Corotating photospheric clouds in the Be star η Cen

From 819 echelle spectra of eta Cen on 13 nights we find strong evidence for corotating gas clouds. Grey-scale images of stacked difference spectra of the combined He I lines at 4144, 4388, 4471,4922 and 5016 Angstrom show diffuse features moving from blue to red with accelerations consistent with the rotation of the star. These are also seen in the Si III 4553 line, but are not very evident in the H beta and H gamma Lines. The same features can be recognized on different nights, from which we infer a rotational period equal to the photometric period of 0.64 d. On many nights emission is present at velocities corresponding to the approaching or receding limb of the star. Although the features are largely stable, clear changes can also be detected. On one night there was a significant increase in emission from a certain region, resulting in the appearance of transient migrating absorption subfeatures sometimes seen in the line profiles of other Be stars. These would normally (incorrectly) be attributed to non-radial pulsation of high degree. These observations are inconsistent with the usually accepted pulsational model for periodic Be stars. We suggest that periodic Be stars can be understood in terms of a model of corotating gas clouds.