Period spacings of gravity modes in rapidly rotating magnetic stars

J. Van Beeck,C. Aerts,D. M. Bowman,C. Neiner,S. Mathis,V. Prat

doi:10.1051/0004-6361/201937398

Abstract

Context.Stellar internal magnetic fields have recently been shown to leave a detectable signature on period spacing patterns of gravity modes.Aims.We aim to investigate the effect of the obliquity of a mixed (poloidal and toroidal) dipolar internal fossil magnetic field with respect to the rotation axis on the frequency of gravity modes in rapidly rotating stars.Methods.We used the traditional approximation of rotation to compute non-magnetic modes, and a perturbative treatment of the magnetic field to compute the corresponding frequency shifts. We applied the new formalism to HD 43317, a magnetic, rapidly rotating, slowly pulsating B-type star, whose field has an obliquity angle of about 80°.Results.We find that frequency shifts induced by the magnetic field on high-radial-order gravity modes are larger with increasing obliquity angle, when the magnetic axis is closer to the equatorial region, where these modes are trapped. The maximum value is reached for an obliquity angle of 90°. This trend is observed for all mode geometries.Conclusions.Our results predict that the signature of an internal oblique dipolar magnetic field is detectable using asteroseismology of gravity modes.

Highlights

Thanks to high-precision space photometry with missions such as CoRoT (Baglin et al 2006), Kepler (Borucki et al 2010), K2 (Howell et al 2014), and TESS (Ricker et al 2015), the study of stellar pulsations has rapidly developed over the last 15 years
We aim to investigate the effect of the obliquity of a mixed dipolar internal fossil magnetic field with respect to the rotation axis on the frequency of gravity modes in rapidly rotating stars
We find that frequency shifts induced by the magnetic field on high-radial-order gravity modes are larger with increasing obliquity angle, when the magnetic axis is closer to the equatorial region, where these modes are trapped

Summary

Introduction

Thanks to high-precision space photometry with missions such as CoRoT (Baglin et al 2006), Kepler (Borucki et al 2010), K2 (Howell et al 2014), and TESS (Ricker et al 2015), the study of stellar pulsations has rapidly developed over the last 15 years It has provided important new information on the internal structure of stars through seismic modelling (see Aerts et al 2019, for a recent review). Donati & Landstreet 2009; Neiner et al 2015; Wade et al 2016) Studying both physical processes (pulsations and magnetism) at the same time offers further opportunities.

Frequency shifts

Zonal modes

Prograde modes

Retrograde modes

Influence of rotation

Findings

Discussion and conclusions