Asteroseismic detection of a predominantly toroidal magnetic field in the deep interior of the main-sequence F star KIC 9244992

TL;DR

This study uses asteroseismology to detect and characterize a strong toroidal magnetic field deep inside a main-sequence F star, revealing insights into stellar magnetism and internal magnetic structures.

Contribution

First detection of a deep interior magnetic field in a main-sequence star using asteroseismic frequency asymmetries, providing constraints on magnetic field strength and structure.

Findings

Detected a strong azimuthal magnetic field (~92 kG) in the star's interior.

Estimated the radial magnetic component to be at least 3.5 kG.

Found the magnetic field strength exceeds dynamo mechanism predictions.

Abstract

An asteroseismic analysis has revealed a magnetic field in the deep interior of a slowly-rotating main-sequence F star KIC9244992, which was observed by the Kepler spacecraft for four years. The star shows clear asymmetry of frequency splittings of high-order dipolar gravity modes, which cannot be explained by rotation alone, but are fully consistent with a model with rotation, a magnetic field and a discontinuous structure (glitch). Careful examination of the frequency dependence of the asymmetry allows us to put constraints on not only the radial component of the magnetic field, but also its azimuthal (toroidal) component. The lower bounds of the root-mean-squares of the radial and azimuthal components in the radiative region within 50 per cent in radius, which have the highest sensitivity in the layers just outside the convective core with a steep gradient of chemical compositions, are estimated to be $\mathsf{B}_{\mathrm{r}}^{\min}=3.5 \pm 0.1$ kG and $\mathsf{B}_{\phi}^{\min}=92 \pm 7$ kG, respectively. The much stronger azimuthal component than the radial one is consistent with the significant contribution of the differential rotation although the star has almost uniform rotation at present. The estimated field strengths are too strong to be explained by dynamo mechanisms in the radiative zone associated with the magnetic Tayler instability. The aspherical glitch is found to be located in the innermost radiative layers where there is a steep gradient of chemical composition. The first detection of magnetic fields in the deep interior of a main-sequence star sheds new light on the problem of stellar magnetism, for which there remain many uncertainties.