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This is the author’s version of the work. It is posted here for personal use, not for redistribution. The definitive version was published in Nature on 04 January 2016, DOI:10.1038/nature16171
Magnetic fields play a role in almost all stages of stellar evolution \citep{Landstreet_1992}. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes \citep{Parker_1955,Donati_2009}. Intermediate-mass stars do not have deep convective envelopes \citep{Kippenhahn_1990}, although 10% exhibit strong surface fields that are presumed to be residuals from the stellar formation process \citep{2008CoSka..38..443P}. These stars do have convective cores that might produce internal magnetic fields \citep{Brun_2005}, 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 \citep{Auri_re_2015}. 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 \citep{Fuller15}, in over 3,600 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 (1.1\(\mathrm{M}_\odot\)), and the occurrence rate is at least 60% for intermediate-mass stars (1.6–2.0\(\mathrm{M}_\odot\)), indicating that powerful dynamos were very common in the convective cores of these stars.