X-ray emission from the Sun comes from radiation from plasma confined by magnetic fields in corona and heated at temperature of millions degrees.
X-ray emission from the Sun comes from radiation from plasma confined by magnetic fields in corona and heated at temperature of millions degrees.
In stars with `solar' activity there is a dynamo mechanism due to the differential rotation and convection, able to generate and amplify the magnetic fields. The rotation generates the magnetic field inside the star. This magnetic field is transported through convective envelope to the surface.
Stars with internal structure not involving both radiative and convective regions cannot sustain `solar' activity. In particular stars with mass significantly higher than the Sun do not have convective region, while very low mass stars do not have radiative core.
Intermediate mass stars ---------------- Very low mass stars
Dynamo should break down at SpT ~ A7...F3,
but X-rays are observed from late-B/A stars : X-rays generated by (unknown) late-type companion stars ???
In absence of different theoretical explanation for X-rays
the more plausible explanation is the companion hypothesis: indeed
many companions around B/A-type stars are found, making plausible this hypothesis, but
we cannot to exclude intrinsic X-ray emission from B/A-type stars due to a new mechanism,
such as intrinsic emission from B/A-type stars related to "fossil" magn.fields ?
