Be Star Disks: Powered by a Nonzero Central Torque

Be stars are rapidly rotating B stars with Balmer emission lines that indicate the presence of a Keplerian, rotationally supported, circumstellar gas disk. Current disk models, referred to as "decretion disks," make use of the zero-torque inner boundary condition typically applied to accretion disks, with the "decretion" modeled by adding mass to the disk at a radius of about 2% larger than the inner disk boundary. We point out that, in this model, the rates at which mass and energy need to be added to the disk are implausibly large. What is required is that the disk has not only a source of mass but also a continuing source of angular momentum. We argue that the disk evolution may be more physically modeled by application of the nonzero torque inner boundary condition of Nixon & Pringle, which determines the torque applied at the boundary as a fraction of the advected angular momentum flux there and approaches the accretion and decretion disk cases in the appropriate limits. We provide supporting arguments for the suggestion that the origin of the disk material is small-scale magnetic flaring events on the stellar surface, which, when combined with rapid rotation, can provide sufficient mass to form, and sufficient angular momentum to maintain, a Keplerian Be star disk. We discuss the origin of such small-scale magnetic fields in radiative stars with differential rotation. We conclude that small-scale magnetic fields on the stellar surface, may be able to provide the necessary mass flux and the necessary time-dependent torque on the disk inner regions to drive the observed disk evolution.