Gravity offset εg : a probe for core magnetic fields in red giants

Magnetic fields are thought of as a promising explanation for the transport of angular momentum in stars. Asteroseismic observations offer the opportunity to test this scenario, especially among red giants. Recently, the first measurements of core magnetic fields have been made for a handful of red giants, bringing valuable data on the intensity of these fields (Li et al 2022). Now, we seek to compose an exhaustive catalog of magnetic Kepler red giants, in order to better understand the prevalence and distribution of these fields. The gravity offset εg ,which appears in the asymptotic expression for g modes, has been shown to be nearly constant for all Kepler red giants (0.28 ± 0.08 ,Mosser et al. 2018). It has been suggested that deviations from this value could indicate the presence of a magnetic field (Li et al. 2022). We analyze around 150 Kepler red giants that show an abnormal value for the gravity offset in the catalogue of Li et al. (2024). For these stars, we identify dipolar mixed mode in their spectra, taking special care in assessing their significance. Then, we use Bayesian inference to fit an asymptotic expression accounting for rotation and magnetic field to the detected modes, and to determine the significance of the detections. We find that we can confidently add around 20 stars to the already existing catalogue of core magnetic field detections, expanding the sample by more than 50%. These measurements offer hints on the mass distribution of magnetic red giants. We investigate what can be inferred from this mass distribution about the origin of the detected fields. We bring information on core rotation for half of the detections, and insight on field topology for a handful of stars. These results will globally help us understand the mechanisms at play in angular momentum transport.