.. _physics-stellarPopulation:

Stellar Populations
===================

Class providing stellar populations---composite descriptions of a coeval group of stars formed with a given initial mass function, metallicity, and age. For a given population, implementations return the mass recycled back to the :term:`ISM` per unit stellar mass formed, the metal yield, and the energy input rate from stellar evolution as a function of age interval. Both explicit (time-resolved) and instantaneous recycling approximations are supported. The spectra method returns the associated :galacticus-class:`stellarPopulationSpectraClass` object for computing integrated luminosities.

**Default implementation:** ``stellarPopulationStandard``

Methods
-------

``rateRecycling`` → ``double precision``
   Return the rate of mass recycling (per unit initial stellar mass formed, per unit time) from this stellar population back to the ISM over the age interval :math:`[\mathrm{ageMinimum}, \mathrm{ageMaximum}]` for the given elemental abundances.

   * ``type (abundances), intent(in ) :: abundances_``

   * ``double precision , intent(in ) :: ageMinimum , ageMaximum``

``uniqueID`` → ``integer(c_size_t)``
   Return the unique integer identifier assigned to this stellar population, which distinguishes populations with different IMFs, metallicities, or other properties and is used to cache pre-computed integrals.

``rateYield`` → ``double precision``
   Return the rate of metal (element) yield (per unit initial stellar mass formed, per unit time) from this stellar population over the age interval :math:`[\mathrm{ageMinimum}, \mathrm{ageMaximum}]`, optionally restricted to a specific element index.

   * ``type (abundances), intent(in ) :: abundances_``

   * ``double precision , intent(in ) :: ageMinimum , ageMaximum``

   * ``integer , intent(in ), optional :: elementIndex``

``rateEnergy`` → ``double precision``
   Return the rate of mechanical energy input (per unit initial stellar mass formed, per unit time) from stellar evolution processes (supernovae, winds) in this population over the age interval :math:`[\mathrm{ageMinimum}, \mathrm{ageMaximum}]`.

   * ``type (abundances), intent(in ) :: abundances_``

   * ``double precision , intent(in ) :: ageMinimum , ageMaximum``

``recycledFractionInstantaneous`` → ``double precision``
   Return the recycled fraction from this population in the instantaneous approximation.

``yieldInstantaneous`` → ``double precision``
   Return the metal yield from this population in the instantaneous approximation.

``spectra`` → ``class(stellarPopulationSpectraClass)``
   Return the :galacticus-class:`stellarPopulationSpectraClass` object associated with this stellar population, which provides methods to compute the integrated spectral energy distribution as a function of wavelength and age.

.. _physics-stellarPopulationStandard:

``stellarPopulationStandard``
-----------------------------

A standard stellar population class, which uses a fully non-instantaneous, metal-dependent calculation of recycling, metal production and rates. These rates are determined from the :galacticus-class:`stellarPopulationClass` object provided by a :galacticus-class:`stellarPopulationSelectorClass`.

It is possible to force this method to operate in the instantaneous recycling approximation limit (which can be useful for testing and comparison) by setting:

.. code-block:: none

   <stellarPopulations value="standard">
   &nbsp;&nbsp;<!-- Force the calculation of recycling, yields etc. to   -->
   &nbsp;&nbsp;<!-- be done assuming instantaneous recycling             -->
   &nbsp;&nbsp;<instantaneousRecyclingApproximation value="true"/>
   &nbsp;&nbsp;<!-- Set the recycled fraction and yield -->
   &nbsp;&nbsp;<recycledFraction value="0.35"/>
   &nbsp;&nbsp;<metalYield       value="0.02"/>
   </stellarPopulationProperties>

