A magnetic rotator wind-disk model for Be stars

We consider a Magnetic Rotator Wind-Disk(MRWD) model for the formation of Keplerian disks around Be stars. Material from low latitudes of the stellar surface flows along magnetic flux tubes and passes through a shock surface near the equatorial plane to form a pre-Keplerian disk region. Initially, the density in this region is small and the magnetic field helps to maintain super-Keplerian rotation speeds. After a fill-up time, the density of the disk is significantly larger and the magnetic force in this region becomes negligible compared with the centrifugal force. The material then moves outwards to form a quasi-steady Keplerian disk. During the fill-up stage, the meridional component B-m,B-star of the magnetic field at the stellar surface must be larger than a minimum value B-m,B-star,B-min. The radial extent of the quasi-steady Keplerian region will be larger when B-m,(star) is larger or when viscosity plays a role. In B-type stars, the values of B-m,(star,min) are of order 1 G to 10 G. We find that a condition for the formation of shock-compressed disk regions is that in faster rotating stars, the wind speed must be correspondingly larger.