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Magnetic fields of 30 to 100 kG in the cores of red giant stars

Abstract

A red giant star is an evolved low- or intermediate-mass star that has exhausted its central hydrogen content, leaving a helium core and a hydrogen-burning shell. Oscillations of stars can be observed as periodic dimmings and brightenings in the optical light curves. In red giant stars, non-radial acoustic waves couple to gravity waves and give rise to mixed modes, which behave as pressure modes in the envelope and gravity modes in the core. These modes have previously been used to measure the internal rotation of red giants1,2, leading to the conclusion that purely hydrodynamical processes of angular momentum transport from the core are too inefficient3. Magnetic fields could produce the additional required transport4,5,6. However, owing to the lack of direct measurements of magnetic fields in stellar interiors, little is currently known about their properties. Asteroseismology can provide direct detection of magnetic fields because, like rotation, the fields induce shifts in the oscillation mode frequencies7,8,9,10,11,12. Here we report the measurement of magnetic fields in the cores of three red giant stars observed with the Kepler13 satellite. The fields induce shifts that break the symmetry of dipole mode multiplets. We thus measure field strengths ranging from about 30 kilogauss to about 100 kilogauss in the vicinity of the hydrogen-burning shell and place constraints on the field topology.

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Fig. 1: Asymmetric splittings of two mixed modes in KIC 8684542.
Fig. 2: Multiplet asymmetries in KIC 8684542 as a function of mode frequency.
Fig. 3: Stretched échelle diagram for KIC 8684542.

Data availability

Kepler data are publicly available from the Mikulski Archive for Space Telescopes (MAST) portal at https://archive.stsci.edu. Spectra are available at https://doi.org/10.5281/zenodo.6818371.

Code availability

This study makes use of the stellar evolution code MESA, which is available at https://docs.mesastar.org.

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Acknowledgements

We acknowledge support from from the project BEAMING ANR-18-CE31-0001 of the French National Research Agency (ANR) and from the Centre National d’Etudes Spatiales (CNES).

Author information

Authors and Affiliations

Authors

Contributions

G.L. discovered the three stars with asymmetric splittings. G.L. and S.D. measured the asymmetries and rotation rates. S.D. measured the absolute magnetic shifts and supervised the whole project. J.B. and F.L. developed the theoretical framework used to interpret the observations. All the authors contributed to writing the manuscript.

Corresponding author

Correspondence to Sébastien Deheuvels.

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Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature thanks Matteo Cantiello, Margarida Cunha and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Shape of the weight function K(m) as a function of the normalized mass.

The function K(m) is shown for the stellar model representative of KIC 11515377. The blue shaded region indicates the hydrogen-burning shell. The vertical dashed line corresponds to the maximal extent of the initial convective core at the beginning of the main sequence.

Extended Data Fig. 2 Multiplet asymmetries in KIC 11515377 as a function of mode frequency.

Symbols have the same meaning as in Fig. 2.

Extended Data Fig. 3 Multiplet asymmetries in KIC 7518143 as a function of mode frequency.

Symbols have the same meaning as in Fig. 2.

Extended Data Fig. 4 Stretched échelle diagram for KIC 11515377.

Symbols have the same meaning as in Fig. 3.

Extended Data Fig. 5 Stretched échelle diagram for KIC 7518143.

Symbols have the same meaning as in Fig. 3.

Supplementary information

Supplementary Information

Supplementary Sections 1–7, including Supplementary Figs. 1–6 and Tables 1–7.

Peer Review File

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Li, G., Deheuvels, S., Ballot, J. et al. Magnetic fields of 30 to 100 kG in the cores of red giant stars. Nature 610, 43–46 (2022). https://doi.org/10.1038/s41586-022-05176-0

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