Advection dominated accretion flow (ADAF) is accretion into a black hole that is carrying energy that might otherwise be radiated. It occurs when emission from the accretion disk is low, which can occur if the accretion material is thin, with few particle collisions to trigger photon generation. The trapped heat puffs up the disk, which is not so much disk-shaped. EMR from an accretion disk may have spectral signatures that suggest this, revealing information about the object. The term ADAF is also sometimes used to mean such a disk.
If the disk is successfully cooling by emitting EMR, the result is an alpha disk.