v_rot represents the rotational velocity which is deduced from the effect of the rotational broadening. Rotational broadening is important for weak spectral lines which are formed in the velocity region around the sonic point (cf. Puls et al. 1996 and Pauldrach et al. 1994; for instance, the He ii line at 1640 Å is considerably influenced by rotational broadening, see Fig. 6).

Hamann (1980) was the first who showed that the assumption of a velocity dispersion v_turb is needed to fit the observed UV P-Cygni profiles of strong lines (see also Puls 1987 and Puls et al. 1993). The physical background of this parameter is well explained by the shock instabilities in the wind flow (cf. Pauldrach et al. 1994) – v_turb can be related to the shock jump velocities. As a consequence of this behavior, the calculated synthetic spectra must account for this velocity dispersion by adopting a depth dependent v_turb law, where the minimum value is usually regarded to be of the order of the sound speed, whereas the maximum value has to be determined from the shape of the strong saturated P-Cygni lines (cf. Pauldrach et al. 1994).