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Advection dominated accretion flow (ADAF) is accretion into a black hole that is carrying in energy that might otherwise be radiated. It occurs when emission from the accretion disk is low, which can happen if the accretion material is relatively diffuse, with few particle collisions to trigger photon generation. The un-radiated heat puffs up the disk, making it less disk-shaped. EMR from an accretion disk may have spectral signatures that suggest this, revealing information about the object. The term ADAF is also used for the model developed for such disks and their accretion, and sometimes used as a term for disks that largely adhere to the model, i.e., calling a particular disk an ADAF.
In contrast, an alpha disk is a disk cooling effectively by radiating EMR.