Effects of velocity and turbulence gradients on A-type supergiant spectra.

Summary. The observed variations of radial velocities of some supergiants are compiled. Measurements of differential line displacements in spectra of cc Cyg and Cas are presented. These observations can be interpreted, if one assumes a depth dependent radial velocity, which is varying with time. The micro turbulence is also depending on optical depth. A series of models for an A2-type supergiant have been computed, assuming different velocity fields fields. Using these models the equivalent widths and the differential displacements of lines of some ions of several elements were calculated. Analysing the results we can conclude: 1. The equivalent widths show differences of the order of 2030 %, if the lines are calculated using models with different velocity fields, but the same micro turbulence field. The shape of the curve of growth remains unchanged. 2. If models with different microturbulence fields, but one velocity field, are used, the equivalent widths can differ by a factor two or more. In addition the shape of the curve of growth is altered. On principle we can represent the observed correlation between micro turbulence and radial velocity The radial velocities of the calculated lines of different ions are discussed and compared with the observed results. Key words: model atmospheres - radial velocity fields - micro turbulence fields - line intensity variations - A-type supergiants