On the oscillation spectrum of a magnetized core in a giant star

¹ Université de Toulouse; UPS-OMP; IRAP ; Toulouse, France
² 2CNRS; IRAP ; 14, avenue Edouard Belin, F-31400 Toulouse, France

^* e-mail: mrieutord@irap.omp.eu

Published online: 27 October 2017

Abstract

The spectrum of gravito-acoustic modes is depleted in dipolar modes for a significant fraction of the giant stars observed by the Kepler mission, a feature that has been explained by the presence of magnetic fields in the core of these stars (Fuller et al. 2015, Cantiello et al. 2016). We further investigate this possible scenario by considering first the oscillation spectrum of the core of a giant star modeled by a stably stratified, self-gravitating fluid of uniform density in a sphere pervaded by a uniform magnetic field. Our results show that the first effect of a magnetic field on the g-modes is to reduce their wavenumber and therefore reduce their damping. The magnetic effect, on this model, is therefore opposite Fuller’s et al scenario. Moreover, the model shows that it is not possible to change the damping rate without changing the frequency of the modes and this latter change is not observed. Because of the simplicity of our model, the magnetized core scenario cannot be dismissed but further investigations are needed, and other ways of explaining the presence of depressed modes should also be considered.