Short-period line profile variations in the Be star mu Centauri

We present new intensive photometric observations of the Be star mu Cen for several seasons which support a period close to 1 d. We also present high- resolution spectroscopic data consisting of 302 spectra in 1999 and 864 spe ctra in 2000, all obtained within a two-week observing run in each season. We use stacked grey-scale plots of spectra, from which the mean line profil e has been removed, to examine the profile variations. We find that most ni ghts show one residual absorption feature, moving from blue to red, visible in all helium and metal lines and also clearly visible in H alpha and othe r lines formed in the circumstellar environment. We therefore conclude that this feature is of circumstellar origin. In addition, a residual absorptio n feature moving from red to blue is sometimes seen at irregular intervals. We find that the residual absorption feature frequently strays outside the projected rotational velocity limit and that on occasions it remains well within this limit. The radial velocity data reproduce only two of the six f requencies previously found in the star. We point out that this by no means implies that the star is a multiperiodic, non-radial pulsator. Photometric data obtained over several seasons indicate a period very close to 1 d and not the 0.5-d period found from the radial velocities. We describe an outb urst which occurred during the run and which resulted in increased H alpha emission two nights later. It is clear that outbursts in Be stars are local ized events and that the gas released by outbursts is probably responsible for localized increased absorption, resulting in periodic light and line pr ofile variations.