Viscous transonic decretion in disks of Be stars

Abstract

We consider the characteristics of the outflow in disks of Be stars, based on the viscous decretion disk scenario. In this scenario, the matter ejected from the equatorial surface of a star drifts outward because of the effect of viscosity, and forms the disk. For simplicity, we adopt the α-prescription for the viscous stress, and assume the disk to be isothermal. Solving the resulting wind equations, we find that a transonic solution exists for any value of α. The sonic point is located at r > 100 R for the plausible values of parameters, where R is the stellar radius. The sonic radius is smaller for a higher temperature and/or a larger radiative force. We also find that the topology of the sonic point is nodal for α ≳ 0.95, while it is of the saddle type for α ≲ 0.9. We expect that the sonic point in the former case is unstable, whereas that in the latter case is stable. The outflow is highly subsonic in the inner part of the disk. Roughly, the outflow velocity increases linearly with r and the surface density decreases as r-2. Interestingly, the disk is near-Keplerian in the inner subsonic region, while it is angular-momentum conserving in the outer subsonic region and in the supersonic region. Our results, together with the observed range of the base density for Be star disks, suggest that the mass-loss rate in the equatorial region is at most comparable with that in the polar region.