The [O I] fine structure line profiles in Mon R2 and M17 SW: The puzzling nature of cold foreground material identified by [12C II] self-absorption

Context. Recent studies of the optical depth comparing [^{12C II] and [13C II] line profiles in Galactic star-forming regions have revealed strong self-absorption in [12C II] by low excitation foreground material. This implies a high column density for C+, corresponding to equivalent A_{V values of a few (up to about 10) mag.Aims. As the nature and origin of such a great column of cold C+ foreground gas are difficult to determine, it is essential to constrain the physical conditions of this material.Methods. We conducted high-resolution observations of [O I] 63 μm and [O I] 145 μm lines in M17 SW and Mon R2. The [O I] 145 μm transition traces warm PDR-material, while the [O I] 63 μm line traces the foreground material, as manifested by the absorption dips.Results. A comparison of both [O I] line profiles with [C II] isotopic lines confirm warm PDR-origin background emission and a significant column of cold foreground material, causing the self-absorption to be visible in the [12C II] and [O I] 63 μm profiles. In M17 SW, the C+ and O0 column densities are comparable for both layers. Mon R2 exhibits larger O0 columns compared to C+, indicating additional material where the carbon is neutral or in molecular form. Small-scale spatial variations in the foreground absorption profiles and the large column density (~1018 cm−2) of the foreground material suggest the emission is coming from high-density regions associated with the cloud complex – and not a uniform diffuse foreground cloud.Conclusions. The analysis confirms that the previously detected intense [C II] foreground absorption is attributable to a large column of low-excitation dense atomic material, where carbon is ionized and oxygen is in a neutral atomic form.}}