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

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.Comment: 6 pages, 1 figure. Accepted for publication in Nature (28 July 2016). Author's version, including Method

Stellar Coronal Activity and Its Impact on Planets

In this chapter the relevance of stellar coronal activity in determining the physical conditions of planetary atmospheres is discussed. We still lack a comprehensive, self-consistent picture of the role of stellar activity, during the star lifespan, in determining and shaping the evolution of planetary atmospheres, after the circumstellar disk has been cleared out, but many efforts in this direction are today ongoing. Here we focus on high-energy radiation since it penetrates deeply the atmosphere ionizing and heating the gas, thus affecting its chemistry with consequences very different from those of optical or UV radiation. Stellar activity is inhomogeneous and variable; flares in particular can be very frequent and intense in young and dM stars. Depending on their duty cycle, they may drive the atmospheric gas toward different chemical regimes