Seismic detection of core magnetic fields in red giants using the gravity offset

¹ IRAP, Université de Toulouse, CNRS, CNES, UPS, 14 Avenue Edouard Belin, 31400 Toulouse, France
² Institut Universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France

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Received: 17 December 2025
Accepted: 13 February 2026

Abstract

Context. Magnetic fields are known to efficiently redistribute angular momentum in stars. Recently, magnetic fields have been measured in the cores of red giant stars using asteroseismology, and it was shown that core magnetic fields, if unaccounted for, can bias the measurements of gravity-mode asymptotic properties, particularly the gravity offset ϵ_g, which is otherwise well characterised for red giants.

Aims. Our aim is to exploit this bias as a way to detect magnetic fields in the cores of red giants within the Kepler catalogue. We wish to increase the number of magnetic field detections in red giants but also establish a method that could be widely applicable to all red giants.

Methods. We selected 218 Kepler red giants showing abnormal measured values of ϵ_g. We used robust statistical criteria based on the expected lifetime of mixed modes to identify significant modes. We then adjusted an asymptotic expression for mixed modes to the observed frequencies, taking magnetic field and rotation into account using Bayesian inference. We then assessed the probability of magnetic field detection and measured the magnetic field intensity using stellar models for the favourable cases.

Results. We found 23 new magnetic red giant stars with fields ranging from 34 to 260 kG. For these stars, we measured values for ϵ_g now in agreement with the expected value. We also placed constraints on the magnetic field topology of seven stars. Adding these new detections to those of previous studies, we show that the mass distribution of magnetic giants is similar to that of the complete catalogue of red giants but different from the mass distribution of red giants with suppressed dipole modes. We also find that the core rotation of magnetic red giants follows a similar distribution as red giants in general. This could either mean that the detected fields do not have a predominant impact on the redistribution of angular momentum or that other red giants also harbour an internal field that is currently non-detectable or has not yet been detected.