Absorption and Self-absorption of [C II] and [O I] Far Infrared Lines toward a Bright Bubble in the Nessie Infrared Dark Cloud

Using the upGREAT instrument on board the Stratospheric Observatory for Infrared Astronomy, we imaged [C II] 157.74 and [O I] 63.18 μm line emission from a bright photodissociation region (PDR) associated with an ionized bubble located in the Nessie Nebula, a filamentary infrared dark cloud. A comparison with Australia Telescope Compact Array data reveals a classic photodissociation region (PDR) structure, with a uniform progression from ionized gas, to photodissociated gas, and to molecular gas from the bubble's interior to its exterior. [O I] line emission from the bubble's PDR reveals self-absorption features. Toward a far-IR bright protostar, both [O I] and [C II] show an absorption feature at a velocity of ‑18 km s^‑1, the same velocity as an unrelated foreground molecular cloud. Since the gas density in typical molecular clouds is well below the [O I] and [C II] critical densities, the excitation temperatures for both lines are low (∼20 K). The Meudon models demonstrate that the surface of a molecular cloud, externally illuminated by a standard G ₀ = 1 interstellar radiation field, can produce absorption features in both transitions. Thus, the commonly observed [O I] and [C II] self-absorption and absorption features plausibly arise from the subthermally excited, externally illuminated photodissociated envelopes of molecular clouds. The luminous young stellar object AGAL337.916-00.477, located precisely where the expanding bubble strikes the Nessie filament, is associated with two shock tracers: NH₃ (3,3) maser emission and SiO 2‑1 emission, indicating an interaction between the bubble and the filament. The interaction of the expanding bubble with its parental dense filament has triggered star formation.