A Comparison of Two- and One-fluid Dust Solvers for Dusty, Supersonic Turbulence

We model 3 and 10 μ m 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 (one fluid) using the diffusion approximation. Simulations are performed in a 3D periodic box using the Phantom code. The gas probability distributions are consistent with a lognormal distribution for both methods and grain sizes. The dust distributions are different in the two methods, showing discrepancies particularly in regions of low density. 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. The narrowing of the one-fluid distribution is also caused by the stopping time limiter used to maintain the terminal velocity approximation. This artificially makes the mixture more coupled in low-density 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 numerical Stokes number regime of the calculation to avoid numerical artifacts.