1994A&A...291..856L

We investigate the effect of stellar mass loss on the photospheric lines of supergiants of spectral types O to late F. High mass loss produces a velocity gradient in the photospheres of supergiants which affects the profiles, the equivalent widths and the radial velocities of the lines. We show that the use of hydrostatic model atmospheres for the study of the equivalent widths of photospheric lines in the spectra of supergiants will result in large apparent microturbulent velocities. The derived microturbulent velocities do not reflect the presence of real microturbulence (i.e. small-scale isotropic motions with a Gaussian velocity distribution), but they reflect the presence of a velocity gradient in the photospheric layers. Therefore the apparent microturbulent velocity can be used to estimate the mass loss rates from supergiants. This explains the empirical relation between the mass loss rate of supergiants and the microturbulent velocity, or the inferred microturbulent pressure, found by Nieuwenhuijzen et al. (1992, 1994). We conclude that the mass loss from A, F and possibly early-G supergiants is not due to the pressure of the photospheric microturbulence, but that the apparent photospheric microturbulence is due to mass loss.