Updated the definition of Lambda in the const-lambda cooling model to have the right units.

examples/AgoraDisk/agora_disk.yml

@@ -49,7 +49,7 @@ InitialConditions:
 
 # Dimensionless pre-factor for the time-step condition
 LambdaCooling:
-  lambda_cgs:                  1.0e-22    # Cooling rate (in cgs units)
+  lambda_nH2_cgs:              1e-22 # Cooling rate divided by square Hydrogen number density (in cgs units [erg * s^-1 * cm^3])
   cooling_tstep_mult:          1.0        # Dimensionless pre-factor for the time-step condition
 
 # Cooling with Grackle 2.0

examples/CoolingBox/coolingBox.yml

@@ -35,7 +35,7 @@ InitialConditions:
 
 # Dimensionless pre-factor for the time-step condition
 LambdaCooling:
-  lambda_cgs:                  1.0e-22    # Cooling rate (in cgs units)
+  lambda_nH2_cgs:              1e-22 # Cooling rate divided by square Hydrogen number density (in cgs units [erg * s^-1 * cm^3])
   cooling_tstep_mult:          1.0        # Dimensionless pre-factor for the time-step condition
 
 # Cooling with Grackle 2.0

examples/CoolingHalo/cooling_halo.yml

@@ -41,5 +41,5 @@ IsothermalPotential:
 
 # Cooling parameters
 LambdaCooling:
-  lambda_cgs:                  1.0e-22    # Cooling rate (in cgs units)
+  lambda_nH2_cgs:              1e-22 # Cooling rate divided by square Hydrogen number density (in cgs units [erg * s^-1 * cm^3])
   cooling_tstep_mult:          1.0    # Dimensionless pre-factor for the time-step condition

examples/CoolingHaloWithSpin/cooling_halo.yml

@@ -41,5 +41,5 @@ IsothermalPotential:
 
 # Cooling parameters
 LambdaCooling:
-  lambda_cgs:                  1.0e-22   # Cooling rate (in cgs units)
+  lambda_nH2_cgs:              1e-22 # Cooling rate divided by square Hydrogen number density (in cgs units [erg * s^-1 * cm^3])
   cooling_tstep_mult:          0.1    # Dimensionless pre-factor for the time-step condition

examples/SmallCosmoVolume_cooling/plotTempEvolution.py

@@ -71,7 +71,7 @@ git = sim["Code"].attrs["Git Revision"]
 cooling_model = sim["/SubgridScheme"].attrs["Cooling Model"]
 
 if cooling_model == "Constant Lambda":
-    Lambda = sim["/SubgridScheme"].attrs["Lambda [cgs]"][0]
+    Lambda = sim["/SubgridScheme"].attrs["Lambda/n_H^2 [cgs]"][0]
     
 # Cosmological parameters
 H_0 = sim["/Cosmology"].attrs["H0 [internal units]"][0]

examples/SmallCosmoVolume_cooling/small_cosmo_volume.yml

@@ -59,4 +59,4 @@ InitialConditions:
 
 # Constant lambda cooling function
 LambdaCooling:
-  lambda_cgs:                  1e-26 # Cooling rate (in cgs units [erg * s^-1 * cm^-3])
+  lambda_nH2_cgs:              1e-26 # Cooling rate divided by square Hydrogen number density (in cgs units [erg * s^-1 * cm^3])

examples/parameter_example.yml

@@ -198,7 +198,7 @@ ConstCooling:
 
 # Constant lambda cooling function
 LambdaCooling:
-  lambda_cgs:                  1e-22 # Cooling rate (in cgs units [erg * s^-1 * cm^-3])
+  lambda_nH2_cgs:              1e-22 # Cooling rate divided by square Hydrogen number density (in cgs units [erg * s^-1 * cm^3])
   cooling_tstep_mult:          1.0   # (Optional) Dimensionless pre-factor for the time-step condition.
 
