 Zero-torque inner boundary condition

The zero-torque inner boundary condition sets the internal disk torque to zero at a chosen inner radius. It still permits a nonzero accretion rate: the gas carries its own specific angular momentum across the boundary. For a steady Keplerian disk, it gives $\nu \Sigma =\dot{M}[1-\sqrt{r_{\mathrm{in}}/r}]/(3\pi )$. A rapid plunge near the innermost stable circular orbit motivates this approximation, but magnetic coupling or a stellar boundary layer requires examination of the actual stresses.^[ref]