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Solar-type dynamo behaviour in fully convective stars without a tachocline

Abstract

In solar-type stars (with radiative cores and convective envelopes like our Sun), the magnetic field powers star spots, flares and other solar phenomena, as well as chromospheric and coronal emission at ultraviolet to X-ray wavelengths. The dynamo responsible for generating the field depends on the shearing of internal magnetic fields by differential rotation1,2. The shearing has long been thought to take place in a boundary layer known as the tachocline between the radiative core and the convective envelope3. Fully convective stars do not have a tachocline and their dynamo mechanism is expected to be very different4, although its exact form and physical dependencies are not known. Here we report observations of four fully convective stars whose X-ray emission correlates with their rotation periods in the same way as in solar-type stars. As the X-ray activity–rotation relationship is a well-established proxy for the behaviour of the magnetic dynamo, these results imply that fully convective stars also operate a solar-type dynamo. The lack of a tachocline in fully convective stars therefore suggests that this is not a critical ingredient in the solar dynamo and supports models in which the dynamo originates throughout the convection zone.

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Figure 1: Rotation–activity relationship diagram for partly and fully convective stars.

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Acknowledgements

N.J.W. acknowledges a Royal Astronomical Society Research Fellowship and an STFC Ernest Rutherford Fellowship. J.J.D. was supported by NASA contract NAS8-03060 to the Chandra X-ray Center. We thank J. Irwin, R. Jeffries and A. West for assistance and comments on an early draft of this paper. This research has made use of the Vizier (http://vizier.u-strasbg.fr/cgi-bin/VizieR) and SIMBAD (http://simbad.u-strasbg.fr/simbad/sim-fid) databases (operated at CDS, Strasbourg, France).

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N.J.W. reduced the Chandra observations, measured the X-ray fluxes and made the necessary calculations to plot the stars in Fig. 1. N.J.W. and J.J.D. wrote the interpretation and discussion of the results.

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Correspondence to Nicholas J. Wright.

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The authors declare no competing financial interests.

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Reviewer Information Nature thanks D. Moss and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Wright, N., Drake, J. Solar-type dynamo behaviour in fully convective stars without a tachocline. Nature 535, 526–528 (2016). https://doi.org/10.1038/nature18638

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