The Kelvin-Helmholtz Instability in the Protoplanetary Disk

Recently, many planetary systems have been found outside the solar system owing to the progress of the observational technology.In order to explain the variety of planetary systems, it is essential to interpret the formation processes of planets.In the core accretion model of planet formation, planetesimals are assumed to form from dust particles in the protoplanetary disk first, and then they grow into planets.However, there are many significant problems.For example, due to gas drag in the protoplanetary disk, dust aggregates migrate inward and fall onto the protostar before growing to planetesimals.This is the radial drift problem.As one of the solutions to the problem, the gravitational instability (GI) of a settled dust layer has been suggested.In this scenario, dust aggregates first settle toward the midplane of the protoplanetary disk and make the dense layer of dust.Then, the dust layer becomes gravitationally unstable, and the dust layer fragments into pieces to form planetesimals before dust aggregates fall onto the protostar.However, as dust aggregates settle toward the midplane, the vertical dust density gradient increases.As a result, vertical shear of the rotational velocity in the dust layer becomes strong.This strong shear has the possibility to induce the Kelvin-Helmholtz instability (KHI), and KHI possibly induces shear-driven turbulence.If the turbulence is sufficiently strong, it prevents dust aggregates from settling toward the midplane, and as a result, GI and the planetesimal formation is possibly suppressed.On the other hand, if GI occurs before KHI, planetesimals will form.However, the condition that GI occurs before KHI during sedimentation is not known.To know this condition, it is necessary to know the dust density at the onset of KHI.Investigation for this condition is essential for understanding i