Letter

Strong dipole magnetic fields in fast rotating fully convective stars

  • Nature Astronomy 1, Article number: 0184 (2017)
  • doi:10.1038/s41550-017-0184
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Abstract

M dwarfs are the most numerous stars in our Galaxy, with masses between approximately 0.5 and 0.1 solar masses. Many of them show surface activity qualitatively similar to our Sun and generate flares, high X-ray fluxes and large-scale magnetic fields1,2,3,4. Such activity is driven by a dynamo powered by the convective motions in their interiors2,5,6,7,8. Understanding properties of stellar magnetic fields in these stars finds a broad application in astrophysics, including theory of stellar dynamos and environment conditions around planets that may be orbiting these stars. Most stars with convective envelopes follow a rotation–activity relationship where various activity indicators saturate in stars with rotation periods shorter than a few days2,6,8. The activity gradually declines with rotation rate in stars rotating more slowly. It is thought that, due to a tight empirical correlation between X-ray radiance and magnetic flux9, the stellar magnetic fields will also saturate, to values around 4 kG (ref. 10). Here we report the detection of magnetic fields above the presumed saturation limit in four fully convective M dwarfs. By combining results from spectroscopic and polarimetric studies, we explain our findings in terms of bistable dynamo models11,12: stars with the strongest magnetic fields are those in a dipole dynamo state, whereas stars in a multipole state cannot generate fields stronger than about 4 kG. Our study provides observational evidence that the dynamo in fully convective M dwarfs generates magnetic fields that can differ not only in the geometry of their large-scale component, but also in the total magnetic energy.

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Acknowledgements

This project was carried out in the framework of the Deutsche Forschungsgemeinschaft funded CRC 963—Astrophysical Flow Instabilities and Turbulence (projects A16 and A17). We acknowledge support from the Deutscher Akademischer Austauschdienst strategic partnership project U4-Network to D.S. and funding through a Heisenberg Professorship, RE 1664/9-2, to A.R. R.Y. is supported by NASA Chandra grant GO4-15011X. O.K. acknowledges funding from the Swedish Research Council and the Swedish National Space Board. We also acknowledge the use of electronic databases (VALD, SIMBAD, NASA Astrophysics Data System). This research is based on observations collected at the Canada–France–Hawaii Telescope (Hawaii) and Télescope Bernard Lyot (France).

Author information

Affiliations

  1. Institute for Astrophysics, Georg-August University, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany.

    • D. Shulyak
    • , A. Reiners
    •  & A. Engeln
  2. Canada–France–Hawaii Telescope Corporation, 65-1238 Mamalahoa Highway, Kamuela, Hawaii 96743, USA.

    • L. Malo
  3. Institut de recherche sur les exoplanétes, Département de Physique, Université de Montréal, Montréal, QC H3C 3J7, Canada.

    • L. Malo
  4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.

    • R. Yadav
  5. LUPM, Université de Montpellier, CNRS, place E. Bataillon, F-34095 Montpellier, France.

    • J. Morin
  6. Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden.

    • O. Kochukhov

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Contributions

D.S. contributed general scientific ideas and conclusions, carried out the processing, modelling and analysis of observed data. A.R. contributed general scientific ideas and conclusions. A.E. carried out modelling and analysis of observed data. L.M. carried out telluric correction on selected targets. R.Y. provided theoretical dynamo models and magnetic maps. J.M. provided and analysed additional Stokes V data on some ESPaDOnS targets. O.K. contributed to development of atomic line analysis methodology. All authors contributed to the text of the paper and discussed the results.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to D. Shulyak.

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    Supplementary Information

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