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

Suppression of dipolar oscillation modes by strong magnetic fields in the cores of intermediate-mass red giant stars reveals that powerful magnetic dynamos were very common in the previously convective cores of these stars. Core magnetic fields in intermediate-mass stars Stellar magnetic fields are present on the surfaces and in the immediate surroundings of stars such as the Sun, and it has been conjectured that magnetic fields also exist deep within stars, where they may have a major effect on stellar evolution. Dennis Stello et al . report observations of dipolar oscillation modes of 3,600 intermediate-mass red giant stars that suggest the presence of strong internal magnetic fields in 60 per cent of the sample. About 20 per cent of the sample show mode suppression from strong magnetic fields in the cores, but this fraction is a strong function of mass. Strong core fields only occur in red giants above 1.1 solar masses. This result demonstrates that strong magnetic fields in stars are much more common than previously thought. Magnetic fields play a part in almost all stages of stellar evolution 1 . Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes 2 , 3 . Intermediate-mass stars do not have deep convective envelopes 4 , although 10 per cent exhibit strong surface fields that are presumed to be residuals from the star formation process 5 . These stars do have convective cores that might produce internal magnetic fields 6 , and these fields might survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface 7 . Here we report the strength of dipolar oscillation modes for a sample of 3,600 red giant stars. About 20 per cent of our sample show mode suppression, by strong magnetic fields in the cores 8 , but this fraction is a strong function of mass. Strong core fields occur only in red giants heavier than 1.1 solar masses, and the occurrence rate is at least 50 per cent for intermediate-mass stars (1.6–2.0 solar masses), indicating that powerful dynamos were very common in the previously convective cores of these stars.