Transport and Structures in Gravitationally Unstable Protoplanetary Disks: Swing and Sling

During the early phases of a protoplanetary disk’s (PPD) life, gravitational instabilities (GIs) can produce significant mass transport, dramatically alter disk structure, mix and shock-process gas and solids, and may be instrumental in planet formation. We present a grid-based 3D-hydrodynamics convergence study of a PPD subject to GIs at a time when it has achieved approximate balance between heating produced by GIs and radiative cooling governed by realistic dust opacities. We follow a 0.07M_⊙ disk orbiting a 0.5M_⊙ star as it settles into a quasi-steady asymptotic state with approximate balance between heating and cooling. Star-disk interactions are explicitly included. We assess disk stability criteria, cooling times, temperatures, strengths of GIs, characteristics of spiral density waves produced by the GIs, evaluate gravitational torques produced by these structures, and determine the radial mass transport arising from these torques. We find that convergence characteristics for most physical and thermal processes display distinct differences between the inner optically thick and outer optically thin regions of the disk. We also examine the level to which transport can be represented as a local (gravitoturbulent) or global process. Torques in the optically thick region are dominated by low-order (m=2-6) Fourier components of the azimuthal mass distribution. These torques are strongly variable on the local dynamical times and driven by recurrent swing amplification. In the outer optically thin region, sling amplified m = 1 torques dominate with moderate contributions from m = 2. We find that ring-like structures, each with mass excesses of several M_Jup, develop between 8 and 14 AU in the simulations. These features exhibit strong noncircular motions and vortices. If real, these may play a role in giant planet formation. The primary message of this work is that GIs are global structures and edges of various sorts (optically thick thin, cooling time gradients, real physical edges, radial Q variations, rings, ...) are important. Edges affect many physical processes that vary in time and space and thus underlie the global nature of PPDs.