2000A&A...363..995M

We have obtained time-series of high-quality Hα spectra with high resolution in wavelength (R=λ/δλ of 15000 to 22000) and time (Δt=1d) of the late-type B supergiant HD 199478. 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 +150km/s) superimposed on almost constant, extended (±1000km/s) 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 σ∼5km/s) and strength, i.e. EW, (up to 13% of the mean) of a sample of three absorption lines (CII λλ6583, 6578 and HeI λ6678) 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.