Commit 971c1ed3 by Matthieu Schaller

### Clarifications in the documentation of the EAGLE SF model

parent ae708c04
 ... ... @@ -424,9 +424,11 @@ pc^2} \right)^{-n} \times \left(\frac{\gamma}{G_{\rm N}}f_gP\right)^{(n-1)/2}, where :math:n is the exponent of the Kennicutt-Schmidt relation (typically :math:n=1.4) and :math:A is the normalisation of the law (typically :math:A=1.515\times10^{-4} {\rm M_\odot}~{\rm yr^{-1}}~{\rm kpc^{-2}} for a Cabrier IMF). :math:m_g is the gas particle mass, :math:\gamma is the Chabrier IMF). :math:m_g is the gas particle mass, :math:\gamma is the adiabatic index, :math:f_g the gas fraction of the disk and :math:P the total pressure of the gas including any subgrid turbulent terms. total pressure of the gas including any subgrid turbulent terms. The star formation rate of the gas particles is stored in the particles and written to the snapshots. Once a gas particle has computed its star formation rate, we compute the probability that this particle turns into a star using :math:Prob= ... ... @@ -434,7 +436,7 @@ probability that this particle turns into a star using :math:Prob= and convert the gas particle into a star or not depending on our luck. The density threshold itself has a metallicity dependence. We use the *smoothed* metallicty (metal mass fraction) of the gas (See :ref:EAGLE_chemical_tracers) metallicity (metal mass fraction) of the gas (See :ref:EAGLE_chemical_tracers) and apply the relation :math:n^*_{\rm H} = n_{\rm H,norm}\left(\frac{Z_{\rm smooth}}{Z_0}\right)^{n_{\rm Z}}, alongside a maximal value. The model is designed such that star formation threshold decreases with increasing ... ... @@ -445,10 +447,10 @@ the figure below. :width: 400px :align: center :figclass: align-center :alt: Metal-dependance of the threshold for star formation in the :alt: Metal-dependence of the threshold for star formation in the EAGLE model. The dependency of the SF threshold density on the metallicty of the gas The dependency of the SF threshold density on the metallicity of the gas in the EAGLE model (black line). The function is described by the four parameters indicated on the figure. These are the slope of the dependency, its position on the metallicity-axis and normalisation ... ... @@ -485,7 +487,7 @@ the relevant YAML parameters defining it is shown on the figure below. ones assumed in the reference EAGLE model. In EAGLE, an entropy floor is already in use, so that the pressure of the gas is mentained high enough to prvent fragmentation of the gas. In such a scenario, maintained high enough to prevent fragmentation of the gas. In such a scenario, there is no need for the internal EoS described above. And, of course, in such a scenario, the gas can have a pressure above the floor. The code hence uses :math:P = \max(P_{\rm gas}, P_{\rm floor}, P_{\rm EoS}). ... ... @@ -515,6 +517,25 @@ The code applying this star formation law is located in the directory src/star_formation/EAGLE/. To simplify things, all constants are converted to the internal system of units upon reading the parameter file. Snapshots contain an extra field to store the star formation rates of the gas particles. If a particle was star forming in the past but isn't any more, the field will contain negative number either corresponding to the last scale-factor where the particle was star forming (cosmological runs) or the last time where it was star forming (non-cosmological runs). +------------------------+--------------------------------------+-------------+-------------------------------------+ | Name | Description | Units | Comments | +========================+======================================+=============+=====================================+ | StarFormationRates` | | Star formation rates of the gas if | [U_M / U_t] | | The quantity is not drifted so | | | | star forming. Negative numbers | | | corresponds to the rate the last | | | | indicate the last time the gas was | | | time the particle was active. | | | | star-forming. | | | | +------------------------+--------------------------------------+-------------+-------------------------------------+ Note that the star formation rates are expressed in internal units and not in solar masses per year as is the case in many other codes. This choice ensures consistency between all the fields written to the snapshots. For a normal EAGLE run, that section of the parameter file reads: .. code:: YAML ... ... @@ -527,8 +548,8 @@ For a normal EAGLE run, that section of the parameter file reads: KS_normalisation: 1.515e-4 # Normalization of the Kennicutt-Schmidt law in Msun / kpc^2 / yr. KS_exponent: 1.4 # Exponent of the Kennicutt-Schmidt law. min_over_density: 57.7 # Over-density above which star-formation is allowed. KS_high_density_threshold_H_p_cm3: 1e3 # Hydrogen number density above which the Kennicut-Schmidt law changes slope in Hydrogen atoms per cm^3. KS_high_density_exponent: 2.0 # Slope of the Kennicut-Schmidt law above the high-density threshold. KS_high_density_threshold_H_p_cm3: 1e3 # Hydrogen number density above which the Kennicutt-Schmidt law changes slope in Hydrogen atoms per cm^3. KS_high_density_exponent: 2.0 # Slope of the Kennicutt-Schmidt law above the high-density threshold. EOS_entropy_margin_dex: 0.5 # (Optional) Logarithm base 10 of the maximal entropy above the EOS at which stars can form. KS_max_density_threshold_H_p_cm3: 1e5 # (Optional) Hydrogen number density above which a particle gets automatically turned into a star in Hydrogen atoms per cm^3. threshold_norm_H_p_cm3: 0.1 # Normalisation of the metal-dependant density threshold for star formation in Hydrogen atoms per cm^3. ... ...
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