¹ Instituto de Astronomia, Geofísica e Ciéncias Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária, 05508-900, São Paulo, SP, BRAZIL
² Ritter Observatory, Department of Physics & Astronomy, Mail Stop 113, University of Toledo, Toledo, OH 43606
³ AAVSO, American Association of Variable Star Observers
⁴ Faculty of Engineering, Hokkai-Gakuen University, Toyohira-ku, Sapporo 062-8605, Japan
⁵ European Organization for Astronomical Research in the Southern Hemisphere, Casilla 19001, Santiago 19, Chile
⁶ European Organization for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany

Be stars possess gaseous circumstellar decretion disks, which are well described using standard α-disk theory. The Be star 28 CMa recently underwent a long outburst followed by a long period of quiescence, during which the disk dissipated. Here we present the first time-dependent models of the dissipation of a viscous decretion disk. By modeling the rate of decline of the V-band excess, we determine that the viscosity parameter α=1.0±0.2, corresponding to a mass injection rate Ṁ=(3.5±1.3)×10^-8M_☉yr^-1. Such a large value of α suggests that the origin of the turbulent viscosity is an instability in the disk whose growth is limited by shock dissipation. The mass injection rate is more than an order of magnitude larger than the wind mass loss rate inferred from UV observations, implying that the mass injection mechanism most likely is not the stellar wind, but some other mechanism.

Accepted by ApJ Letters
Preprints from carciofi@usp.br
or on the web at http://arxiv.org/abs/1112.0053