A Magnetically Torqued Disk Model for Be Stars

Despite extensive study, the mechanisms by which Be star disks acquire highdensities and angular momentum while displaying variability on many time scalesare still far from clear. In this paper, we discuss how magnetic torquing mayhelp explain disk formation with the observed quasi-Keplerian (as opposed toexpanding) velocity structure and their variability. We focus on the effects ofthe rapid rotation of Be stars, considering the regime where centrifugal forcesprovide the dominant radial support of the disk material. Using a kinematicdescription of the angular velocity, vphi(r), in the disk and a parametricmodel of an aligned field with a strength B(r) we develop analytic expressionsfor the disk properties that allow us to estimate the stellar surface fieldstrength necessary to create such a disk for a range of stars on themain-sequence. The model explains why disks are most common for main-sequencestars at about spectral class B2 V. The earlier type stars with very fast andhigh density winds would require unacceptably strong surface fields (> 10^3Gauss) to form torqued disks, while the late B stars (with their low mass lossrates) tend to form disks that produce only small fluxes in the dominant Bediagnostics. For stars at B2 V the average surface field required is about 300Gauss. The predicted disks provide an intrinsic polarization and a flux atHalpha comparable to observations. We also discuss whether the effect on fieldcontainment of the time dependent accumulation of matter in the flux tubes/diskcan help explain some of the observed variability of Be star disks.Comment: ApJ, in press. 46 pages, 12 figure