Title: [OI] fine structure line profiles in Mon R2 and M17 SW: the puzzling nature of cold foreground material identified by [12CII] self-absorption

Abstract: Context. Recent studies of the optical depth comparing [12CII] and [13CII] line profiles in Galactic star-forming regions revealed strong self-absorption in [12CII] by low excitation foreground material, implying a large column density of C+ corresponding to an equivalent AV of a few, up to about 10 mag.
Aims. As the nature and origin of such a large column of cold C+ foreground gas are difficult to explain, it is essential to constrain the physical conditions of this material.
Methods. We conducted high-resolution observations of [OI] 63 um and [OI] 145 um lines in M17 SW and Mon R2. The [OI] 145 um transition traces warm PDR-material, while the [OI] 63 um line traces foreground material as manifested by absorption dips.
Results. Comparison of both [OI] line profiles with [CII] isotopic lines confirms warm PDR-origin background emission and a significant column of cold foreground material causing self-absorption visible in [12CII] and [OI] 63 um profiles. In M17 SW, the C+ and O column densities are comparable for both layers. Mon R2 exhibits larger O columns compared to C+, indicating additional material where the carbon is neutral or in molecular form. Small-scale spatial variation of the foreground absorption profiles and the large column density (around 1E18 cm-2 ) of the foreground material suggest emission from high-density regions associated with the cloud complex, not a uniform diffuse foreground cloud.
Conclusions. The analysis confirms that the previously detected intense [CII] foreground absorption is attributable to a large column of low excitation dense atomic material, where carbon is ionized, and oxygen is in neutral atomic form.