $10^5\,\rm G$ leads to signatures that can be detectable thanks to modern space photometry. More complex hemispheric configurations are more difficult to observe.