Cosmic Ray Two-Fluid Model¶

This section documents the two-fluid cosmic ray model implemented in Enzo, which was first used (and is described in detail) in Salem & Bryan 2014 . For relevant parameters, please also see Cosmic Ray Two-Fluid Model Parameters. The bulk of the code itself can be found in Grid_ZeusSolver.C

This module models the dynamical role of cosmic rays via a set of two-fluid hydro equations (see Jun et. al. 1994 ). Central to the effort is a new baryon field, CREnergyDensity, which is in units of ergs/cm^3, and is advected along with the gas. Gradients in the CR field result in a pressure felt by the gas. The CR gas is also diffusive and rays can be produced during star formation. See Cosmic Ray Two-Fluid Model Parameters for information on how to control all these options. But most important are:

• CRModel - Switches on the CR physics (0 = off, 1 = on)
• CRgamma - For polytropic equation of state. 4/3 = relativistic, adiabatic gas (default)
• CRDiffusion - turns on diffusion of CREnergyDensity field
• CRkappa - Diffusion coefficient (currently constant, isotropic)
• CRFeedback - Controls production of rays in star forming regions

For this model to run properly you must be running the Zeus Hydro Solver! (HydroMethod = 2). The model has not yet been implemented for any of the other fluid solvers in Enzo.

If you plan on including cosmic rays, definitely first verify the solver is working by running the Cosmic Ray Shocktube problem, which ought to match the analytic solution described in Pfrommer 2006 . See CR Shock Tube (250: unigrid and AMR) for more detailed information on this test problem.

Cosmic Rays have also been implemented in the isolated galaxy simulation. They initialize with a profile equal to the density of the thermal gas, multiplied by a constant, GalaxySimulationCR, typically set to 0.1 (all in code units).