Accretion disk

Remarks

Example: take a steady, thin, optically thick disk with accretion rate $\dot M$ and inner edge $r_{\text{in}}$, where the stress vanishes. If the energy released locally is radiated from both faces, the surface temperature satisfies $\sigma T^4(r) = \frac{3 G M \dot M}{8\pi r^3} \left( 1 - \sqrt{r_{\text{in}} / r} \right)$. Here $\sigma$ is the Stefan–Boltzmann constant. Non-example: gas without angular momentum falling radially onto the central body, as in Bondi accretion, is accreted but forms no disk. It is not an accretion disk, because it neither rotates nor is concentrated near a plane, so clauses 1 and 2 fail. Molecular viscosity is far too small to explain observed inflow rates, so the stress is attributed to turbulence. The Shakura–Sunyaev alpha disk model models it, and the magnetorotational instability is thought to cause it.