The luminosities of early-type B stars are high enough to drive mass outflows in the form of radiation-driven stellar winds, and this may extend to the later-type B stars if rapid rotation is also present to oppose the surface gravity.
The wind-compressed disk model seeks to explain how the wind may produce a circumstellar disk by tracing the trajectories of gas particles flowing out from the star and noting that streams from both hemispheres must eventually cross the equatorial plane, resulting in an accumulation there.
Hydrodynamical simulations have shown that localized enhancements in mass loss at the surface of the star can give rise to spiral streams in the wind. These corotating interaction regions may account for the variations in wind density commonly observed as absorption features in the ultraviolet spectra on time scales similar to the stellar rotation period.
Although both of these hypotheses are consistent with some of the observations, neither one includes a mechanism for the fundamental long term cycles of formation and dissipation of the disk which is the root of the Be phenomenon.
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