The coexistence of the streaming instability and the vertical shear instability in protoplanetary disks

The streaming instability is a leading candidate mechanism to explain the formation of planetesimals. However, the role of this instability in the driving of turbulence in protoplanetary disks, given its fundamental nature as a linear hydrodynamical instability, has so far not been investigated in detail. We study the turbulence that is induced by the streaming instability as well as its interaction with the vertical shear instability. For this purpose, we employ the FLASH Code to conduct two-dimensional axisymmetric global disk simulations spanning radii from 1 to 100 au, including the mutual drag between gas and dust as well as the radial and vertical stellar gravity. If the streaming instability and the vertical shear instability start their growth at the same time, we find the turbulence in the dust midplane layer to be primarily driven by the streaming instability. The streaming instability gives rise to vertical gas motions with a Mach number of up to similar to 10(-2). The dust scale height is set in a self-regulatory manner to about 1% of the gas scale height. In contrast, if the vertical shear instability is allowed to saturate before the dust is introduced into our simulations, then it continues to be the main source of the turbulence in the dust layer. The vertical shear instability induces turbulence with a Mach number of similar to 10(-1) and thus impedes dust sedimentation. Nonetheless, we find the vertical shear instability and the streaming instability in combination to lead to radial dust concentration in long-lived accumulations that are significantly denser than those formed by the streaming instability alone. Therefore, the vertical shear instability may promote planetesimal formation by creating weak overdensities that act as seeds for the streaming instability.