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

Abstract

Magnetic fields are known to efficiently redistribute angular momentum in stars. They have been recently measured in the cores of red giant stars using asteroseismology. It was shown that core magnetic fields, if unaccounted for, can bias the measurements of the gravity offset $ε_g$, which is otherwise well characterised for red giants. Exploiting this bias as a way to detect magnetic fields in the cores of red giants, we wish to increase the number of magnetic field detections in red giants, but also to establish a method that could be widely applicable to all red giants. 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. We found 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. Adding these new detections to those of previous studies, we showed 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 found 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.