What is the definition of $[\alpha/{\rm H}]$?

The notation $[A/B]$ denotes

\begin{equation} [A/B] = \text{log}_{10}\left[\frac{(\text{number of A atoms/number of B atoms})_{\star}}{(\text{number of A atoms/number of B atoms})_{\odot}}\right] \end{equation}

or

\begin{equation} [A/B]\equiv\text{log}_{10}\left(\frac{A}{B}\right)_{\star}-\text{log}_{10}\left(\frac{A}{B}\right)_{\odot} \end{equation}

where $\star$ refers to the star and $\odot$ refers to the Sun. $[A/B]$ means the log abundance of $A/B$ relative to solar.

Therefore, $[{\rm Fe}/{\rm H}]$ is the log abundance of ${\rm Fe}/{\rm H}$ relative to solar. In a star with $[{\rm Fe}/{\rm H}]=-2$, iron is 1% as abundant as in the Sun.

In SDSS SEGUE papers, one finds $[\alpha/{\rm Fe}]$. What does this $\alpha$ denote? Helium?

• alpha particles? – John M Sep 8 '15 at 15:52

The $\alpha$ refers to "alpha process elements" which are the result of a class of fusion processes involving helium. The $\alpha$ elements are roughly speaking a bunch of helium nuclei ("$\alpha$ particles") stuck together, so they have an integer multiple of 4 nucleons. The $\alpha$ elements are Ne, Mg, Si, S, Ar, Ca and Ti.
A measurement of any one of these elements can be called "$\alpha$", it's not some combination of their abundances. Lumping them together means that measurements that use different tracers can be combined in one comparison. For instance, Type II supernovae produce a certain ratio of $\alpha$ elements and "iron peak" elements (Ti, V, Cr, Mn, Fe, Co and Ni), while Type Ia supernovae produce primarily iron peak elements. The metallicity (e.g. [Fe/H]) at which [$\alpha$/Fe] begins to drop is thus indicative of when in a galaxy's history Type Ia supernovae began contributing to chemical enrichment.