Magnetic signatures on mixed-mode frequencies II. Period spacings as a probe of the internal magnetism of red giants

Context. Theoretical works have looked into the various topologies and amplitudes, as well as the stability of the magnetic field that is expected to be present in the radiative interior of stars evolving after the main sequence. From these studies, we know that strong stable fossil fields might be trapped inside evolved stars. These could trigger the strong transport of angular momentum from the core to the envelope, a process that is not generally included in state-of-the-art stellar models. This may therefore have a substantial impact on the mixing and the inferred stellar parameters. Such internal magnetic fields have never been observed in evolved stars. As a result, there is a major piece missing from our global picture of stars as dynamical bodies. Aims. Asteroseismology has opened a window onto stellar internal dynamics, as oscillation frequencies, amplitudes, and lifetimes are affected by processes that are taking place inside the star. The detection of buried magnetic fields could therefore be possible through the measurement of their impact on the oscillations of stars. This advancement would be groundbreaking for our knowledge of stellar dynamics. In this scope, magnetic signatures on mixed-mode frequencies have recently been characterized, but the task of detection remains challenging as the mixed-mode frequency pattern is highly complex and affected by rotational effects, while modes of different radial orders are often intertwined. In this work, we aim to build a bridge between theoretical prescriptions and complex asteroseismic data analysis to facilitate a future search and characterization of internal magnetism with asteroseismology. Methods. We investigated the effect of magnetic fields inside evolved stars with solar-like oscillations on the estimation of the period spacing of gravity-mode (g-mode) components of simulated mixed gravito-acoustic modes. We derived a new corrected stretching function of the power spectrum density to account for the presence of magnetic signatures on their frequencies. Results. We demonstrate that the strong dependency of the amplitude of the magnetic signature with mixed-mode frequencies leads to biased estimates of period spacings towards lower values. We also show that a careful analysis of the oscillation frequency pattern through various period spacing estimates and across a broad frequency range might lead to the first detection of magnetic fields inside red giants and at the same time, we adjust the measured value of g-mode period spacing.

Automated summary

In this study, we investigate the effect of magnetic fields on the internal dynamics of evolved stars using asteroseismic data. We propose a new method to account for magnetic signatures in the estimation of period spacings. Our results show that the presence of magnetic fields leads to biased estimates of period spacings and careful analysis of multiple estimates and frequency patterns may result in the first detection of internal magnetic fields in red giants.