NEW COMPOSITE MODELS OF PARTIALLY IONIZED PROTOPLANETARY DISKS

We study an accretion disk in which three different regions can coexist: MHD turbulent regions, dead zones, and gravitationally unstable regions. Although the dead zones are stable, there is some transport due to the Reynolds stress associated with waves emitted from the turbulent layers. We model the transport in each of the different regions by its own alpha parameter, which is 10-10(3) times smaller in dead zones than in active layers. In gravitationally unstable regions, alpha is determined by the fact that the disk self-adjusts to a state of marginal stability. We construct steady-state models of such disks. We find that for uniform mass flow, the disk has to be more massive, hotter, and thicker at the radii where there is a dead zone. In disks in which the dead zone is very massive, gravitational instabilities are present. Whether such models are realistic or not depends on whether hydrodynamical fluctuations driven by the turbulent layers can penetrate all the way inside the dead zone. If the disk is not in a steady state at some stage of its evolution, then the surface density will evolve toward the steady-state solution. However, if the value of alpha in the dead zone is much smaller than that in the active zone, the timescale for the parts of the disk that are beyond a few AU to reach a steady state can become longer than the disk lifetime. Steady-state disks with dead zones are a more favorable environment for planet formation than are standard disks, since the dead zone is typically 10 times more massive than a corresponding turbulent zone at the same location.