Studies of high velocity outflows on an expanding mesh

High velocity outflows are ubiquitous in astrophysics. Since they often expand over several orders of magnitude in size and time, high resolutions or high dynamic ranges are required to model them numerically. A jet from an active galactic nucleus (AGN) is a prominent example. I will begin with a description of its morphological properties, investigated using the moving mesh code Jet. The Fanaroff-Riley (edge brightened vs center brightened) morphological dichotomy arises naturally in our models, as a function of the jet-to-surroundings energy density contrast rather than the jet power itself, as previously thought. Our models present recollimation shocks as a possible explanation for “knots” observed in AGN jets. The number of  knots provides a prediction for the jet energetics, which are found to be consistent with the jets in M87 and Cygnus A. I will also describe my novel moving mesh hydro code Sprout, which solves the equations of hydrodynamics on an expanding Cartesian mesh. This provides high dynamic range for many orders of magnitude of expansion with little numerical diffusion, capturing shocks and fine structures accurately. Finally, we will discuss the application of Sprout in modeling hydrodynamic instabilities in a young supernova remnant, which is another example of a high velocity outflow. These instabilities form a typical size of structures in the remnant. This is a direct diagnostic of the density profile of the progenitor and may be related to the cauliflower-like structures in Tycho’s SNR. It is also found that inhomogeneities in the ejecta or the surrounding medium do not influence structure formation in the remnant, unless they have a very large amplitude and form large-scale coherent structures.