vice.singlezone.tau_star
========================

Type : real number or ``<function>``

Default : 2.0

The star formation rate per unit gas supply in Gyr, defined by

.. math:: \tau_\star \equiv M_\text{g}/\dot{M}_\star

where :math:`M_\text{g}` is the ISM gas mass and :math:`\dot{M}_\star`
is the star formation rate. Numbers will be interpreted as a constant
value. Functions must accept either one or two parameters, the first
of which will always be time in Gyr. In infall and gas modes, the
second parameter will always be interpreted as the gas mass in
:math:`M_\odot`, but in star formation mode, it will be interpreted as
the star formation rate in :math:`M_\odot/yr`. This approach allows
this attribute to vary with either the gas mass or the star formation
rate in simulation (depending on which mode the model is ran in).

.. versionadded:: 1.2.0
	Prior to version 1.2.0, a functional form for this attribute had
	to accept only one numerical parameter, always interpreted as
	time in Gyr.

.. tip::

	In infall and gas modes, this parameter can be set to infinity to
	forcibly shut off star formation.

.. tip::

	When adopting a functional form for this attribute which depends
	on the gas supply itself via a pure power-law, we recommend users
	make use of the attributes ``schmidt``, ``schmidt_index``, and
	``MgSchmidt``. These control the parameters of the power-law and
	allow VICE to calculate the values internally, resulting in
	shorter integration times.

.. note::

	When the attribute ``schmidt == True``, this is interpreted as the
	prefactor on gas-dependent star formation efficiency:

	.. math:: \tau_\star^{-1} = \tau_{*,\text{specified}}^{-1}
		\left(
		\frac{M_\text{g}}{M_\text{g,Schmidt}}
		\right)^{\alpha}

	where :math:`\alpha` is the power-law index on gas-dependent
	star formation efficiency, set by the attribute ``schmidt_index``,
	and :math:`\tau_{*,\text{specified}}` is the value of this
	attribute.

.. note::

	Saving functional attributes with VICE outputs requires the
	package dill_, an extension to ``pickle`` in the Python_ standard
	library. It is recommended that VICE user's install dill_
	>= 0.2.0.

	.. _dill: https://pypi.org/project/dill/
	.. _Python: https://docs.python.org/library/

.. note::

	In the interstellar medium and star formation literature, this
	parameter is often referred to as the depletion timescale. In this
	documentation and in much of the galactic chemical evolution
	literature, it is often referred to as the "star formation
	efficiency timescale."

.. note::

	If the user assigns this attribute a function which is ran through
	a Cython_ compiler, the corresponding Cython_ source code must be
	compiled with the ``binding = True`` directive. This allows
	VICE to inspect the signature of the compiled function; otherwise,
	assigning the function to this attribute will raise a
	``ValueError``.

	.. _Cython: https://cython.org/

Example Code
------------
>>> import vice
>>> sz = vice.singlezone(name = "example")
>>> sz.tau_star = 1
>>> def f(t):
	if 5 <= t <= 6:
		return 1
	else:
		return 2
>>> sz.tau_star = f