A comparison of two-fluid and one-fluid dust solvers for dusty, supersonic turbulence

Dust is an important observational tracer of gas in molecular clouds and different dust solvers have led to conflicting conclusions about dust distributions. For this reason, We model 3 and 10 μm dust grains in supersonic, turbulent molecular cloud conditions to compare two different numerical methods solving dust coupled to gas through a drag term. One method models dust and gas as two separate species (two-fluid) and the other the combination of dust and gas as a single mixture (onefluid). Simulations are performed in a 3D periodic box using the Phantom code. The gas probability distributions are consistent with a log-normal distribution for both methods and grain sizes. The dust distributions are different in the two methods, showing discrepancies especially in low densities. The most significant difference between the two methods is in the dust-to-gas ratio distributions. Both methods peak at the mean dust-to-gas ratio of 0.01, but the two-fluid method has wider distributions than the one-fluid method suggesting excess dust concentration in dense filaments. Filaments are where the one-fluid method results are most accurate, but the narrowing of the distribution is also caused by the one-fluid limiter used to maintain the terminal velocity approximation. This artificially makes the mixture more coupled in lowdensity regions. Our overall conclusion is that both methods are viable for the study of dust in molecular clouds, but that the correct method should be chosen based on the Stokes number regime of the calculation to avoid numerical artefacts.