Effects of the Disk Settling Instability at Small Scales

The fundamental mechanisms governing planetesimal formation are widely debated. Once dust grains reach millimeter-sizes, they tend to either inspiral toward their star due to gas drag or collisionally fragment before they can grow to larger sizes. The Streaming Instability (SI) has been shown to efficiently form overdense dust regions that could then potentially gravitationally collapse into planetesimals — but it has also been shown that this is one of many “Resonant Drag Instabilities” (RDIs) that may play a key role in disk evolution. The “disk settling instability” is one such RDI that occurs as dust vertically settles onto the midplane of a protoplanetary disk. In the small grain regime, this instability can grow orders-of-magnitude faster than the SI. We use the multi-physics simulation code, GIZMO, to examine whether the disk settling instability can play a significant role in instigating grain clumping. Our simulations probe the smallest scales of the protoplanetary disk by modeling vertical dust grain settling in shearing boxes of lengths that are much smaller than the pressure scale height of the disk. We also vary the driven turbulence in each run to understand whether turbulence suppresses or enhances grain clustering and the growth of the RDIs. Exploring the various clumping structures that emerge in our simulations can help us piece together the early stages of planet formation and thermal disk evolution.