The Astrophysical Journal (Jan 2024)

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

  • James M. Jackson,
  • J. Scott Whitaker,
  • Edward Chambers,
  • Robert Simon,
  • Cristian Guevara,
  • David Allingham,
  • Philippa Patterson,
  • Nicholas Killerby-Smith,
  • Jacob Askew,
  • Thomas Vandenberg,
  • Howard A. Smith,
  • Patricio Sanhueza,
  • Ian W. Stephens,
  • Lars Bonne,
  • Fiorella L. Polles,
  • Anika Schmiedeke,
  • Netty Honigh,
  • Matthias Justen

DOI
https://doi.org/10.3847/1538-4357/ad2849
Journal volume & issue
Vol. 965, no. 2
p. 187

Abstract

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Using the upGREAT instrument on board the Stratospheric Observatory for Infrared Astronomy, we imaged [C ii ] 157.74 and [O i ] 63.18 μ m 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: NH _3 (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.

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