  We present synthetic FeH band spectra in the z-filter range for several
M-dwarf models with logg=3.0-5.0 [cgs] and Teff=2800K -3450K. Our aim is to
characterize convective velocities in M-dwarfs and to give a rough estimate of
the range in which 3D-atmosphere treatment is necessary and where 1D-atmosphere
models suffice for the interpretation of molecular spectral features. This is
also important in order to distinguish between the velocity-broadening and the
rotational- or Zeeman-broadening. The synthetic spectra were calculated using
3D CO5BOLD radiative-hydrodynamic (RHD) models and the line synthesis code
LINFOR3D. We used complete 3D-models and high resolution 3D spectral synthesis
for the detailed study of some well isolated FeH lines. The FeH line strength
shows a dependence on surface gravity and effective temperature and could be
employed to measure both quantities in M-type objects. The line width is
related to the velocity-field in the model stars, which depends strongly on
surface gravity. Furthermore, we investigate the velocity-field in the 3D
M-dwarf models together with the related micro- and macro-turbulent velocities
in the 1D case. We also search for effects on the lineshapes.
