Is a 1D perturbative method sufficient for asteroseismic modelling of $\beta$ Cephei pulsators? Implications for measurements of rotation and internal magnetic fields

TL;DR

This study compares 1D perturbative and 2D non-perturbative methods for modeling pulsations in $eta$ Cephei stars, assessing their accuracy in measuring internal magnetic fields and rotation profiles.

Contribution

It quantifies the differences between 1D and 2D pulsation predictions, highlighting the limitations of 1D methods for magnetic field measurements in main-sequence pulsators.

Findings

Good agreement between 1D and 2D oscillation frequencies.

Magnetic asymmetries are small compared to rotational asymmetries.

Accurate magnetic field measurements are challenging with current methods.

Abstract

Asymmetries in the observed rotational splittings of a multiplet contain information about the star's rotation profile and internal magnetic field. However, to exploit this information, highly accurate theoretical predictions are needed. We aim to quantify the difference in the predicted mode asymmetries between a 1D perturbative method, and a 2D method that includes a 2D stellar structure model, which takes rotation into account. We then put these differences in the context of asteroseismic measurements of internal magnetic fields. We couple the 1D pulsation codes GYRE and StORM to the 2D stellar structure code ESTER and compare the oscillation predictions with the results from the 2D TOP pulsation code. We focus on ZAMS models representative of rotating $\beta$~Cephei pulsators, going up to 20% of the critical rotation rate. We find a generally good agreement between the oscillation frequencies resulting from the 1D and 2D pulsation codes. Since the magnetic asymmetries are small compared to the differences in the rotational asymmetries resulting from the 1D and 2D predictions, accurate measurements of the magnetic field are in most cases challenging. Differences in the predicted mode asymmetries between 1D perturbative methods and 2D non-perturbative methods can greatly hinder accurate measurements of internal magnetic fields in main-sequence pulsators with low-order modes. Nevertheless, reasonably accurate measurements could be possible with $n_{pg} \ge 2$ modes if the internal rotation is roughly below 10% of the critical rotation frequency for (aligned) magnetic fields on the order of a few hundred kG. While the differences between the 1D and 2D predictions are mostly too large for internal magnetic field detections, the rotational asymmetries predicted by StORM are in general accurate enough for asteroseismic modelling of the stellar rotation in main-sequence stars.