Fourier‐Doppler Imaging of Line Profile Variations in ζ Ophiuchi

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Abstract
We present, for the first time, the Fourier-Doppler Imaging (FDI) analysis of periodic line profile variations in a ζ Oph-type star. For this purpose 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 ζ Oph itself. Using the FDI technique, we examine the variations in both time and wavelength 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, considering it the more likely explanation for the observed line profile variability. Two distinct groups of modes are detected: medium (4 ≤ l ≈ |m| ≤ 8) and high-degree modes (which could be associated with 13 ≤ l ≈ |m| ≤ 17). It is shown that the high-frequency oscillations were strongly confined to an equatorial belt narrower than 20° and that the line profile variability was caused predominantly by sectoral modes, although tesseral modes |m| = l - 1 are not excluded in taking into account the effect of fast rotation. We discuss the modal nature of the waves with respect to the characteristic oscillation periods in the corotating frame and the high amplitude of the projected rotational velocity variations (≈20 km s-1).

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