Evidence for one-armed oscillations in the equatorial disk of ζ Tauri from GI2T spectrally resolved interferometry

We report sub-mas observations of the Be Shell star ζ Tauri with the Grand Interféromètre à 2 Télescopes (GI2T) on November ’93 and October ’94. In ’93, the Hα line presented a V/R ratio of 0.57 with a central absorption shell component. On October ’94 the Hα line presented a reversed V/R ratio of 1.26 with a shallower absorption component. For both epochs we analysed the amplitude and phase of the fringe signal relative to the local continuum as a function of Doppler-shift across Hα. We clearly resolve the Hα emitting envelope on October ’93. We find that the bulge of the emission which occurs around RV=+130 km.s−1 has a N-S projected position of 0.7 mas to the South of the continuum source. This value corresponds to a linear separation of 3.6 photospheric radii. For October 94, the same analysis shows that the projected position of this bulge, occuring around RV=-70 km.s−1 has moved to 0.5 mas, i.e. 2.6 photospheric radii, North of the continuum source. On account of the opposite V/R values between 93 and 94 and the long term Hα cyclic variability of ζ Tauri this apparent motion corresponds to the first interferometric detection of an axiasymmetric envelope around a Be star that we interpret as direct evidence for a prograde one-armed oscillation of its equatorial disk.