Spectral variability of luminous early type stars. I. Peculiar supergiant HD199478

We have obtained time-series of high-quality Hα spectra with high resolution in wavelength (R = lambda /delta lambda of 15000 to 22000) and time (Delta t =1d) of the late-type B supergiant HD199478. The spectra were analysed in terms of line-profile variability (lpv) using contemporary techniques of time-series analysis, such as Temporal Variance Spectrum and the 2d- Discrete Fourier Transform. The Hα profile is found to consist of a highly variable emission core (between -280 and +150 km s^-1) superimposed on almost constant, extended (+/-1000 km s^-1) emission wings. Due to the lack of strong line-emission, the latter is attributed to electron-scattering in deep atmospheric layers. The Hα variability manifests itself by variations in velocity and intensity of blue- and red-shifted emission peaks, which result in drastic alterations in the shape of the profile from almost symmetric and unshifted emission, with respect to the stellar rest frame, through blue- or red-shifted asymmetric emission, to double-peaked emission or a reverse P Cygni-type profile. Significant variations in total emissivity (i.e. EW) of the line are also noted, but these variations do not appear to be obviously linked to changes in the line-profile shape. The pattern of variability resembles that in Be-stars - though on a much shorter time scale - and suggests interpretation in terms of an axially symmetric and perturbed stellar wind. Since the time-scale of the V/R variations is found to be 3 to 5 times longer than the radial fundamental pulsation period but consistent with rotational period, rotational modulation as a possible cause for this variability is considered. Besides variations in Hα , continuous changes in velocity (typical dispersion of sigma ~ 5 km s^-1) and strength, i.e. EW, (up to 13% of the mean) of a sample of three absorption lines (CII lambda lambda6583 , 6578 and HeI lambda6678 ) were also observed. The phenomenon observed is more likely connected to changes in velocity and temperature structures of the stellar photosphere. Pulsation instability as a possible cause of photospheric variability is suggested.