Phase Changes in Be Stars: The Be-shell and Be phases of Pleione

Phase changes from Be to Be-shell and/or B normal, and conversely, presently remain unpredictable in Be stars (except in some binaries where the transition to a shell phase seems to be associated with the orbital period). Because of their unpredictable character, these phenomena have been monitored in very few cases and are still very poorly understood. However, the existence of phase transitions have strong modelling implications. Under the disk model, the inclination of the star’s rotation axis determines the type of spectrum, Be or Be-shell, that a given Be star may exhibit. It is obvious that this picture is contradicted by phase transitions (Doazan 1982).