Be stars

Be stars show H emission that is strongly variable and usually double-peaked. They are also known to be rapid rotators, which led Struve (1931) to suggest that the emission arises in a circumstellar disk of ejected matter. This model has not been universally accepted (Doazan, 1987), but optical interferometry has now confirmed that Be star envelopes are indeed flattened (Mourard et al. 1987; Quirrenbach et al. 1993, 1994; Stee et al. 1995).

Ad hoc models which assume a disk geometry have been successful in describing the winds of Be stars (e.g., Marlborough, 1987), but a theoretical mechanism for disk formation only came through the work of Bjorkman & Cassinelli 1993. They showed that Coriolis forces in the radiation-driven wind of a rapidly rotating star will force the flow of gas towards the equatorial plane and create a very thin, dense disk. Many questions remain unsolved, however. This model underestimates the amount of matter in the disk by a factor of about 100 and predicts a disk opening angle which is much smaller than that derived from the statistics of shell stars. Also, the important variable character of Be stars (short-, middle- and long-term) is not understood. Indeed different mechanisms could give the same classical measurements. Fortunately, one can show that the analysis of the interferometic data through the different variation cycles allows the determination of the correct processes.

VLTI at optical and infrared wavelengths is very well suited to resolving the disk structure of Be stars and monitoring time variability. There are more than 100 Be stars brighter than 6th magnitude and they have already been well studied by classical techniques (spectroscopy, photometry, polarimetry). Interferometry brings new constraints on the size and morphology of the disk (including velocity and density fields), on the central star itself (radius, ellipticity, surface activity and limb-darkening - see Cidale & Vazquez, 1995) and on the effects of a binary companion.

Be stars make good targets for long-baseline interferometers, thanks to the simultaneous presence of a point-like continuum source (the central star) and a resolved structure (the emitting envelope). The program demands a good spectral resolution (R=10000 in the visible and R=100-1000 in the near-IR). Moderate (u,v) coverage is sufficient because the geometry is simple - even without images, and strong constraints can be placed on the physical processes involved in the Be phenomenon. The large apertures of the VLTI will be vital to achieve high spectral resolution maps in a few nights.