Magnetically-Fed Hot Star `Keplerian' Disks with Slow Outflow

J. C. Brown$^{1}$, J. P. Cassinelli$^{2}$ and M. Maheswaran$^{3}$

$^{1}$Dept. of Physics and Astronomy,
University of Glasgow, Glasgow, G12 8QQ,UK;
$^{2}$Dept. of Astronomy, University of
Wisconsin-Madison, WI 53711, USA;
$^{3}$Dept. of Mathematics, University of Wisconsin-Wausau, WI
54401, USA;

The puzzle of the origin of Be star disks is discussed. Contrary to recently published claims, it is argued that the magnetically torqued disk (MTD) type models of Cassinelli et al (2002) offer a viable scenario for a successful model with all the key ingredients. MTD models involve disk compression by equatorial collision of stellar wind streams that are steered and torqued by a dipole-like magnetic field. While the growing disk density tends to lead to the gas breaking out centrifugally from the field, it is proposed that the onset of viscous effects can lead to an eventual stable, slowly outflowing, Keplerian disk. It is then shown that the resulting very dense (wind compressed) disk need have only a very slow subsonic outflow to satisfy mass continuity. Consequently, line profile data do not preclude steadily expanding disks of high density. It is also shown that the time taken to reach the steady state would typically be of the order of 10^4 wind flow times R/v_infty. This is far longer than the run times of recent numerical MHD simulations that displayed bursty breakout behavior, which may therefore only be transients induced by unrealistic initial conditions.

Reference: Published in Ap.J., v 688, p 1320
Status: Manuscript has been accepted