The [OI] Fine Structure Line Profiles in Mon R2 and M17 SW: the Puzzling Nature of Cold Foreground Material Identified by [$^{12}$CII] Self-Absorption

Recent studies of the optical depth comparing [12CII] and [13CII] line profiles in Galactic star-forming regions have revealed strong self-absorption in [12CII] by low excitation foreground material. This implies a high column density for C+, corresponding to equivalent AV values of a few (up to about 10) mag. 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. We conducted high-resolution observations of [OI] 63 micron and 145 micron lines in M17 SW and Mon R2. The [OI] 145 micron transition traces warm PDR material, while the 63 micron line traces the foreground material, as manifested by the absorption dips. 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 [CII] and [OI] 63 micron profiles. In M17 SW, the C+ and atomic oxygen column densities are comparable for both layers. Mon R2 exhibits larger atomic oxygen 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 (~1e18 /cm2) 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. 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 a neutral atomic form.