Modeling of the Wind/Disk Outflow from Be Stars II: Formation of the Keplerian Disk

Abstract

Computer modeling of the outflow from Be stars is performed. In our approach, processes of turbulence excitation and turbulent viscosity are added to the conventional model of the radiation driven winds. The objective of our study is to reproduce from the first principles the main features of the outflow from Be stars: a fast polar wind and a slow viscous Keplerian disk at the equator. At sub-critical velocity of rotation up to 0.999 of the critical velocity, our model reproduces the formation of the fast polar wind together with a slow highly turbulent outflow at the equatorial region. This outflow, however, does not reassemble a Keplerian disk. We link this to the absence of the angular moment transfer from the star to the disk. This process provides an increase of the angular momentum of the disk matter with radius. We consider a star with super critical rotation as the simplest way to supply the angular momentum to the disk. In this case, the star surface has a higher azimuthal speed than the matter at the inner edge of the disk. The angular momentum transfer becomes unavoidable. Already at rotation velocity 0.5% above the critical one, a quasi Keplerian disk at the equator is formed with size ∼10 stellar radius. At rotation 1% higher than the critical speed, the disk reaches ∼15 stellar radius. The main conclusion following from our work is that the conventional model of the radiation driven winds is able to reproduce the main features of the outflow from Be stars provided that the process of turbulence excitation and a process of angular momentum supply of the disk from the central source are added in to this model.