Basic model (others)
Sinks: Simple Bondi-Hoyle accretion
The Basic sink model provides a foundation on which new sink implementations could be built. It includes a prescription for Bondi-Hoyle gas accretion, and a method for sink-sink mergers that is a slightly simplified version of the implementation used in GEAR.
No other physics is implemented for this model. Sinks cannot form - to use this model, sink particles must already be present in the initial conditions. They also cannot spawn stars from the gas they accrete.
Bondi-Hoyle accretion can be done in one of two ways:
Gas particles within the sink’s kernel are stochastically swallowed entirely, with a probability set by the Bondi-Hoyle rate. Specifically, the probability is set by the current difference between the sink’s subgrid mass (determined by the accretion rate) and its dynamical mass (which tracks the number of particles/sinks actually swallowed). This mode is equivalent to the EAGLE black hole accretion model.
Gas particles within the sink’s kernel are “nibbled” down to some minimal mass, which can be specified by the user. This method is equivalent to the black hole accretion model of Bahe et al. 2022.
This model has only two parameters that must be specified in your parameter yml file:
BasicSink:use_nibbling: determines whether accretion is done by “nibbling” or by swallowing outright.
BasicSink:min_gas_mass_for_nibbling_Msun: if using “nibbling”, the minimum mass to which gas particles can be nibbled. A good default is half the original particle mass.
For an even more bare-bones starting point, the Default sink model contains no physics at all, and is a totally blank canvas on which to build your sink model.
Cooling: Analytic models
Currently, we have 3 different simple cooling models (const-lambda, const-du and Compton). These are all based on analytic formulas and can be used to quickly understand how the cooling interacts with the rest of the code before moving to more complex models.
Equations
The first table compares the different analytical cooling while the next ones are specific to a given cooling. The quantities are the internal energy (\( u \)), the density \( rho \), the element mass fraction (\( X_i \)), the cooling function (\(\Lambda\), the proton mass (\( m_H \)) and the time step condition (\( t_\text{step}\)). If not specified otherwise, all cooling contains a temperature floor avoiding negative temperature.
Variable |
Const-Lambda |
Const-du |
|---|---|---|
\( \frac{ \mathrm{d}u }{ \mathrm{d}t } \) |
\( -\Lambda \frac{\rho^2 X_H^2}{\rho m_H^2} \) |
const |
\( \Delta t_\text{max} \) |
\( t_\text{step} \frac{u}{\left|\frac{ \mathrm{d}u }{ \mathrm{d}t }\right|} \) |
\( t_\text{step} \frac{u}{\ \left| \frac{ \mathrm{d}u }{ \mathrm{d}t }\right|} \) |
TODO: Add description of the parameters and units.
TODO: Add Compton cooling model
How to Implement a New Cooling
- The developer should provide at least one function for:
writing the cooling name in HDF5
cooling a particle
the maximal time step possible
initializing a particle
computing the total energy radiated by a particle
initializing the cooling parameters
printing the cooling type
For implementation details, see src/cooling/none/cooling.h
See General information for adding new schemes for the full list of changes required.