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Cooling
Cholla includes several equilibrium cooling and heating functions that act on the gas in a hydrodynamic simulation. These include: a piecewise-parabolic fit to a collisional ionization equilibrium (CIE) cooling function at solar metallicity (see Appendix A.2 in Schneider & Robertson, 2018 for a description); a cooling / heating function based on a solar metallicity Cloudy model with a Hardt & Madau 2005 UV background (see Appendix C in Schneider & Robertson, 2017 for a description), and H/He chemistry/cooling based on the Grackle library with a uniform UVB background for photoheating and photoionization. Each of these cooling and heating options are turned on via compile-time macros, listed below.
The COOLING_GPU flag activates gpu-native cooling, which is either the default CIE cooling or Cloudy Cooling.
The CLOUDY_COOL flag overrides default CIE cooling with Cloudy Cooling from cooling tables.
The CHEMISTRY_GPU flag turns on non-equilibrium H+He chemistry and cooling
The COOLING_GRACKLE flag activates CPU-native Grackle cooling. (deprecated but code still exists)
CIE cool provides an analytic fit to a solar metallicity CIE cooling curve calculated using Cloudy, with no cooling below 1e4 K.
Cloudy cooling loads data from relative paths ./cloudy_coolingcurve.txt or src/cooling/cloudy_coolingcurve.txt (see src/cooling/load_cloudy_texture.cu). A copy of the Cloudy table can be found here.
The provided table spans densities log(n) = -6 to log(n) = 6 (cm^-3) and temperatures from log(T) = 1 to log(T) = 9 (K) with no cooling for temperatures below 10 K.
Cooling is computed from the table using bilinear interpolation. A custom function is used for double precision because the CUDA built-in performs interpolation using 8-bit accuracy.
Tracks non-equilibrium ionization states of hydrogen and helium and calculates cooling/heating rates based on a uniform UV background. Time-dependent UVB phothoheating and photoionization rates are passed as a text file, with an input parameter specifying the text file name.