M43, Horsehead, MonR2, M17SW [CII]158um spectra

We first derive the [^12^CII] optical depth and the [CII] column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multi-component velocity fit allows to derive the physical conditions of the [CII] gas: column density and excitation temperature.

We find moderate to high [^12^CII] optical depths in all four sources, and self-absorption of [^12^CII] in Mon R2 and M17 SW. The high column density of the warm background emission corresponds to an equivalent Av of up to 41mag. The foreground absorption requires substantial column densities of cold and dense [CII] gas, with an equivalent Av ranging up to about 13mag.

The column density of the warm background material requires multiple PDR surfaces stacked along the line of sight and in velocity. The substantial column density of dense and cold foreground [CII] gas detected in absorption cannot be explained with any known scenario and we can only speculate about its origin.