where the recycled fraction and metal yield are specified directly, or

.. code-block:: none

   &nbsp;&nbsp;<stellarPopulations value="standard">
   &nbsp;&nbsp;<!-- Force the calculation of recycling to be done       -->
   &nbsp;&nbsp;<!-- assuming the instantaneous recycling approximation  -->
   &nbsp;&nbsp;<instantaneousRecyclingApproximation value="true"/>
   &nbsp;&nbsp;<!-- Set the mass of stars which should be used as the    -->
   &nbsp;&nbsp;<!-- dividing line between long-lived and instantaneously -->
   &nbsp;&nbsp;<!-- evolving in this approximation.                      -->
   &nbsp;&nbsp;<massLongLived value="1.0"/>
   &nbsp;&nbsp;<!-- Set the effective age of populations to use in this -->
   &nbsp;&nbsp;<!-- approximation when computing SNe numbers.           -->
   &nbsp;&nbsp;<ageEffective value="13.8"/>
   </stellarPopulationProperties>

in which case the recycled fraction and metal yield will be computed that all stars with mass greater than ``massLongLived`` have fully evolved, and energy input (from stellar winds and supernovae) will be computed assuming that stellar populations instantaneously reach an age of ``ageEffective``.

Similar options are available to control whether metal yields and energy input from stellar populations are computed using the fully non-instantaneous or instantaneous approximations, e.g.:

.. code-block:: none

   &nbsp;&nbsp;<stellarPopulations value="standard">
   &nbsp;&nbsp;<!-- Force the calculation of recycling to be done       -->
   &nbsp;&nbsp;<!-- assuming the instantaneous recycling approximation  -->
   &nbsp;&nbsp;<instantaneousRecyclingApproximation value="true"/>
   &nbsp;&nbsp;<!-- Force the calculation of yields to be done          -->
   &nbsp;&nbsp;<!-- assuming the instantaneous recycling approximation  -->
   &nbsp;&nbsp;<instantaneousYieldApproximation value="true"/>
   &nbsp;&nbsp;<!-- Force the calculation of energy input to be done    -->
   &nbsp;&nbsp;<!-- assuming the instantaneous recycling approximation  -->
   &nbsp;&nbsp;<instantaneousEnergyInputApproximation value="true"/>
   &nbsp;&nbsp;<!-- Set the mass of stars which should be used as the    -->
   &nbsp;&nbsp;<!-- dividing line between long-lived and instantaneously -->
   &nbsp;&nbsp;<!-- evolving in this approximation.                      -->
   &nbsp;&nbsp;<massLongLived value="1.0"/>
   &nbsp;&nbsp;<!-- Set the effective age of populations to use in this -->
   &nbsp;&nbsp;<!-- approximation when computing SNe numbers.           -->
   &nbsp;&nbsp;<ageEffective value="13.8"/>
   </stellarPopulationProperties>

**(Default implementation)**

**Methods**

* ``factors`` — Compute factors needed for tidal tensor calculation.
* ``tidalTensorGet`` — Get the tidal tensor.

**Parameters**

* ``[instantaneousRecyclingApproximation]`` (boolean; default ``.false.``) — If true, then use an instantaneous recycling approximation when computing recycling rates.
* ``[instantaneousYieldApproximation]`` (boolean; default ``.false.``) — If true, then use an instantaneous recycling approximation when computing yield rates.
* ``[instantaneousEnergyInputApproximation]`` (boolean; default ``.false.``) — If true, then use an instantaneous recycling approximation when computing energy input rates.
* ``[massLongLived]`` (real; default ``1.0d0``) — The mass below which stars are assumed to be infinitely long-lived in the instantaneous approximation for stellar evolution.
* ``[ageEffective]`` (real; default ``13.8d0``) — The effective age to use for computing SNeIa yield when using the instantaneous stellar evolution approximation.
* ``[recycledFraction]`` (real; default ``0.0d0``) — The recycled fraction to use in the instantaneous stellar evolution approximation. (If not specified it will be computed internally.)
* ``[metalYield]`` (real; default ``0.0d0``) — The metal yield to use in the instantaneous stellar evolution approximation. (If not specified it will be computed internally.)