2024A&A...690A.294G

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.