Self-sustained Ionization and Vanishing Dead Zones in Protoplanetary Disks

We analyze the ionization state of the magnetohydrodynamically turbulentprotoplanetary disks and propose a new mechanism of sustaining ionization.First, we show that in the quasi-steady state of turbulence driven bymagnetorotational instability in a typical protoplanetary disk with dustgrains, the amount of energy dissipation should be sufficient for providing theionization energy that is required for activating magnetorotationalinstability. Second, we show that in the disk with dust grains the energeticelectrons that compose electric currents in weakly ionized gas can providecollisional ionization, depending on the actual saturation state ofmagnetorotational turbulence. On the other hand, we show that in theprotoplanetary disks with the reduced effect of dust grains, the turbulentmotion can homogenize the ionization degree, leading to the activation ofmagnetorotational instability even in the absence of other ionizationprocesses. The results in this Letter indicate that most of the regions inprotoplanetary disks remain magnetically active, and we thus require a changein the theoretical modeling of planet formation.Comment: 11 pages, 2 figures. Accepted for publication in The Astrophysical Journal Letter