# Science Documentation¶

Disclaimer: This section of VICE’s documentation is not intended to provide users with a holistic scientific background in galactic chemical evolution models. Rather, some knowledge of the science at hand is assumed. Where possible, we have linked additional references to peer reviewed journal articles with more in-depth scientific justification and disucssion.

In this documentation we adopt the notation where a lower-case $$m$$ implicitly represents the mass ratio of the star to the sun, a unitless mass measurement. When relevant, we refer to the mass of a star with units with an upper-case $$M$$. In a similar fashion, $$l$$ and $$u$$ refer to the lower and upper mass limits of star formation, respectively.

All nucleosynthetic yields in chemical evolution models provided by VICE are defined as fractional net yields. That is, they quantify the mass of stellar material that is processed into a given element and subsequently ejected to the ISM in units of the star or stellar population’s initial mass, and they do not quantify the mass fraction of nucleosynthetic material that a star or stellar population was born with. We denote these values with a lower-case $$y$$ with subscripts and superscripts denoting the element and the enrichment channel, respectively.

A capital $$Z$$ refers always to the metallicity by mass:

$Z \equiv \frac{M_x}{M}$

Where $$M_x$$ refers to the mass of some element $$x$$ and $$M$$ to the mass of either the interstellar gas or a star.

The logarithmic abundance measurement $$[X/H]$$ is defined by:

$[X/H] \equiv \log_{10}\left(\frac{Z_x}{Z_x^\odot}\right)$

This approximation assumes hydrogen mass fractions are similar to the sun always. Relaxing this assumption would require subtracting the term $$\log_{10}(X/X_\odot)$$ where $$X$$ is the hydrogen mass fraction. However, this is generally a negligible correction as hydrogen mass fractions vary only a little, especially on a logarithmic scale ($$\lesssim$$ 0.05 dex), and neither the types of models that VICE provides nor observationally derived abundances can claim this level of precision anyway. The logarithmic abundance ratios $$[X/Y]$$ follow accordingly:

$[X/Y] = [X/H] - [Y/H] = \log_{10}\left(\frac{Z_x}{Z_x^\odot}\right) - \log_{10}\left(\frac{Z_y}{Z_y^\odot}\right)$

The symbols $$\odot$$ and $$\tau$$ refer to the sun and a timescale, respectively, and the term “zone models” refers to both one-zone and multi-zone models in the general sense.