Fourier-Doppler Imaging of line profile variations in zeta Ophiuchi

We present, for the first time, the Fourier-Doppler Imaging (FDI) analysis of periodic line profile variations in a zeta Oph-type star. For this purpo se we obtained, in the period from 1996 May 3 to May 5, a total of 242 high -resolution, high signal-to-noise ratio spectra of the Be star zeta Oph its elf. Using the FDI technique, we examine the variations in both time and wa velength and complement it with time series analysis. This kind of analysis is valid for both the nonradial pulsator model and the rotation modulation model, but we discuss the results in terms of the former model, considerin g it the more likely explanation for the observed line profile variability. Two distinct groups of modes are detected: medium (4 less than or equal to l approximate to \ m \ less than or equal to 8) and high-degree modes (whi ch could be associated with 13 less than or equal to l approximate to \ m \ less than or equal to 17). It is shown that the high-frequency oscillation s were strongly confined to an equatorial belt narrower than 20 degrees and that the line profile variability was caused predominantly by sectoral mod es, although tesseral modes \ m \ = l - 1 are not excluded in taking into a ccount the effect of fast rotation. We discuss the modal nature of the wave s with respect to the characteristic oscillation periods in the corotating frame and the high amplitude of the projected rotational velocity variation s (approximate to 20 km s(-1)).