Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes

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Abstract

When the core hydrogen is exhausted during stellar evolution, the central region of a star contracts and the outer envelope expands and cools, giving rise to a red giant. Convection takes place over much of the star’s radius. Conservation of angular momentum requires that the cores of these stars rotate faster than their envelopes; indirect evidence supports this1,2. Information about the angular-momentum distribution is inaccessible to direct observations, but it can be extracted from the effect of rotation on oscillation modes that probe the stellar interior. Here we report an increasing rotation rate from the surface of the star to the stellar core in the interiors of red giants, obtained using the rotational frequency splitting of recently detected ‘mixed modes’3,4. By comparison with theoretical stellar models, we conclude that the core must rotate at least ten times faster than the surface. This observational result confirms the theoretical prediction of a steep gradient in the rotation profile towards the deep stellar interior1,5,6.

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Figure 1: Oscillation spectrum of KIC 8366239.
Figure 2: Contributions to the total rotational splitting.
Figure 3: Rotational splitting versus mode linewidth for KIC 8366239.

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Acknowledgements

We acknowledge the work of the team behind Kepler. Funding for the Kepler Mission is provided by NASA's Science Mission Directorate. P.G.B. and C.A. were supported by the European Community’s Seventh Framework Programme (ERC grant PROSPERITY); J.D.R. and T.K. were supported by the Fund for Scientific Research, Flanders. S.H. was supported by the Netherlands Organisation for Scientific Research. J.M. and M.V. were supported by the Belgian Science Policy Office. The work is partly based on observations with the High Efficiency and Resolution Mercator Echelle Spectrograph at the Mercator Telescope, which is operated at La Palma in Spain by the Flemish Community.

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P.G.B., T.K., J.D.R., C.A., R.A.G., S.H., B.M., Y.E., S.F., F.C. and M.G. measured the mode parameters, and derived and interpreted the rotational splitting and period spacings. J.M., M.-A.D., P.E., J.C.-D. and A.M. calculated stellar models and provided theoretical interpretation of the rotational splitting. M.H. and M.V. observed and analysed the spectra. J.D.R., S.H., S.F., Y.E., D.S., T.R.B., H.K., F.R.G., J.R.H. and K.A.I. contributed to the coordination of the project, including the acquisition and distribution of the data. C.A. defined and supervised the research. All authors discussed the results and commented on the manuscript.

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Correspondence to Paul G. Beck.

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The authors declare no competing financial interests.

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This file contains Supplementary Notes and Data , Supplementary References, Supplementary Tables 1-2 and Supplementary Figures 1-11 with legends. (PDF 2049 kb)

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Beck, P., Montalban, J., Kallinger, T. et al. Fast core rotation in red-giant stars as revealed by gravity-dominated mixed modes. Nature 481, 55–57 (2012). https://doi.org/10.1038/nature10612

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