Protoplanetary Disk Dead Zone
A dead zone is an annular region of a circumstellar disk where the local viscosity is essentially zero, leading to a negligible infall of disk material toward the central star. Viscosity is needed to remove the orbital energy and angular momentum from gas and dust in the disk, allowing material to spiral inward and accrete onto the central star (as observed in young stellar systems with known circumstellar material). The source of this disk viscosity is uncertain, but the most likely source is electromagnetic interactions between ionized species, known as the magnetorotational instability (MRI). Dead zones may be created when the local gas is insufficiently ionized to support the MRI. They can occur, for example, when the gas column density is large enough to protect the inner zones of the disk from ionization by cosmic rays.