¹ Dept. of Astronomy, University of Wisconsin-Madison, 475 N. Charter St. Madison WI 53706;   cassinelli@astro.wisc.edu,   nmiller@astro.wisc.edu
² Dept. of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK;   john@astro.gla.ac.uk,   deborah@astro.gla.ac.uk
³ University of Wisconsin-Marathon County, 518 S. 7th Av. Wausau, WI 54401,   mmaheswa@uwc.edu

Despite extensive study, the mechanisms by which Be star disks acquire high densities and angular momentum while displaying variability on many time scales are still far from clear. In this paper, we discuss how magnetic torquing may help explain disk formation with the observed quasi-Keplerian (as opposed to expanding) velocity structure and their variability. We focus on the effects of the rapid rotation of Be stars, considering the regime where centrifugal forces provide the dominant radial support of the disk material.

Using a kinematic description of the angular velocity, , in the disk and a parametric model of an aligned field with a strength B(r) we develop analytic expressions for the disk properties that allow us to estimate the stellar surface field strength necessary to create such a disk for a range of stars on the main-sequence. The fields required to form a disk are compared with the bounds previously derived from photospheric limiting conditions. The model explains why disks are most common for main-sequence stars at about spectral class B2 V. The earlier type stars with very fast and high density winds would require unacceptably strong surface fields (> 10³ Gauss) to form torqued disks, while the late B stars (with their low mass loss rates) tend to form disks that produce only small fluxes in the dominant Be diagnostics. For stars at B2 V the average surface field required is about 300 Gauss. The predicted disks provide an intrinsic polarization and a flux at H comparable to observations. The radial extent of our dense quasi-Keplerian disks is compatible with typical estimates. We also discuss whether the effect on field containment of the time dependent accumulation of matter in the flux tubes/disk can help explain some of the observed variability of Be star disks.

Accepted by ApJ
Preprints from cassinelli@astro.wisc.edu
or by anonymous ftp to
ftp.astro.wisc.edu/outgoing/joecas/MagTorqDisk.ps
or on the web at http://xxx.lanl.gov/abs/astro-ph/0207370