Abundances

Korg's functions take arguments specifying abundances in two different ways. Korg.synthesize takes a vector of $A(X)$ abundances, one for every element from H to U. (Korg.format_A_X is a convenient way to create this vector.) Other functions (synth, Korg.Fit.fit_spectrum, etc.) take arguments specifying abundances more indirectly. In either case, it's important to be clear about how the arguments are interpreted.

The "abundance" of an element, $X$, can refer to both $A(X)$ ("absolute") and [$X$/H] ("solar relative"). These are defined as:

\[A(X) = \log_{10} \left( \frac{n_X / n_\mathrm{H}} \right) + 12\]

and

\[[X / \mathrm{H}] = A(X) - \left(A(X)\right)_\odot = \log_{10} \left ( \frac{n_X n^\odot_\mathrm{H}}{n_\mathrm{H} n^\odot_X} \right )\]

More fraught than "abundance" is "metallicity", which is routinely used to refer to the metal mass fraction ($Z$), the iron abundance (solar relative or absolute), or to

\[[M / \mathrm{H}] = \log_{10} \left( \frac{\sum_\mathrm{X \in M} n_X}{n_\mathrm{H}} \frac{n^\odot_\mathrm{H}}{\sum_\mathrm{X \in M} n^\odot_X} \right),\]

where the set $M$ can refer to everything heavier than He, or to a subset of elements that researchers were able to measure. Korg.get_metals_H computes this.

Korg.get_metals_H(Korg.default_solar_abundances)

0.0

To generate abundances with a different metallicity, do:

A_X = Korg.format_A_X(-1)
Korg.get_metals_H(A_X)

-1.0

But watch out! If we also specify [N/H], we get a different result:

A_X = Korg.format_A_X(-1, Dict(["N" => -0.5]))
Korg.get_metals_H(A_X)

-0.8485012484862304

That's because the first argument of Korg.format_A_X specifies what the "default" abundance for each element should be, not the total metallcity. It would be easy for it to scale the abundances of everything except N so that the resulting metallicity was still -1.0, but this would be confusing in its own way.