Effect of dust on Kelvin-Helmholtz instabilities

Centre for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
e-mail: tom.hendrix@wis.kuleuven.be

Received: 19 July 2013
Accepted: 27 November 2013

Abstract

Context. Dust is present in a large variety of astrophysical fluids, ranging from tori around supermassive black holes to molecular clouds, protoplanetary discs, and cometary outflows. In many such fluids, shearing flows are present, which can lead to the formation of Kelvin-Helmholtz instabilities (KHI) and may change the properties and structures of the fluid through processes such as mixing and clumping of dust.

Aims. We study the effects of dust on the KHI by performing numerical hydrodynamical dust+gas simulations. We investigate how the presence of dust changes the growth rates of the KHI in 2D and 3D and how the KHI redistributes and clumps dust. We investigate if similarities can be found between the structures in 3D KHI and those seen in observations of molecular clouds.

Methods. We perform numerical multifluid hydrodynamical simulations in addition to the gas a number of dust fluids. Each dust fluid represents a portion of the particle size-distribution. We study how dust-to-gas mass density ratios between 0.01 and 1 alter the growth rate in the linear phase of the KHI. We do this for a wide range of perturbation wavelengths, and compare these values to the analytical gas-only growth rates. As the formation of high-density dust structures is of interest in many astrophysical environments, we scale our simulations with physical quantities that are similar to values in molecular clouds.

Results. Large differences in dynamics are seen for different grain sizes. We demonstrate that high dust-to-gas ratios significantly reduce the growth rate of the KHI, especially for short wavelengths. We compare the dynamics in 2D and 3D simulations, where the latter demonstrates additional full 3D instabilities during the non-linear phase, leading to increased dust densities. We compare the structures formed by the KHI in 3D simulations with those in molecular clouds and see how the column density distribution of the simulation shares similarities with log-normal distributions with power-law tails sometimes seen in observations of molecular clouds.