Fast rotating stars with rotation periods less than 12 days (BLUE) are distinguished from
slow rotators (RED) by their X-ray luminosity. The Sun is yellow. X-ray luminosity is normalized to the
solar one. The Rossby number gives the rotation period in units of the eddy's lifetime.
The red dwarf activity, as it is revealed by
the X-ray emission of cool main-sequence stars, is mainly
magnetically driven. But a
turbulence is needed from a convection zone to produce stellar magnetic fields.
Stellar rotation and turbulence in a convection zone are
responsible not only for the generation of a magnetic
field but also lead to differential rotation . Observations indicate
the normalized surface differential rotation decreasing for fast velocity stars.
The inclusion of meridinonal flow into the computations indeed yields a
systematic reduction of the resulting differential surface rotation .
To
understand the solar-stellar connection long-term
monitoring of stars in different spectral lines is of vital
importance. This is the aim of our telescope project STELLA .
The internal rotation law of a T Tauri star (WTTS) proves to be
close to rigid-body rotation hence the
WTTS dynamo should work in the alpha^2 regime.
The resulting magnetic field must therefore
have nonaxisymmetric geometry and will not oscillate.
