18 research outputs found
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