abstract

We study the tidal truncation of disks around Be stars in Be/X-ray binaries, assuming that the Be-star disks are formed by viscous decretion. With Shakura-Sunyaev's $\alpha$-viscosity, we find that a critical value of viscosity parameter, $\alpha_{\rm crit}$, exists at each resonance radius. The disk is truncated at a given resonance if $\alpha < \alpha_{\rm crit}$. With the disk radii estimated from the H$\alpha$ observations for A0535+262 and 4U0115+634, we evaluate the tidal truncation criterion and find that $\alpha \sim 0.1$ is compatible with those disk radii. By the viscous effect, the decretion disks are overstable for eccentric (m=1) modes.