vice.primordial
===============

The VICE dataframe: primordial abundances

Stores the abundance by mass of each element following big bang
nucleosynthesis. Stored values are of type ``float``, and are zero for all
elements with the exception of helium, which is assigned a value of
:math:`Y_\text{p} = 0.24721 \pm 0.00014` (Pitrou et al. 2021 [1]_).

.. versionadded:: 1.1.0
	Previous versions of VICE did not implement helium, and therefore did
	not require any information on primordial abundances.

Indexing
--------
- ``str`` [case-insensitive]
	The symbol of a chemical element as it appears on the periodic table.

Item Assignment
---------------
This instance of the VICE dataframe does not support item assignment.

Functions
---------
- keys
- todict

Notes
-----
In versions >= 1.3.0, the primordial abundance of helium is taken to be
:math:`Y_\text{p} = 0.24721 \pm 0.00014` (Pitrou et al. 2021), which updates
the value of :math:`Y_\text{p} = 0.24672 \pm 0.00017` (Planck Collaboration
et al. 2016 [2]_; Pitrou et al. 2018 [3]_; Pattie et al. 2018 [4]_) from
previous versions of VICE based on updates to the neutron lifetime and the
:math:`\text{D}(p, \gamma)^3\text{He}` reaction.

Example Code
------------
>>> import vice
>>> vice.primordial['o']
0.0
>>> vice.primordial['he']
0.24721
>>> vice.primordial['c']
0.0

.. [1] Pitrou et al. (2021), MNRAS, 502, 2474
.. [2] Planck Collaboration et al. (2016), A&A, 594, A13
.. [3] Pitrou et al. (2018), Phys. Rep., 754, 1
.. [4] Pattie et al. (2018), Science, 360, 627