Macroturbulence is defined as a scale of turbulence in the stellar atmosphere in which the size of the turbulent cell is greater than the mean-free path of the photon. This is in contrast to microturbulence in which the size of a turbulent cell is smaller than the mean-free path. With microturbulence, a photon will tend to be emitted in one turbulent cell and absorbed in another; relative motions between these turbulent cells will cause a photon emitted in line center to be absorbed in the line wing. As a consequence, microturbulence both broadens and strengthens the line, as it leads to a desaturation of the line core. The effects of microturbulence are fully taken into account in the spectral synthesis calculation. This is not the case with macroturbulence. Macroturbulence will broaden the spectral line, but not desaturate it, and so it has no effect on the strength of the line. However, it is necessary to apply macroturbulent broadening to the computed spectrum when making a detailed comparison between the synthetic spectrum and a high-resolution observed spectrum. The program MACTURB implements the macroturbulent broadening formulation of David Gray (Gray, 2008). MACTURB requires as input the name of the spectral synthesis file, an output file name, the wavelength spacing in the input file and the macroturbulent velocity. The output wavelength spacing will be the same as the spacing in the input file. MACTURB may be used in the command line mode thus:
macturb input_file output_file spacing velocityMACTURB operates only on ascii synthetic spectra. There is no equivalent (yet) for SPECTRUM binary files. Macroturbulent and rotational broadening should be applied before the spectrum is smoothed with SMOOTH2. The macroturbulent velocity must be determined by trial and error, or via the techniques outlined in David Gray's book. Typically, the macroturbulent velocity is on the order of a few km/s.