The dynamical disks of Be stars and their importance to stellar evolution and the astrophysics of disks

Stellar evolution with rotation is still poorly understood. Be stars are massive stars with the fastest rotation rates among non-degenerate stars,

and they occasionally eject matter from their equatorial regions forming disks of hot gas around them. In the last decade, the viscous decretion disk (VDD) model, in which matter and angular momentum (AM) are carried away due to viscous forces,has been shown to adequately explain most of the observations of Be stars. However, up until now, the magnitude of the disk's viscosity and the AM carried away from the star through the disk, still remains poorly constrained. We have recently shown that the light curves of Be stars that undergo events of disk formation and dissipation offer an opportunity to constrain the disks' fundamental properties. We have showed, using a sample of dozens of Be stars from the OGLE survey of the Small Magellanic Cloud (SMC), that the AM lost by the stars, even for the events with the densest disks, were still smaller than the required by the best stellar evolutionary models. We, therefore, offer, for the first time, constraints on the internal AM transport mechanisms of fast rotating massive stars. In addition to this, our values found for the viscosity parameter are consistent with the ones found in disks of dwarf novae, but larger than the current magnetohydrodynamical (MHD) simulations predict.