Evolution in circumstellar envelopes of Be stars: From disks to rings?

New series of echelle spectra were obtained to study the medium- and long-t erm evolution of the disks of several Be stars. Subtle variations in the wi ngs of optically thin and thick emission lines suggest that the conventiona l, static picture of the disk being in quasi-contact with the central star is justified primarily (or perhaps only) after an outburst event. Some week s to months later, a low-density region seems to develop above the star and slowly grows outwards. A subsequent outburst may later replenish this cavi ty. In fact, in two stars this more ring-like structure is apparently at ti mes detached far enough from the star to allow for the formation of a secon dary inner disk from the ejecta of a later outburst. This behaviour is not necessarily representative of Be stars in general because in the later spec tral sub-types, discrete mass loss events have not so far been observed to play a major role. In the light of the apparent life cycle of such disks, a brief discussion is given of the differences in strength and variability b etween the winds of Be and normal B stars. It seems possible to attribute t hese differences to matter that was initially in the disk and therefore lar gely shielded from the stellar radiation, but that during the course of the inner excavation (or even complete distruction) of the disk becomes expose d.