VISCOUS EXCRETION DISKS AROUND BE STARS

We discuss the properties of geometrically thin viscous Keplerian excretion discs around Be stars. The inner boundary of the disc is assumed to be located at the equatorial surface of the central star. We assume that angular momentum is transferred by some mechanism from the interior to the equatorial surface of the central star and hence matter is supplied at the inner boundary of the disc at a certain rate (excretion rate). The matter drifts outward in the disc because the angular momentum of the matter is exchanged by the effect of viscous stress. For mass excretion rates similar to the mass-loss rate estimated for Be stars, the irradiative heating by the central star is as important for the disc structure as the viscous heating in the disc. Although the irradiative heating has a stabilizing effect on thermal instability which is familiar in accretion discs in dwarf novae, we show that the irradiated excretion discs around Be stars have a thermally unstable region when the mass excretion rate is approximately 10(-7) M./yr-1. The inner part of the disc is optically thick, while the outer low temperature region (T(eff) less-than-or-similar-to 7000 K) is optically thin. The optically thick region is larger for a larger excretion rate.