The Abundance Scale

It was mentioned above that the way the abundance scale is expressed in stdatom.dat (or for that matter, Kurucz's model atmosphere programs ATLAS9 and 12) is not the standard way that is usually found in the literature. Normally, abundances of elements are expressed with respect to hydrogen, on a scale in which the log of the abundance of hydrogen is set equal to 12.0. SPECTRUM uses a scale in which the abundances are expressed with respect to the total number density of atoms (and ions). To convert between the two scales is easy; use the following formula

$$\log(A/N_H) = 12.0 + \log(A/N_{\rm Total}) - \log(N_H/N_{\rm Total})$$

For $\log(N_H/N_{\rm Total})$, SPECTRUM uses the hydrogen abundance in the atomic and molecular data file (stdatom.dat or a corresponding custom file). The value in the current version of stdatom.dat is -0.0360.

It should be noted that SPECTRUM has been used to calculate successfully spectra for hydrogen-poor (helium-rich) models. To introduce a non-standard hydrogen and/or helium abundance, it is necessary to edit stdatom.dat AND use a model calculated for those abundances. If you do not use a model with hydrogen and helium abundances consistent with those found in stdatom.dat, it is likely that SPECTRUM will fail, as the equilibrium equations will have no solution.