A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars

TL;DR

This study reveals that a significant proportion of intermediate-mass stars possess strong magnetic fields generated by dynamos in their convective cores, as evidenced by asteroseismic measurements of mode suppression in Kepler data.

Contribution

It provides the first large-scale observational evidence of prevalent core magnetic fields in intermediate-mass stars using asteroseismology.

Findings

Approximately 20% of red giants show mode suppression.

Strong core magnetic fields are found mainly in stars above 1.1 solar masses.

At least 60% of intermediate-mass stars (1.6-2.0Msun) have strong core magnetic fields.

Abstract

Magnetic fields play a role in almost all stages of stellar evolution. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes. Intermediate-mass stars do not have deep convective envelopes, although 10% exhibit strong surface fields that are presumed to be residuals from the stellar formation process. These stars do have convective cores that might produce internal magnetic fields, and these might even survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface. Here we use asteroseismology to study the occurrence of strong magnetic fields in the cores of low- and intermediate-mass stars. We have measured the strength of dipolar oscillation modes, which can be suppressed by a strong magnetic field in the core, in over 3600 red giant stars observed by Kepler. About 20% of our sample show mode suppression but this fraction is a strong function of mass. Strong core fields only occur in red giants above 1.1 solar masses (1.1Msun), and the occurrence rate is at least 60% for intermediate-mass stars (1.6--2.0Msun), indicating that powerful dynamos were very common in the convective cores of these stars.