  We carried out 3D radiative magnetohydrodynamic simulations of the convective
and magnetic structure in the surface layers (uppermost part of the convection
zone and photosphere) of main-sequence stars of spectral types F3 to M2. The
simulation results were analyzed in terms of sizes and properties of the
convection cells (granules) and magnetic flux concentrations as well as
velocity, pressure, density, and temperature profiles.
  Our numerical simulations show for the first time a qualitative difference in
the magneto-convection between solar-like stars and M dwarfs. Owing to higher
surface gravity, lower opacity (resulting in higher density at optical depth
unity), and more stable downflows, small-scale magnetic structures concentrate
into pore-like configurations of reduced intensity. This implies that in very
cool stars magnetic surface structures like plage regions and starspots
significantly differ from the solar example. Such a difference would have major
impact on the interpretation of Doppler imaging data and the analysis of M
dwarf spectra.
