2. Viscous Descretion Disk Model for Be Stars

Lee et al. (1991) proposed a viscous decretion disk model for Be stars. In this model, the matter supplied from the equatorial surface of the star drifts outward by the viscous effect, and forms the disk. Basic equations for viscous decretion disks are the same as those for viscous accretion disks, except that the sign of $\dot{M}$(mass decretion/accretion rate) is opposite. Viscous decretion disks around Be stars are thus geometrically thin and near Keplerian as cool accretion disks are. The outflow is highly subsonic near the star. These features agree well with observed features of Be-star disks. For simplicity, we assume in this paper that the decretion disk is isothermal with the temperature of $0.8 T_{\rm eff}$, where $T_{\rm eff}$ is the effective temperature of the Be star.