Modelling ripples in Orion with coupled dust dynamics and radiative
  transfer
  In light of the recent detection of direct evidence for the formation of
Kelvin-Helmholtz instabilities in the Orion nebula, we expand upon previous
modelling efforts by numerically simulating the shear-flow driven gas and dust
dynamics in locations where the H$_{II}$ region and the molecular cloud
interact. We aim to directly confront the simulation results with the infrared
observations. Methods: To numerically model the onset and full nonlinear
development of the Kelvin-Helmholtz instability we take the setup proposed to
interpret the observations, and adjust it to a full 3D hydrodynamical
simulation that includes the dynamics of gas as well as dust. A dust grain
distribution with sizes between 5-250 nm is used, exploiting the gas+dust
module of the MPI-AMRVAC code, in which the dust species are represented by
several pressureless dust fluids. The evolution of the model is followed well
into the nonlinear phase. The output of these simulations is then used as input
for the SKIRT dust radiative transfer code to obtain infrared images at several
stages of the evolution, which can be compared to the observations. Results: We
confirm that a 3D Kelvin-Helmholtz instability is able to develop in the
proposed setup, and that the formation of the instability is not inhibited by
the addition of dust. Kelvin-Helmholtz billows form at the end of the linear
phase, and synthetic observations of the billows show striking similarities to
the infrared observations. It is pointed out that the high density dust regions
preferentially collect on the flanks of the billows. To get agreement with the
observed Kelvin-Helmholtz ripples, the assumed geometry between the background
radiation, the billows and the observer is seen to be of critical importance.
