We present synthetic FeH band spectra in the z -filter range for several M-dwarf models with \log { g } = 3.0 - 5.0 [ cgs ] and T _ { eff } = 2800 ~ { } { K } -3450 ~ { } { K } .
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 .