  We present an investigation of the velocity fields in early to late M-type
star hydrodynamic models, and we simulate their influence on FeH molecular line
shapes. The M star model parameters range between log g of 3.0 - 5.0 and Teff
of 2500 K and 4000 K. Our aim is to characterize the Teff- and log g
-dependence of the velocity fields and express them in terms of micro- and
macro-turbulent velocities in the one dimensional sense. We present also a
direct comparison between 3D hydrodynamical velocity fields and 1D turbulent
velocities. The velocity fields strongly affect the line shapes of FeH, and it
is our goal to give a rough estimate for the log g and Teff parameter range in
which 3D spectral synthesis is necessary and where 1D synthesis suffices. In
order to calculate M-star structure models we employ the 3D
radiative-hydrodynamics (RHD) code CO5BOLD. The spectral synthesis on these
models is performed with the line synthesis code LINFOR3D. We describe the 3D
velocity fields in terms of a Gaussian standard deviation and project them onto
the line of sight to include geometrical and limb-darkening effects. The micro-
and macro-turbulent velocities are determined with the "Curve of Growth" method
and convolution with a Gaussian velocity profile, respectively. To characterize
the log g and Teff dependence of FeH lines, the equivalent width, line width,
and line depth are regarded. The velocity fields in M-stars strongly depend on
log g and Teff. They become stronger with decreasing log g and increasing Teff.
