Long-term spectroscopic monitoring of BA-type supergiants III. Variability of photospheric lines?

We obtained time series of spectra with high S/N and high resolution in wavelength and time of early-type A and late-type B supergiants (cf. Kaufer et al. 1996a, Paper i, and Kaufer et al. 1996b, Paper ii for the analysis of the variability of the stellar envelopes). In this work we inspect the time variations of the numerous photospheric line profiles in the optical spectrum. We find complex cyclical variations of the radial velocities with a typical velocity dispersion of σ ≈ 3 km s−1. The corresponding equivalent-width variations are less than 1% of their mean if we assume a common modulation mechanism for both radial velocities and equivalent width. We do not find any depth dependence of the velocity fields in the metallic lines. For αCyg the Balmer lines show an increase of the radial velocity from H27 to H8 by 3 km s−1, which is identified with the onset of the radially accelerating velocity field of the stellar wind. TheCleaned periodograms of the radial-velocity curves show the simultaneous excitation of multiple pulsation modes with periods longer and shorter than the estimated radial fundamental periods of the objects, which might indicate the excitation of non-radial and radial overtones, respectively. The analysis of the line-profile variations (LPV) of the photospheric line spectrum reveals prograde travelling features in the dynamical spectra. The travelling times of these features are in contradiction to the possible rotation periods of these extended, slowly rotating objects. Therefore, we suggest that these features should be identified with non-radial pulsation modes, possibly g-modes, of low order (l = |m| ∼ 5).