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

TL;DR

This study shows that fully convective stars without a tachocline exhibit solar-like magnetic activity linked to rotation, indicating they operate a similar dynamo mechanism as Sun-like stars.

Contribution

It provides observational evidence that fully convective stars can have a solar-type dynamo without a tachocline, challenging previous assumptions about dynamo requirements.

Findings

X-ray emission correlates with rotation in fully convective stars

Fully convective stars exhibit solar-like magnetic activity patterns

Tachocline not essential for solar-type dynamo

Abstract

In solar-type stars (with radiative cores and convective envelopes), 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 rotation. The shearing has long been thought to take place in a boundary layer known as the tachocline between the radiative core and the convective envelope. Fully convective stars do not have a tachocline and their dynamo mechanism is expected to be very different, 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 Sun-like 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.