 # Cooling with Grackle 3.0

src/cooling/const_lambda/cooling.h

@@ -47,8 +47,10 @@
 /**
  * @brief Calculates du/dt in CGS units for a particle.
  *
- * The cooling rate is \f$\frac{du}{dt} = -\Lambda \frac{n_H^2}{\rho} \f$ and
- * the returned value is in physical [erg * g^-1 * s^-1].
+ * The cooling rate is \f$\frac{du}{dt} = -\frac{\Lambda}{n_H^2}
+ * \frac{n_H^2}{\rho} \f$, where \f$ \frac{\Lambda}{n_H^2} \f$ is a constant in
+ * this model (lambda_nH2_cgs in #cooling_function_data).
+ * The returned value is in physical [erg * g^-1 * s^-1].
  *
  * @param cosmo The current cosmological model.
  * @param hydro_props The properties of the hydro scheme.
@@ -61,18 +63,19 @@ __attribute__((always_inline)) INLINE static double cooling_rate_cgs(
     const struct cosmology* cosmo, const struct hydro_props* hydro_props,
     const struct cooling_function_data* cooling, const struct part* p) {
 
-  /* Get particle density */
+  /* Get particle density [g * cm^-3] */
   const double rho = hydro_get_physical_density(p, cosmo);
   const double rho_cgs = rho * cooling->conv_factor_density_to_cgs;
 
   /* Get Hydrogen mass fraction */
   const double X_H = hydro_props->hydrogen_mass_fraction;
 
-  /* Hydrogen number density (X_H * rho / m_p) */
+  /* Hydrogen number density (X_H * rho / m_p) [cm^-3] */
   const double n_H_cgs = X_H * rho_cgs * cooling->proton_mass_cgs_inv;
 
-  /* Calculate du_dt (Lambda * n_H^2 / rho) */
-  const double du_dt_cgs = -cooling->lambda_cgs * n_H_cgs * n_H_cgs / rho_cgs;
+  /* Calculate du_dt ((Lambda / n_H^2) * n_H^2 / rho) */
+  const double du_dt_cgs =
+      -cooling->lambda_nH2_cgs * n_H_cgs * n_H_cgs / rho_cgs;
 
   return du_dt_cgs;
 }
@@ -238,8 +241,8 @@ static INLINE void cooling_init_backend(struct swift_params* parameter_file,
                                         struct cooling_function_data* cooling) {
 
   /* Read in the cooling parameters */
-  cooling->lambda_cgs =
-      parser_get_param_double(parameter_file, "LambdaCooling:lambda_cgs");
+  cooling->lambda_nH2_cgs =
+      parser_get_param_double(parameter_file, "LambdaCooling:lambda_nH2_cgs");
   cooling->cooling_tstep_mult = parser_get_opt_param_float(
       parameter_file, "LambdaCooling:cooling_tstep_mult", FLT_MAX);
 
@@ -265,9 +268,10 @@ static INLINE void cooling_print_backend(
     const struct cooling_function_data* cooling) {
 
   message(
-      "Cooling function is 'Constant lambda' with Lambda=%g [erg * s^-1 * "
-      "cm^-3]",
-      cooling->lambda_cgs);
+      "Cooling function is 'Constant lambda' with Lambda/n_H^2=%g [erg * s^-1 "
+      "* "
+      "cm^3]",
+      cooling->lambda_nH2_cgs);
 
   if (cooling->cooling_tstep_mult == FLT_MAX)
     message("Cooling function time-step size is unlimited");

src/cooling/const_lambda/cooling_io.h

@@ -45,7 +45,7 @@ __attribute__((always_inline)) INLINE static void cooling_write_flavour(
     hid_t h_grp, const struct cooling_function_data* cooling) {
 
   io_write_attribute_s(h_grp, "Cooling Model", "Constant Lambda");
-  io_write_attribute_d(h_grp, "Lambda [cgs]", cooling->lambda_cgs);
+  io_write_attribute_d(h_grp, "Lambda/n_H^2 [cgs]", cooling->lambda_nH2_cgs);
 }
 #endif
 

src/cooling/const_lambda/cooling_struct.h

@@ -33,8 +33,8 @@
  */
 struct cooling_function_data {
 
-  /*! Cooling rate in physical cgs units [erg * s^-1 * cm^-3] */
-  double lambda_cgs;
+  /*! Cooling rate / nH^2 in physical cgs units [erg * s^-1 * cm^3] */
+  double lambda_nH2_cgs;
 
   /*! Conversion factor from internal units to cgs for density */
   double conv_factor_density_to_cgs;