The Effect of Rotational Gravity Darkening on Magnetically Torqued Be Star Disks

In the magnetically torqued disk (MTD) model for hot star disks, as proposed and formulated by Cassinelli et al. (2002), stellar wind mass loss was taken to be uniform over the stellar surface. Here account is taken of the fact that as stellar spin rate So (= √ ΩoR 3/GM) is increased, and the stellar equator is gravity darkened, the equatorial mass flux and terminal speed are reduced, compared to the poles, for a given total Ṁ . As a result, the distribution of equatorial disk density, determined by the impact of north and southbound flows, is shifted further out from the star. This results, for high So (& 0.5), in a fall in the disk mass and emission measure, and hence in the observed emission line EW, scattering polarization, and IR emission. Consequently, contrary to expectations, critical rotation So → 1 is not the optimum for creation of hot star disks which, in terms of EM for example, is found to occur in a broad peak around So ≈ 0.5−0.6 depending slightly on the wind velocity law. The relationship of this analytic quasi-steady parametric MTD model to other work on magnetically guided winds is discussed. In particular the failures of the MTD model for Be-star disks alleged by Owocki & ud-Doula (2003) are shown to revolve largely around open observational tests, rather in the basic MTD physics, and around their use of insufficiently strong fields. Subject headings: stars: emission-line, Be – stars: magnetic fields – stars: mass loss – stars: rotation – stars: winds, outflows – polarization. Send offprint requests to: [email protected](JCB); [email protected](QL)