Self-Sustained Ionization and Vanishing Dead Zones in Protoplanetary Disks

We analyse the ionization state of the magnetohydrodynamically turbulent protoplanetary disks and propose a new mechanism of sustaining ionization. First, we show that in the quasi-steady state of turbulence driven by magnetorotational instability in a typical protoplanetary disk with dust grains the amount of energy dissipation should be sufficient for providing the ionization energy that is required for activating magnetorotational instability. Second, we show that in the disk with dust grains the energetic electrons in weakly ionized gas can provide collisional ionization, depending upon the actual saturation state of magnetorotational turbulence. Finally, we show that in the protoplanetary disks with the reduced effect of dust grains, the turbulent motion can homogenize the ionization degree, which can solely activate magnetorotational instability. The results in this letter indicate that the most of the regions in protoplanetary disks remain magnetically active, thus, require a change in theoretical modeling of planet formation. Subject headings: protoplanetary disks — planetary systems: formation — instabilities — MHD — turbulence — accretion, accretion disks