Perturbative analysis of the effect of a magnetic field on gravito-inertial modes

Abstract: Magnetic fields have been measured recently in the core of red giant stars thanks to their effects on stellar oscillation frequencies. The search for magnetic signatures in pulsating stars, such as $\gamma$ Doradus or Slowly Pulsation B stars, requires to adapt the formalism developed for the slowly rotating red giants to rapidly rotating stars. We perform a theoretical analysis of the effects of an arbitrary magnetic field on high radial order gravity and Rossby modes in a rapidly rotating star. The magnetic effects are treated as a perturbation. For high radial order modes, the contribution of the radial component of the magnetic field is likely to dominate over the azimuthal and latitudinal components. The rotation is taken into account through the traditional approximation of rotation. General expressions of the frequency shift induced by an arbitrary radial magnetic field are derived. Approximate analytical forms are obtained in the high-order high-spin-parameter limits for the modes most frequently observed in $\gamma$ Dor stars. We propose simple methods to detect seismic magnetic signatures and measure possible magnetic fields in such stars. These methods offer new possibilities to look for internal magnetic fields in future observations, such as the ones of the PLATO mission, or to revisit existing Kepler or TESS data.