...CD

This is actually not obvious in the figure for light elements and small ionization parameters. Indeed when $\xi$ is small and the column density is large, the intermediate zone is dominated by weakly ionized species (CII, OII, etc.) and our code Titan does not converge correctly, owing to the oversimplified description of these ions. This is why the column densities of CV, CVI and OVII, do not display the saturation for $\xi =10$ and 30, though $CD_{{\rm CV}}$, $CD_{{\rm CVI}}$ and  $CD_{{\rm OVII}}$ cannot exceed those shown on the figure.

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... slabs

These EWs correspond to a non-radial view of the objects.

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