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 [ 12 C II ] and [ 13 C II ] line profiles in Galactic star-forming regions have revealed strong self-absorption in [ 12 C 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 [ 12 C II ] and [O I ] 63 μm profiles. In M17 SW, the C + and O 0 column densities are comparable for both layers. Mon R2 exhibits larger O 0 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 (~10 18 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.