In solar-type stars (with radiative cores and convective envelopes), themagnetic field powers star spots, flares and other solar phenomena, as well aschromospheric and coronal emission at ultraviolet to X-ray wavelengths. Thedynamo responsible for generating the field depends on the shearing of internalmagnetic fields by differential rotation. The shearing has long been thought totake place in a boundary layer known as the tachocline between the radiativecore and the convective envelope. Fully convective stars do not have atachocline and their dynamo mechanism is expected to be very different,although its exact form and physical dependencies are not known. Here we reportobservations of four fully convective stars whose X-ray emission correlateswith their rotation periods in the same way as in Sun-like stars. As the X-rayactivity - rotation relationship is a well-established proxy for the behaviourof the magnetic dynamo, these results imply that fully convective stars alsooperate a solar-type dynamo. The lack of a tachocline in fully convective starstherefore suggests that this is not a critical ingredient in the solar dynamoand supports models in which the dynamo originates throughout the convectionzone.
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