Circumstellar material is a characteristic of hot luminous stars (Dougherty, 1993); it is ubiquitous and often remains present throughout their entire life (Waters et al., 2000). The stars’ spectra show complex hydrogen Balmer line profiles with emission components. In addition to Be stars, stars such as O and B supergiants, luminous blue variables, Wolf-Rayet stars Oe and Ae stars all exhibit circumstellar matter.
Be stars have two distinct regions of circumstellar matter: a diffuse polar stellar wind and a denser, Balmer line emitting, component in the equatorial plane of the central star (see e.g., Dachs, 1987; Dachs et al., 1986; Porter, 1998; Slettebak, 1988). The fast diffuse polar wind is well described by standard line-driven wind theory physics (see Castor et al., 1975; Lucy and Solomon, 1970), however, it has been much more difficult to describe the equatorial disc (Porter, 1998).
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Optical interferometry is now able to provide milli-arc-second spatial resolution and can be
thus used to probe the circumstellar environments of Be stars. Of note is Figure 1.1 which
shows direct observational evidence of extended H
emission around the star 
Tauri
(Quirrenbach et al., 1994).
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A sub-class of the Be stars are shell stars, which are interpreted as Be stars oriented so that the circumstellar disc is viewed edge-on see Figure 1.2. Shell stars have spectra that show Balmer emission with sharp absorption cores, narrow absorption lines of ionized metals, and broad HeI absorption, see Figure 1.3.
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Observations of Be star discs have shown that they vanish and re-appear in a apparently random fashion with time-scales of 100s of days (e.g., Hanuschk et al., 1993; Hirata, 1995). Such observations are interpreted as an increase in the density of the circumstellar material, for line generation, and a rarefaction for line dispersal Dachs (1987). A large rarefaction can cause a Be star spectrum to resemble a normal B-star spectrum, before once again entering a Be phase. Figure 1.4 shows observations of Pleione over ~30 year period, the disappearance and reappearance of a Be star disc can be easily seen. To be noted is the sudden fading of the disc whilst the re-growth is evident over a much longer time scale. Be stars may go through periods of disc, no-disc and disc reappearance.
In Figure 1.4 the closed circular symbols indicate a period when the Be star went through a period of being a shell star. Shell stars are accepted to be Be stars viewed edge on. The reason why a Be star should fluctuate between being a shell star and not is unclear. One possible explanation is that the flaring factor of the disc is not a constant. Therefore if a Be star is viewed from a low-latitude a flared disc would be interpreted as a shell star. Should the flaring decrease then the star would be viewed as a “normal” Be star. The method which would give rise to any change of flaring in the disc is not clear.
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