= Primordial atomic cooling curve
{title2=$\Lambda_{\rm H,He}(T)$}
The <primordial atomic cooling curve> describes hydrogen-helium gas without heavy elements. Near the <hydrogen> excitation threshold it rises sharply, then shows <hydrogen> and <helium> excitation/<ionization> features. Bound-state losses weaken when both elements become highly ionized. At sufficiently high temperatures <thermal bremsstrahlung> gives a slowly rising tail. Composition, density convention and <ionization> assumptions fix the numerical normalization.
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