A Magnetic Rotator Wind Disk Model for Be Stars

Abstract

A Magnetic Rotator Wind-Disk(MRWD) model is considered 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 to form a pre-Keplerian disk region. Initially, the density in this region is small. After a fill-up time, the density of the disk is significantly larger and the magnetic force becomes negligible compared with the centrifugal force. The material then expands to form a Keplerian disk. During disk fill-up, the meridional component B-star of the magnetic field at the stellar surface must be larger than a minimum value B-star-min. The radial extent of the Keplerian region will be larger when B-star is larger or when viscosity plays a role. In B-type stars, B-star-min must be of order 1 G to 10 G. In faster rotating stars, the wind speed must be correspondingly larger for the formation of shock-compressed disk regions.