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

Abstract

New series of echelle spectra were obtained to study the medium- and long-term evolution of the disks of several Be stars. Subtle variations in the wings of optically thin and thick emission lines suggest that the conventional, static picture of the disk being in quasi-contact with the central star is justied primarily (or perhaps only) after an outburst event. Some weeks to months later, a low-density region seems to develop above the star and slowly grows outwards. A subsequent outburst may later replenish this cavity. In fact, in two stars this more ring-like structure is apparently at times detached far enough from the star to allow for the formation of a secondary inner disk from the ejecta of a later outburst. This behaviour is not necessarily representative of Be stars in general because in the later spectral 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 dierences in strength and variability between the winds of Be and normal B stars. It seems possible to attribute these dierences to matter that was initially in the disk and therefore largely shielded from the stellar radiation, but that during the course of the inner excavation (or even complete distruction) of the disk becomes exposed.