4. Eccentric (m=1) Modes in Viscous Decretion Disks

Figure 1(a) shows the steady structure of the viscous decretion disk around the Be star of A0535+262. The disk structure of the Be star of 4U0115+634 is quite similar to that of A0535+262. In viscous, isothermal decretion disks, the surface density decreases as r^-2, the radial velocity, which is highly subsonic, increases as r, and the rotaion velocity is near Keplerian.

By the viscous effect, decretion disks are overstable for eccentric (m=1) modes, of which the growth rate is of the order $\alpha (H/r)^2 \Omega$. Figure 1(b) shows the fundamental eccentric mode in the Be-star disk of A0535+262. The eccentric mode in the Be-star disk of 4U0115+634 is similar to that of A0535+262. The characteristics of the modes agree with the long-term ($1 \sim 2$yr) variabilities of the Balmer line profiles observed for these Be/X-ray binaries.