For column densities greater than
cm
,
corresponding to a Thomson depth of unity, neglect of
`Comptonization', spectral modification arising from Compton scattering,
makes model results incorrect.
In addition, the upper bound on
volume density declines as the total column density increases. The
reason is that the number of transitions in detailed balance increases
rapidly as the product of volume density and column density rises. Once
many transitions are in detailed balance, the thermal balance becomes
determined by transitions involving rarer species and excited states
which are not included in the calculation. As a rough rule of thumb,
the product of volume density and column density should not exceed
cm
.