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Description
The [CII] 158um FIR fine-structure line is one of the most important cooling lines of the star-forming ISM. It is used as a tracer of star formation efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the line profiles of the [CII] emission. To determine whether the complex profiles observed in [^12^CII] are due to individual velocity components along the line-of-sight or due to self-absorption, one has to compare the [^12^CII] and isotopic [^13^CII] line profiles. Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in the sources M43, Horsehead PDR, Monoceros R2 and M17 SW allow to detect with high S/N the optically thin [^13^CII] and simultaneously the [^12^CII] emission lines. 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.
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